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micheleandClaude Sonnet 5 ee5a68f6e6 exp: SDRAM CDC bridge + open-row policy (EXP-0053/54/55) -- open-row is a real ~5% D-Stress win, CDC bridge measured net-negative once integrated 
EXP-0053: sdram_cdc_bridge.v decouples the SDRAM clock (115.2MHz, real
value derived from the board's own existing PLL VCO=576MHz, verified
via ecppll) from the 64MHz compute domain. Isolated: 137/137 tests, 0
errors, but real measured speedup is only 1.095x (not the naive 1.8x
clock-ratio estimate) -- the CDC handshake's own synchronizer
round-trip is a fixed per-transaction tax.

EXP-0054: sdram_controller_openrow.v implements the page-hit/
keep-row-open optimization sdram_controller.v's own header had always
deferred. weight_prefetch_engine_wide.v's real production traffic is
strictly sequential per job and mostly stays within one SDRAM row --
closing/reopening it every tile (today's fixed auto-precharge policy)
wastes tRP+tRCD for no reason. Isolated: 154/154 tests, 0 errors, 0
protocol violations (including the new refresh-while-row-open hazard,
fixed via an explicit precharge-before-refresh path). Real measured
speedup on the actual sequential access pattern: 1.141x.

EXP-0055: composed both, then integrated into the real D-Stress
benchmark (N=4/N=8, 256/256 bit-exact in every config). Result:
open-row ALONE gives a real, consistent ~5% cycle-count improvement
(47445/47468 vs baseline 49927/49909). CDC alone is a real ~8%
REGRESSION. Combined is still a ~4% regression -- the CDC's fixed tax
is paid on every transaction regardless of row-hit, and real D-Stress
traffic interleaves weight-fetch/activation-result access far more
than the isolated same-row test exercised, so open-row's real saving
doesn't offset it. Decision: do not adopt the CDC approach; open-row
alone is the disclosed, real win worth considering for production
next, pending an explicit go-ahead (not applied to the real board top
in this commit -- all additive, existing production RTL untouched).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MUG92aM9m68TRc4rG55BcC
2026-09-16 07:07:57 +02:00

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# V2 experiments log -- REGISTRO PRINCIPALE (docs/v2-description.md §25).
# Un EXP-XXXX per ogni esperimento end-to-end (config -> sim/synth/timing ->
# risultato). ID mai riutilizzati, anche per i FAIL. Ogni EXP rimanda a
# reports/experiments/EXP-XXXX/ (experiment.yaml, simulation.log,
# synthesis.log, timing.log, results.txt, notes.md).
#
# Nessun esperimento ancora eseguito.
EXP-0001
timestamp: 2026-09-05T12:03:12Z
git_commit: 07a48e401f56d395f9d1a6e22f1b5ebc6b598e64 (+ uncommitted M1 work)
session: v2-M1-neural-processor
module: hardware/v2/rtl/neural_processor.v
configuration: DATA_WIDTH=8, P_IN=8, ACC_WIDTH=32
action: first M1 implementation -- 8-stage pipelined perceptron unit,
bit-exact vs hardware/v1/rtl/neuron_parallel.v + mac8.v + mac_unit.v
command (sim, Verilator): verilator --binary --timing -j 0 -Wno-fatal
--top-module tb -o /tmp/vtb_np hardware/v1/rtl/neuron_parallel.v
hardware/v1/rtl/mac8.v hardware/v1/rtl/mac_unit.v
hardware/v2/rtl/neural_processor.v hardware/v2/sim/tb_neural_processor.v
&& /tmp/vtb_np
command (synth): yosys -p "synth_ecp5 -json
hardware/v2/synthesis/neural_processor_p8/top.json -top neural_processor"
hardware/v2/rtl/neural_processor.v
command (timing): nextpnr-ecp5 --45k --package CABGA381 --speed 8
--freq 80 --json hardware/v2/synthesis/neural_processor_p8/top.json
--lpf-allow-unconstrained --textcfg .../top.config
result:
SIMULATED: 7/7 tests PASS, bit-exact vs V1 (functional +
extreme-INT8 + back-to-back-tiles + 64-input/8-tile coverage).
SYNTHESIZED: 0 CHECK problems, 8 MULT18X18D, 533 TRELLIS_FF,
96 CCU2C, 55 LUT4 (36 benign integer-loop-variable warnings, see
hardware/v2/logs/synthesis.log).
POST-P&R: Fmax = 183.12 MHz, PASS at 80MHz target (real
nextpnr-ecp5 measurement, hardware/v2/synthesis/neural_processor_p8/
nextpnr.log). Compare V1 isolated PARALLEL=8: 61.71 MHz
(hardware/v1/synthesis/p8/) -- ~3x higher Fmax for the pipelined
single-processor datapath alone (not yet a system-level
comparison -- no Memory Manager/Director/multi-processor overhead
included at this milestone).
errors: ERR-0001, ERR-0002, ERR-0003 (see errors.log) encountered and
resolved/worked around during development.
decision: see decisions.log DEC-0002, DEC-0003, DEC-0004.
next_action: EXP-0002 (ACC_WIDTH=24 comparison), then M2 (Processor
Array, N_PROCESSORS sweep).
EXP-0002
timestamp: 2026-09-05T12:03:12Z
git_commit: 07a48e401f56d395f9d1a6e22f1b5ebc6b598e64 (+ uncommitted M1 work)
session: v2-M1-neural-processor
module: hardware/v2/rtl/neural_processor.v
configuration: DATA_WIDTH=8, P_IN=8, ACC_WIDTH=24 (vs EXP-0001's
ACC_WIDTH=32 baseline) -- requested explicitly by the user as a test
variant ("come test nel perceptrone crea una versione con
accumulatori a 24 bit anziche' a 32 bit").
reason: ACC_WIDTH is already fully parametric throughout
neural_processor.v (no code duplication needed); 24 bits is a real,
non-degenerate choice -- worst-case realistic accumulation (e.g. 256
INT8 inputs all at +-127/+-128) needs at most 23 signed bits
(256 * 128 = 32768, |value| <= 2^22 for the largest single-tile-
chain sums this project's configs reach), so 24 bits carries a
legitimate 1-bit margin, not an arbitrary/unsafe truncation.
command (sim, Verilator): same as EXP-0001 with a copy of
tb_neural_processor.v with ACC_WIDTH localparam changed to 24 (V1's
own neuron_parallel instantiated at ACC_WIDTH=24 too, for a true
bit-exact comparison at the SAME width -- V1's ACC_WIDTH is also a
module parameter, no V1 modification needed).
command (synth): yosys -p "read_verilog
hardware/v2/rtl/neural_processor.v; chparam -set ACC_WIDTH 24
neural_processor; synth_ecp5 -json
hardware/v2/synthesis/neural_processor_p8_acc24/top.json -top
neural_processor"
command (timing): nextpnr-ecp5, same flags as EXP-0001, json from
the ACC_WIDTH=24 build.
result:
SIMULATED: 7/7 tests PASS, bit-exact vs V1 at ACC_WIDTH=24 (no
overflow in any test vector, as expected from the margin analysis
above).
SYNTHESIZED: 0 CHECK problems, 8 MULT18X18D (unchanged, multiplier
width is DATA_WIDTH-driven not ACC_WIDTH-driven), 509 TRELLIS_FF
(was 533 at ACC_WIDTH=32, -24 FF as expected for 8 fewer
accumulator bits carried through ~3 pipeline-stage copies),
88 CCU2C (was 96), 49 LUT4 (was 55).
POST-P&R: Fmax = 176.21 MHz (was 183.12 MHz at ACC_WIDTH=32) --
REAL measurement, both from nextpnr-ecp5. The 24-bit build is
~4% SLOWER despite fewer resources -- almost certainly placement
noise (consistent with this project's established finding,
hardware/v1/docs/WORKLOG.md "Timing closure", that seed-to-seed
placement variance on this device dominates small logic-width
differences), NOT attributed to a real architectural effect
without a seed sweep to confirm. Reported as-measured, not
overinterpreted -- see hardware/v2/logs/benchmark.log.
errors: none.
decision: ACC_WIDTH=32 remains the M1 default (matches V1 exactly for
ongoing bit-exact comparisons); ACC_WIDTH=24 confirmed as a viable,
correctly-functioning, slightly-smaller alternative, not adopted as
default without a proper seed sweep (out of scope for this single
comparison run).
next_action: revisit ACC_WIDTH choice during M10 (Optimization) with a
real seed sweep, not before.
EXP-0003
timestamp: 2026-09-05T14:30:00Z
git_commit: dc0b331 (+ uncommitted M2 work)
session: v2-M2-processor-array
module: hardware/v2/rtl/neural_processor_array.v
configuration: N_PROCESSORS in {1,2,4,8}, P_IN=8, ACC_WIDTH=32 each
action: M2 -- functional array + real N_PROCESSORS resource/timing sweep
command (sim, Verilator): verilator --binary --timing -j 0 -Wno-fatal
--top-module tb -o /tmp/vtb_array hardware/v2/rtl/neural_processor.v
hardware/v2/rtl/neural_processor_array.v
hardware/v2/sim/tb_neural_processor_array.v && /tmp/vtb_array
command (synth/timing): see hardware/v2/synthesis/harness_n{1,2,4,8}/
(yosys.log, nextpnr.log) -- synthesized via
hardware/v2/synthesis/harness_neural_processor_array.v, a
synthesis-only timing harness (see its own header comment and
errors.log ERR-0005 for why the array cannot be synthesized as a
bare top-level module beyond N_PROCESSORS=1 without it).
result:
SIMULATED (N_PROCESSORS=4, tb_neural_processor_array.v): 7/7 tests
PASS -- single-processor sanity, 4 processors launched the SAME
cycle with different tile counts (finish at different times,
proving true concurrency), and a staggered-start test (processor 1
launched mid-way through processor 0's 6-tile job, both complete
correctly and independently, confirming §18/§34's "un processor
bloccato non deve bloccare gli altri").
SYNTHESIZED (resource scaling, harness): perfectly linear in
N_PROCESSORS, confirming no unintended resource sharing:
N=1: LUT=59 FF=409 MULT18X18D=8 CCU2C=96
N=2: LUT=106 FF=786 MULT18X18D=16 CCU2C=192
N=4: LUT=207 FF=1540 MULT18X18D=32 CCU2C=384
N=8: LUT=374 FF=3048 MULT18X18D=64 CCU2C=768
0 CHECK problems in every configuration.
POST-P&R (real nextpnr-ecp5, --45k --package CABGA381 --speed 8
--freq 80): Fmax PASS at 80MHz in every configuration:
N=1: 159.11 MHz N=2: 149.59 MHz
N=4: 151.01 MHz N=8: 134.70 MHz
Fmax decreases gently with N (routing congestion), never close to
failing the 80MHz target up to N=8.
REAL RESOURCE CEILING FOUND (not assumed, measured via nextpnr's own
device utilisation report): MULT18X18D usage is 22%/44%/88% of the
LFE5U-45F's 72 total DSPs at N=2/4/8 respectively, while LUT4/FF
usage stays under 6% even at N=8. **DSP (not LUT/FF/routing) is
the first hard ceiling as N_PROCESSORS grows at P_IN=8** --
N_PROCESSORS=9 would already exceed the device's 72 MULT18X18D
budget at P_IN=8, before accounting for any multipliers the rest
of a real system (Memory Manager, PSRAM path, etc.) might need.
See decisions.log DEC-0005 and benchmark.log.
errors: ERR-0005 (toplevel-pin-count synthesis artifact, worked around
with the timing harness -- see errors.log); a first harness attempt
fed every processor and every MAC lane identical LFSR-derived data,
which Yosys correctly (from pure logic-equivalence) collapsed via
CSE down to 1 processor's worth of multipliers regardless of N --
fixed by giving each processor AND each of its P_IN MAC lanes a
distinct bit-rotated data source, confirmed by the corrected,
properly-linear MULT18X18D counts above.
decision: see decisions.log DEC-0005 (DSP is the binding constraint,
not LUT/FF -- informs how the N_PROCESSORS x P_IN trade-off should
be explored going forward).
next_action: M3 -- activation_buffer.v / weight_buffer.v /
result_buffer.v.
EXP-0004
timestamp: 2026-09-05T15:15:00Z
git_commit: 3026dcd (+ uncommitted M3 work)
session: v2-M3-buffers
module: hardware/v2/rtl/activation_buffer.v, weight_buffer.v,
result_buffer.v
configuration: activation_buffer/result_buffer DEPTH in {4096, 256};
weight_buffer DEPTH (in tiles) in {512, 64}, P_IN=8, DATA_WIDTH=8
(TILE_WIDTH=64 bits)
action: M3 -- three parametric dual-port BRAM-inferring buffers
(Input/Weight/Result of the §12 data-plane diagram), reusing the
proven inference idiom from hardware/v1/rtl/act_buffer.v (sync
write, sync REGISTERED read, no reset on the read register).
command (sim, Verilator): verilator --binary --timing -j 0 -Wno-fatal
--top-module tb -o /tmp/vtb_buffers hardware/v2/rtl/
activation_buffer.v hardware/v2/rtl/weight_buffer.v
hardware/v2/rtl/result_buffer.v hardware/v2/sim/tb_buffers.v &&
/tmp/vtb_buffers
command (synth): yosys -p "read_verilog <file>; chparam -set DEPTH
<N> <module>; synth_ecp5 -json .../top.json -top <module>" for each
of the 6 (module, depth) combinations.
command (timing): nextpnr-ecp5 --45k --package CABGA381 --speed 8
--freq 80 --lpf-allow-unconstrained, default-depth configs only.
result:
SIMULATED: 10/10 tests PASS -- write-then-read correctness incl.
extreme INT8 (-128/127) round-tripping exactly, back-to-back
writes to different addresses not disturbing earlier entries, and
weight_buffer's full 64-bit TILE_WIDTH round-tripping (not just
one lane).
SYNTHESIZED (real BRAM mapping, not assumed):
activation_buffer DEPTH=4096: 2 DP16KD, 37 LUT4, 30 FF
activation_buffer DEPTH=256: 1 DP16KD, 21 LUT4, 26 FF
weight_buffer DEPTH=512: 2 DP16KD, 88 LUT4, 139 FF
weight_buffer DEPTH=64: 2 DP16KD, 73 LUT4, 136 FF
result_buffer DEPTH=4096: 2 DP16KD, 37 LUT4, 30 FF
result_buffer DEPTH=256: 1 DP16KD, 21 LUT4, 26 FF
0 CHECK problems in all 6 configurations -- every one correctly
inferred DP16KD block RAM, none fell back to LUT-RAM.
REAL, NON-OBVIOUS FINDING (§14: "non assumere che buffer piu' grandi
siano automaticamente migliori"): shrinking weight_buffer's DEPTH
8x (512->64) did NOT reduce DP16KD usage at all (2->2) -- its
64-bit TILE_WIDTH (P_IN*DATA_WIDTH), not its depth, is what forces
2 physical block RAMs (a single DP16KD's usable port width in the
density this needs is narrower than 64 bits). Depth-only buffer
sizing decisions are the wrong lever for THIS buffer; width
(P_IN) is. activation_buffer/result_buffer, being byte-wide, DO
show the expected depth-proportional DP16KD count (2 -> 1).
POST-P&R (default-depth configs): all PASS at 80MHz with large
margin (287-367 MHz range, real place&route) -- these buffers are
not a timing concern in isolation.
errors: none.
decision: keep DEPTH parametric as specified, but document (this
entry + benchmark.log) that weight_buffer's real BRAM cost is driven
by P_IN (its width), not DEPTH -- relevant when M4/M9 size these
buffers against a real workload.
next_action: M4 -- memory_manager.v + prefetch_engine.v (wires these
three buffers + the array together, PSRAM backend reused unmodified
from V1 per §15).
EXP-0005
timestamp: 2026-09-05T16:00:00Z
git_commit: 5f0d7f1 (+ uncommitted M4 work)
session: v2-M4-memory-manager
module: hardware/v2/rtl/memory_manager.v, prefetch_engine.v
configuration: P_IN=8, ADDR_WIDTH=23, real V1 backend chain
(int8_memory_access -> memory_interface -> psram_controller ->
psram_model, ALL unmodified), real M1 neural_processor
action: M4 -- end-to-end integration: memory_manager double-buffers
prefetch of X/W tiles from PSRAM, feeds a real neural_processor,
writes the computed result back to PSRAM.
command (sim, Verilator): verilator --binary --timing -j 0 -Wno-fatal
--top-module tb -o /tmp/vtb_mm hardware/v1/rtl/int8_memory_access.v
hardware/v1/rtl/memory_interface.v hardware/v1/rtl/psram_controller.v
hardware/v1/sim/psram_model.v hardware/v2/rtl/neural_processor.v
hardware/v2/rtl/prefetch_engine.v hardware/v2/rtl/memory_manager.v
hardware/v2/sim/tb_memory_manager.v && /tmp/vtb_mm
command (synth): yosys -p "synth_ecp5 -json .../top.json -top
memory_manager" hardware/v2/rtl/memory_manager.v
hardware/v2/rtl/prefetch_engine.v (standalone, for real resource
counts); harness_memory_manager.v (see errors.log ERR-0005 pattern)
for real P&R Fmax, since the bare module exceeds the device's
TRELLIS_IO budget as a top-level (same class of artifact as the
Processor Array, not a logic limit).
result:
SIMULATED: 3/3 jobs PASS end-to-end -- 3-tile (bias=0, ACT_RELU,
saturates to 127), 1-tile (no saturation, y=32), and 5-tile
(steady-state double-buffer swap across more than 2 tiles, y=40)
-- each verified by an INDEPENDENT PSRAM read-back of the
written result byte (not just internal signal inspection), with
"poison" bytes surrounding the real operand regions to catch any
off-by-one addressing (none found).
Cycle counts (real, PSRAM power-up already excluded): 3-tile job =
446 cycles, 1-tile job = 166 cycles, 5-tile job = 728 cycles.
Roughly ~140-150 cycles/tile at this PSRAM timing (dominated by
the real ~70ns TAA access latency modeled in psram_model.v, not
by memory_manager's own control overhead) -- a real, measured
number, not estimated.
SYNTHESIZED (standalone, real resource count): 0 CHECK problems,
851 LUT4, 789 TRELLIS_FF, 108 CCU2C, 0 DSP (expected -- no
multiplication in this module).
POST-P&R (via harness, real Fmax): 165.86 MHz, PASS at 80MHz with
large margin.
errors: ERR-0006 (three real RTL bugs found and fixed during bring-up
-- see errors.log for full detail: a missing single-in-flight-
request discipline, a one-cycle pf_busy blind spot, and an
off-by-one state mux for the write-back path). ERR-0005's pin-count
artifact recurred for this module too (worked around the same way).
decision: see decisions.log DEC-0006 (single prefetch engine + pending
register is sufficient for this milestone's scope; a real backend
arbiter is deferred until multiple processors/jobs actually need to
share one memory_manager).
next_action: M5 -- neural_director.v (first-free scheduling), wiring
job dispatch to potentially multiple (memory_manager, neural_
processor) pairs instead of the single hardcoded pair tested here.
EXP-0006
timestamp: 2026-09-05T17:00:00Z
git_commit: 175f697 (+ uncommitted M5 work)
session: v2-M5-neural-director
module: hardware/v2/rtl/neural_director.v
configuration: N_SLOTS=2 (sim), N_SLOTS=4 (synth default),
QUEUE_DEPTH=4 (sim), ADDR_WIDTH=23
action: M5 -- first-free job scheduler dispatching to N_SLOTS
(memory_manager, neural_processor) pairs, with a parametric-depth
ready queue.
command (sim, Verilator): verilator --binary --timing -j 0 -Wno-fatal
--top-module tb -o /tmp/vtb_dir hardware/v2/rtl/neural_processor.v
hardware/v2/rtl/prefetch_engine.v hardware/v2/rtl/memory_manager.v
hardware/v2/rtl/neural_director.v hardware/v2/sim/tb_neural_director.v
&& /tmp/vtb_dir
command (synth): yosys -p "synth_ecp5 -json .../top.json -top
neural_director" hardware/v2/rtl/neural_director.v (standalone, real
resource counts); harness_neural_director.v (see errors.log
ERR-0005 pattern) for real P&R Fmax.
result:
SIMULATED (N_SLOTS=2, tb_neural_director.v): 4/4 tests PASS --
3 jobs submitted to 2 slots (first two dispatch immediately,
first-free; third correctly WAITS in the ready queue until a slot
frees, then auto-dispatches), each result independently verified;
a deliberate 2-long-job burst forces the ready queue to genuinely
fill (job_in_ready correctly deasserts after exactly QUEUE_DEPTH
queued jobs while both slots are kept busy) and recover once
slots/queue drain.
SYNTHESIZED (standalone, N_SLOTS=4): 0 CHECK problems, 382 LUT4,
366 TRELLIS_FF, 4 CCU2C, 0 DSP.
POST-P&R (via harness, real Fmax): 250.50 MHz, PASS at 80MHz with
large margin.
errors: two real testbench bugs found and fixed during bring-up (not
RTL bugs): (1) sim_byte_mem's DEPTH parameter (1024) was smaller
than the test's own address range (up to 0x703=1795), an
out-of-bounds array access silently returning garbage; (2) the
initial completion-wait loop exited as soon as ANY ONE of three
jobs' result bytes changed, not all three -- fixed by counting
job_out_done pulses instead of polling result memory directly.
decision: see decisions.log DEC-0007 (reduced 4-state FSM; dependency
handling deferred to M6; per-slot independent behavioral memory
instead of shared real PSRAM, deferred to when backend arbitration
is actually needed).
next_action: M6 -- dependency_manager.v (ready/waiting queue,
dependency counters, wake-up, producer tracking) -- the first
milestone where job READINESS itself, not just free-slot dispatch,
becomes the Director's actual gating condition.
EXP-0007
timestamp: 2026-09-05T18:00:00Z
git_commit: 2e4cedc (+ uncommitted M6 work)
session: v2-M6-dependency-manager
module: hardware/v2/rtl/dependency_manager.v
configuration: N_NODES=8 (sim), N_NODES=16 (synth default),
MAX_DEPS=4, ADDR_WIDTH=23
action: M6 -- dependency-count tracking table (node_id/state/
required_dependencies/resolved_dependencies/producer_ids, §10
exact field list), first-found-ready dispatch to the Director.
command (sim, Verilator): verilator --binary --timing -j 0 -Wno-fatal
--top-module tb -o /tmp/vtb_dep hardware/v2/rtl/dependency_manager.v
hardware/v2/sim/tb_dependency_manager.v && /tmp/vtb_dep
command (synth/timing): yosys -p "synth_ecp5 -json .../top.json -top
dependency_manager" hardware/v2/rtl/dependency_manager.v;
nextpnr-ecp5 --45k --package CABGA381 --speed 8 --freq 80
--lpf-allow-unconstrained (no timing harness needed this time --
module's ports fit within the TRELLIS_IO budget as a bare top-level).
result:
SIMULATED: 4/4 tests PASS on a small hand-built DAG (node0, node1:
no dependencies; node2: depends on BOTH node0 and node1 --
"dipendenze multiple"; node3: depends on node0 ALONE -- "risultati
condivisi... piu' consumer"): node0/node1 dispatch immediately;
node3 becomes READY the cycle node0's producer_done arrives (before
node1 completes); node2 stays WAITING until BOTH node0 AND node1
have completed, confirmed by an explicit negative check (still
WAITING after only one of its two dependencies resolved).
SYNTHESIZED: 0 CHECK problems, 763 LUT4, 474 TRELLIS_FF, 0 DSP,
0 CCU2C.
POST-P&R (real, no harness needed): Fmax = 155.30 MHz, PASS at
80MHz.
errors: one testbench syntax error (nested nonblocking nested-
replication `{(N){M{1'b0}}}` needs an extra brace level, Verilator
correctly rejected it) -- fixed by building reg_producer_ids via
explicit bit-slice assignment instead of one big concatenation
expression. Not an RTL bug.
decision: see decisions.log DEC-0008 (no value forwarding yet, no
slot reclamation yet -- both explicitly deferred, not missing by
oversight).
next_action: M7 -- dataflow_core.v, integrating Director + Dependency
Manager + Memory Manager + Processor Array + Buffers into one top-
level module for the first time.
[2026-09-05] EXP-0008 -- hardware/v2/rtl/dataflow_core.v (M7, full
M1-M6 integration)
test: hardware/v2/sim/tb_dataflow_core.v -- a 3-node DAG (node0/node1
independent, node2 depends on BOTH) run through the REAL
dependency_manager -> neural_director -> N_SLOTS x (memory_manager +
neural_processor) chain end-to-end for the first time, each slot
backed by its own independent behavioral byte memory (shared real
PSRAM arbitration explicitly deferred to M8, decisions.log DEC-0009)
simulator: Verilator 5.050 (--binary --timing)
PASS/FAIL:
SIMULATED: 4/4 PASS -- node0=48, node1=8 (correct real
neural_processor computations via the full stack), node2=40
dispatched only after BOTH node0 and node1 genuinely completed
(continuously polled every cycle, not just checked at the end).
SYNTHESIZED (via harness_dataflow_core.v -- see errors.log ERR-0005):
N_SLOTS=2: 0 CHECK problems, LUT4=2127, CCU2C=248, TRELLIS_FF=2505,
MULT18X18D=16, DP16KD=0.
N_SLOTS=4: 0 CHECK problems, LUT4=3953, CCU2C=500, TRELLIS_FF=4688,
MULT18X18D=32, DP16KD=0.
POST-P&R (real, harness-based): N_SLOTS=2 Fmax=165.15 MHz,
N_SLOTS=4 Fmax=133.19 MHz -- both PASS at 80MHz.
errors: one Yosys build-script usage quirk (chparam ordering against
a non-top module vs synth_ecp5's own internal re-hierarchy pass) --
see errors.log ERR-0007. Not an RTL bug; no dataflow_core.v or
harness_dataflow_core.v source change needed, only the build command
itself.
decision: see decisions.log DEC-0009 (no M3 buffers wired in yet, no
shared-PSRAM arbitration across slots yet -- both explicitly
deferred to M8/a future measurement-driven decision, not missing by
oversight).
next_action: M8 -- PSRAM integration. Wire the real (unmodified) V1
PSRAM backend chain (int8_memory_access -> memory_interface ->
psram_controller) end-to-end through dataflow_core, and design/
measure whatever N_SLOTS>1 arbitration across ONE physical PSRAM
port actually requires.
[2026-09-05] EXP-0009 -- hardware/v2/rtl/neural_multiprocessor.v (M8,
real PSRAM integration)
test: hardware/v2/sim/tb_neural_multiprocessor.v -- same 3-node DAG as
EXP-0008, now routed through the REAL, UNMODIFIED V1 PSRAM backend
chain shared across N_SLOTS=2 genuinely concurrent memory_manager
instances via the new slot_mem_arbiter.v
simulator: Verilator 5.050 (--binary --timing)
PASS/FAIL:
SIMULATED: 4/4 PASS after fixing a real dropped-request bug in the
arbiter's first draft (errors.log ERR-0008) -- 444 cycles
end-to-end. hardware/v2/sim/tb_memory_manager.v (M4, untouched)
re-confirmed 3/3 PASS, no regression.
SYNTHESIZED (real standalone top-level, no harness needed -- 157
port bits, real PSRAM pins keep it under the TRELLIS_IO budget):
0 CHECK problems, LUT4=3145, CCU2C=388, TRELLIS_FF=3659,
MULT18X18D=16, DP16KD=0.
POST-P&R (real): Fmax=142.45 MHz -- PASS at 80MHz.
errors: one real RTL bug (errors.log ERR-0008) -- the first arbiter
draft silently dropped a request pulse arriving during contention;
fixed with a per-port pending-request latch, the same "queue, don't
drop" idiom already used by memory_manager's own pf_pending register
(ERR-0006).
decision: see decisions.log DEC-0010 (fixed lowest-index priority
arbitration, not fairness-balanced -- consistent with every other
scheduling policy chosen so far in this roadmap; revisit only if
M9's real measurement shows starvation actually matters).
next_action: M9 -- Full benchmark. Produce the V1-vs-V2 comparison
table mandated by §32 (Fmax, LUT, FF, DSP, BRAM, MAC/cycle,
cycles/neuron, neurons/s, stall %, memory/processor utilization,
effective MAC/s), each number labeled THEORETICAL/SIMULATED/
SYNTHESIZED/POST-P&R per §30.
[2026-09-05] EXP-0010 -- M9 Full benchmark: V1 vs V2 comparison table
(docs/v2-description.md §32)
test: not a new simulation -- a consolidation of real, already-
measured data from hardware/v1/ (frozen, pre-certified) and V2's own
M1-M8 logs (EXP-0001..EXP-0009), placed side-by-side on an
apples-to-apples basis (both full-system, both PARALLEL=8/P_IN=8,
both using the real unmodified V1 PSRAM backend).
result: see benchmark.log's own M9 entry for the full 12-row table.
Headline: V2's full system (neural_multiprocessor, N_SLOTS=2) is
142.45 MHz POST-P&R (PASS @80MHz) vs V1's 68.65 MHz (FAIL @80MHz);
166 vs 209 real simulated cycles for one neuron's 8-input dot
product through the real PSRAM chain -- 2.6x wall-clock speedup,
measured, not assumed.
errors: none this milestone (pure data consolidation, no new RTL).
decision: see decisions.log DEC-0011 -- 3 of the 12 §32 rows (stall %,
memory utilization, processor utilization) are reported as NOT
MEASURED rather than approximated, since a real number would require
dedicated cycle-accounting instrumentation neither system has had
built for it yet; approximating from partial data would violate
§30's "no invented results" rule.
next_action: M10 -- Optimization, using the REAL data gathered in M1-
M9 (not blind guessing): revisit memory_manager's +1-cycle/tile
overhead (DEC-0006), the ACC_WIDTH 24-vs-32 Fmax question (EXP-0002,
inconclusive without a seed sweep), the N_SLOTS x P_IN trade-off
given the DSP ceiling (DEC-0005), dependency_manager's node-slot
reclamation (DEC-0008), and slot_mem_arbiter's fixed-priority
fairness question (DEC-0010) -- plus building the stall %/
utilization instrumentation DEC-0011 deferred, since M10 is exactly
where that data becomes actionable.
[2026-09-05] EXP-0011/EXP-0012/EXP-0013 -- M10 Optimization, only on
data already gathered (docs/v2-description.md M10: "Solo sulla base
dei dati: pipeline; P_IN; numero processor; buffer; FIFO; scheduling;
prefetch; routing; memoria.")
test/result summary (full detail in synthesis.log/timing.log/
simulation.log/benchmark.log under the same EXP numbers):
EXP-0011 ("numero processor" axis): dataflow_core N_SLOTS=8 real
synthesis+P&R (via harness_dataflow_core.v) -- 92.63 MHz POST-P&R,
PASS at 80MHz, DSP 64/72 (88.9%). Extends M7's N_SLOTS=2/4 sweep
to the real ceiling DEC-0005 predicted. See decisions.log DEC-0012
(N_SLOTS=8 recommended practical ceiling for P_IN=8).
EXP-0012 ("pipeline" axis): ACC_WIDTH 24-vs-32, 6 real placement
seeds each (reusing already-synthesized netlists, nextpnr-ecp5
P&R only) -- ACC_WIDTH=24 wins on both mean Fmax (+6.2%) and
variance (~3.4x tighter), resolving EXP-0002's single-seed
inconclusiveness. See decisions.log DEC-0013 (ACC_WIDTH=24
recommended new default).
EXP-0013 ("scheduling"/"memoria" axes): testbench-only cycle-
accounting instrumentation added to tb_neural_multiprocessor.v (no
RTL touched) -- closes decisions.log DEC-0011's deferred stall %/
utilization gap with real SIMULATED numbers (shared PSRAM port
81.7% utilized, slot 0 95.2%, slot 1 65.2%, N_SLOTS=2 3-node DAG).
No conclusive evidence of harmful fixed-priority starvation in
this small a test (slot 0's higher utilization is at least partly
explained by serving 2 sequential jobs vs slot 1's 1) --
decisions.log DEC-0010's arbiter fairness question remains
correctly deferred pending a larger, longer-running workload.
simulator: Verilator 5.050 (--binary --timing) for EXP-0013; Yosys +
real nextpnr-ecp5 for EXP-0011/EXP-0012 (no new simulation needed,
reused already-verified netlists/functional results).
PASS/FAIL: EXP-0013 4/4 PASS (unchanged from EXP-0009, instrumentation
is additive-only). EXP-0011/EXP-0012 are synthesis/P&R measurements,
not functional tests -- 0 synthesis problems, all P&R runs PASS at
80MHz.
errors: none this milestone.
decision: see decisions.log DEC-0012 (N_SLOTS=8 ceiling) and DEC-0013
(ACC_WIDTH=24 new default).
next_action: none mandated by docs/v2-description.md's own roadmap
(§33 ends at M10) -- the 10-milestone V2 roadmap is now complete
end-to-end, from M1's single neural_processor through M9's full
V1-vs-V2 benchmark to M10's data-driven optimization findings.
Remaining open items (all explicitly deferred by their own DEC
entries, not oversights): dependency_manager node-slot reclamation
(DEC-0008), slot_mem_arbiter fairness under sustained/larger
contention (DEC-0010, now informed by EXP-0013's small-scale data),
a P_IN<4 sweep at higher N_SLOTS (DEC-0012's alternative #2), M3
buffer reuse as a shared cache once real bandwidth pressure is
measured (DEC-0009), and V1-side stall%/utilization instrumentation
to complete the M9 table's V1 column (DEC-0011).
[2026-09-05] EXP-0014 -- FINAL BENCHMARK CAMPAIGN (post-M10, real
end-to-end characterization, not an isolated functional test -- see
hardware/v2/docs/benchmarks/final-benchmark.md for the full 21-section
report)
test: hardware/v2/sim/tb_benchmark_suite.v -- 6 representative
workloads (Small/Medium/Large/Stress/Multilayer/DAG, up to 256
independent neurons and a 6-node 2-hop dependency diamond), each
bit-exact verified against a software golden model, run through the
REAL full neural_multiprocessor.v (real V1 PSRAM chain, real
slot_mem_arbiter) at N_SLOTS=1, 2, 4, and 8 (24 total workload/config
runs)
simulator: Verilator 5.050 (--binary --timing)
PASS/FAIL:
SIMULATED: 24/24 workload/config combinations PASS bit-exact, zero
errors, zero timeouts, zero deadlocks -- AFTER fixing 3 real issues
found during this campaign (errors.log ERR-0009: 1 real RTL bug in
neural_director.v never exercised at N_SLOTS=1 before, 2 testbench
sizing bugs in tb_benchmark_suite.v itself).
SYNTHESIZED + POST-P&R (real, no harness needed, full system incl.
real PSRAM pins): N_SLOTS=1 -> 152.46 MHz/LUT4=2642/FF=2240/DSP=8;
N_SLOTS=2 -> 142.45 MHz/LUT4=4191/FF=3659/DSP=16 (EXP-0009);
N_SLOTS=4 -> 113.38 MHz/LUT4=7552/FF=6495/DSP=32.
errors: see errors.log ERR-0009 (1 real RTL bug + 2 testbench bugs,
all found and fixed).
decision: see decisions.log DEC-0014 -- N_SLOTS=2 recommended as the
default configuration (supersedes DEC-0012's N_SLOTS=8 "practical
ceiling" framing for general use): real measured parallel scaling is
essentially flat for memory-bound workloads regardless of N_SLOTS
(shared PSRAM port is the real bottleneck, ~91% utilized regardless
of N_SLOTS>=2), and N_SLOTS=4 is actually 21% SLOWER in real
wall-clock time than N_SLOTS=1 for the Stress workload once real
Fmax degradation is accounted for.
next_action: none mandated by the roadmap (this campaign was
requested directly by the user, post-M10, as a final
characterization before deciding N_SLOTS and writing the V2
datasheet). Full report: hardware/v2/docs/benchmarks/final-
benchmark.md.
[2026-09-05] EXP-0015 -- word-level burst-read implementation (user-
requested optimization #1, following final-benchmark.md's own
recommendation: exploit psram_controller.v's already-implemented
page-mode support by fetching multiple bytes per real transaction
instead of one at a time)
test: prefetch_engine.v/memory_manager.v rewritten to speak
memory_interface.v's 16-bit word protocol directly (bypassing
int8_memory_access.v, still frozen/unmodified -- just no longer
instantiated in this datapath); slot_mem_arbiter.v/dataflow_core.v/
neural_multiprocessor.v widened to match. Re-verified: M4's own
testbench (updated to skip int8_memory_access), M7's own testbench
(sim_byte_mem -> sim_word_mem), M8's own testbench (UNCHANGED,
black-box), and the full final benchmark campaign (UNCHANGED,
black-box) at N_SLOTS=1/2/4/8.
simulator: Verilator 5.050 (--binary --timing)
PASS/FAIL:
SIMULATED: M4 3/3 PASS, cycles reduced 49-56% (166->84, 446->204,
728->322). M7 4/4 PASS. M8 4/4 PASS, cycles 684->337. Final
campaign 24/24 PASS bit-exact, D-Stress cycles reduced from
780298/736402/736823/738751 to 348682/307602/307346/307874
(N=1/2/4/8) -- roughly 2.2-2.4x fewer real cycles.
SYNTHESIZED + POST-P&R (real, full system incl. real PSRAM pins):
N=1 152.44 MHz (was 152.46), N=2 133.58 MHz (was 142.45, -6.2%),
N=4 112.07 MHz (was 113.38, -1.2%) -- small real Fmax cost.
Combined real wall-clock speedup (cycles / real Fmax): 2.24-2.37x
across N_SLOTS=1/2/4.
errors: none found (clean implementation, no regressions).
decision: see decisions.log DEC-0015.
next_action: user-requested optimization #2 -- a shared on-chip cache
for the activation (X) vector, so N independent neurons sharing one
input vector (the dense-layer shape used throughout this benchmark
suite) fetch it from PSRAM ONCE instead of once per neuron.
[2026-09-05] EXP-0016 -- shared activation_cache implementation (user-
requested optimization #2, following final-benchmark.md's own
recommendation: eliminate redundant per-neuron re-fetching of a
shared input vector)
test: new module activation_cache.v (single-tag, tile-granular,
N_SLOTS request ports, real word-level PSRAM backend via its own
arbiter port); memory_manager.v's activation half redirected through
it (weight half unchanged from DEC-0015); dataflow_core.v/
neural_multiprocessor.v/slot_mem_arbiter.v widened to N_SLOTS+1
ports to arbitrate the cache's own traffic alongside the N_SLOTS
memory_managers' weight traffic. Re-verified: M4 (updated
testbench), M7 (updated testbench), M8 (unchanged, black-box), and
the full final benchmark campaign (unchanged, black-box) at
N_SLOTS=1/2/4/8.
simulator: Verilator 5.050 (--binary --timing)
PASS/FAIL:
SIMULATED: M4 3/3 PASS, M7 4/4 PASS, M8 4/4 PASS, final campaign
24/24 PASS bit-exact. D-Stress cycles: 348682/307602/307346/
307874 (post-DEC-0015) -> 174610/185428/184795/184797 (N=1/2/4/8,
post-DEC-0016) -- a further 1.66-2.00x real cycle reduction from
the cache alone, ~4x combined with DEC-0015 vs the original
byte-level baseline.
SYNTHESIZED + POST-P&R (real, full system incl. real PSRAM pins):
N=1 131.79 MHz (was 152.44), N=2 87.72 MHz (was 133.58, -34.3%),
N=4 65.01 MHz (was 112.07, -42.0% -- NOW FAILS the 80MHz target).
Combined real wall-clock speedup vs the ORIGINAL byte-level
baseline: N=1 3.86x, N=2 2.45x, N=4 2.29x (but a real regression
vs DEC-0015-alone once N=4's own now-failing Fmax is used).
errors: 2 real bugs found and fixed (errors.log ERR-0010): a target-
bank/pending-bank race (ERR-0006's bug class, new instance) and a
repeat of ERR-0009's N_SLOTS=1 zero-width replication bug.
decision: see decisions.log DEC-0016 -- real net win confirmed at
N_SLOTS=1/2 (the recommended range, DEC-0014), real regression to a
failing timing state at N_SLOTS=4 (never the recommended default,
but a real, honestly-reported cost of this optimization). Cache
pipelining flagged as real follow-up work if N_SLOTS>2 with the
cache active is ever needed.
next_action: none further requested by the user for this round. Real,
concrete follow-up flagged in DEC-0016: pipeline the cache's own
hit-detection/broadcast logic to recover Fmax margin if higher
N_SLOTS configurations are ever needed with the cache active.
EXP-0017
timestamp: 2026-09-05T21:50:00Z
git_commit: 63cac6a7e5e0126bb13bea89dfe21966d5c9a1a5 (+ uncommitted NMS STEP1 work)
session: v2-NMS-STEP1-bandwidth-study
module: hardware/v2/nms/rtl/ideal_memory_model.v, hardware/v2/nms/sim/tb_bandwidth_study.v
configuration: real, unmodified hardware/v2/rtl/neural_processor.v
(P_IN=8, DATA_WIDTH=8, ACC_WIDTH=32) driven by an idealized,
SIMULATION-ONLY backing-store model (never synthesized) with
runtime-configurable latency and bandwidth. NTILES=2048 synthetic
tiles/job (steady-state dominated). Swept: N_SLOTS in {1,2,4,8}
(compile-time), PREFETCH_DEPTH in {2,4,8,16}, LATENCY in
{0,1,2,4,8,16} cycles, BANDWIDTH in {1,2,4,8,16,32,64,128}
bytes/cycle (runtime, all combinations swept inside 4 compiled
Verilator binaries, one per N_SLOTS -- 768 total data points).
action: NMS (Neural Memory System) roadmap STEP1 -- bandwidth
requirement study, run BEFORE any NMS RTL architecture decision, per
the user's own explicit ordering (no bank count/SRAM depth/bus
width/prefetch policy assumed a priori).
reason: the frozen V2 datapath's final benchmark campaign
(hardware/v2/docs/benchmarks/final-benchmark.md) found the shared
PSRAM port saturating ~91% utilization with N_SLOTS>=2 delivering
essentially no real scaling -- this study measures, independent of
any specific memory architecture, how much aggregate bandwidth and
how much prefetch depth the REAL compute fabric actually needs to
approach its own compute-only throughput ceiling.
command (sim, Verilator, per N_SLOTS in 1 2 4 8):
verilator --binary --timing -j 0 -Wno-fatal -GN_SLOTS=<N>
--top-module tb_bandwidth_study --Mdir /tmp/objdir_bw<N>
hardware/v2/rtl/neural_processor.v
hardware/v2/nms/rtl/ideal_memory_model.v
hardware/v2/nms/sim/tb_bandwidth_study.v && ./Vtb_bandwidth_study
result (RTL SIMULATION, idealized memory model, NOT a real hardware
measurement -- see hardware/v2/nms/reports/experiments/EXP-0017/
bandwidth_study.csv for the full 768-row raw data):
Three real bugs were found and fixed in the harness itself before
trusting its output -- see errors.log ERR-0011.
MINIMUM AGGREGATE BANDWIDTH (latency=0, PREFETCH_DEPTH>=4, i.e. once
latency is fully hidden) for >=90/95/99% of compute-only throughput
scales EXACTLY LINEARLY with N_SLOTS at 16 bytes/cycle/slot:
N_SLOTS=1: 16 B/cycle N_SLOTS=2: 32 B/cycle
N_SLOTS=4: 64 B/cycle N_SLOTS=8: 128 B/cycle
(16 B/cycle/slot = TILE_BYTES = 2*P_IN, i.e. exactly the raw
activation+weight demand of one neural_processor.v consuming one
tile/cycle at its own maximum pipelined rate -- this is a hard
floor, not a design margin; the real backing store's actual PAGE
BANDWIDTH, not just this floor, still needs separate real
measurement against psram_controller.v's own timing).
PREFETCH_DEPTH required to actually REACH that bandwidth-implied
ceiling scales with round-trip LATENCY, not with N_SLOTS or
bandwidth (measured at N_SLOTS=2, BW=128 -- ample):
latency=0-1 cycles : PREFETCH_DEPTH=4 -> 99.5% utilization
latency=2 cycles : PREFETCH_DEPTH=8 -> 99.4%
latency=4 cycles : PREFETCH_DEPTH=8 -> 99.3%
latency=8 cycles : PREFETCH_DEPTH=16 -> 99.1%
latency=16 cycles : PREFETCH_DEPTH=16 -> 83.4% (not yet enough;
PREFETCH_DEPTH>16 not tested this round)
Rule of thumb confirmed by the data: PREFETCH_DEPTH (in tiles) must
be roughly >= round-trip latency (in cycles) + a small margin to
sustain near-compute-only throughput -- an artificially small
PREFETCH_DEPTH silently caps utilization even when bandwidth is
generous (e.g. PREFETCH_DEPTH=2 caps utilization at ~50% even at
BW=128, latency=0 -- NOT a bandwidth problem, a lookahead-depth
problem).
errors: see ERR-0011 (3 bugs, all in the new harness, none in the
frozen V2 RTL -- fixed before trusting any of this result).
decision: see DEC-0017.
next_action: STEP2 (mathematical traffic model: activation/weight/
result bytes/cycle as closed-form functions of N_SLOTS, P_IN,
workload shape) is now largely closed-form-derivable from this
measured floor; then STEP3 (bank/bandwidth architectural sweep in
simulation) using these bandwidth/prefetch-depth requirements as the
design target, not an assumption.
EXP-0018
timestamp: 2026-09-05T22:15:00Z
git_commit: 63cac6a7e5e0126bb13bea89dfe21966d5c9a1a5 (+ uncommitted NMS STEP1/STEP3 work)
session: v2-NMS-STEP3-bank-contention
module: hardware/v2/nms/rtl/ideal_banked_activation.v, hardware/v2/nms/sim/tb_bank_contention.v
configuration: real, unmodified hardware/v2/rtl/neural_processor.v x
N_SLOTS (compile-time, 1/2/4/8), all consuming tiles of ONE SHARED
activation vector (the realistic "one layer dispatched together"
case), each with an assumed-private/instant weight supply (justified
analytically, not simulated -- see architecture.log's STEP2 note:
weight is never shared, so a private per-slot bank has zero
contention by construction). Activation vector modeled as already
resident (steady-state consumption; PSRAM fill/latency is STEP1's
own separate concern, EXP-0017). Swept N_BANKS in {1,2,4,8} x
STAGGER (cycles between successive job starts, modeling the Neural
Director's own real non-instantaneous dispatch) in {0,1,2,4,8}.
NTILES=1024/job. 80 total data points (4 N_SLOTS x 20 combos).
action: NMS STEP3 -- architectural bank/bandwidth sweep IN SIMULATION,
targeting specifically whether banking the shared ACTIVATION SRAM
(with broadcast-on-same-address, avoiding one-read-per-consumer) lets
N_SLOTS actually scale, per the user's own explicit question ("Voglio
vedere se il nuovo memory system permette finalmente N=2>N=1 e
N=4>N=2").
reason: V2's frozen final benchmark showed real parallel scaling flat
(1.05-1.06x, N=1 to N=8) because every slot's activation traffic
serialized through ONE shared arbitrated port. This experiment tests
the most direct fix: give the shared activation enough CONCURRENT
read bandwidth (via banking) that same-cycle requests from different
slots for different tile offsets of the shared vector don't
serialize.
command (sim, Verilator, per N_SLOTS in 1 2 4 8):
verilator --binary --timing -j 0 -Wno-fatal -GN_SLOTS=<N>
--top-module tb_bank_contention --Mdir /tmp/objdir_bank<N>
hardware/v2/rtl/neural_processor.v
hardware/v2/nms/sim/tb_bank_contention.v && ./Vtb_bank_contention
result (RTL SIMULATION, idealized zero-latency banked-SRAM model, NOT
a real hardware measurement -- see hardware/v2/nms/reports/
experiments/EXP-0018/bank_contention.csv for the full 80-row raw
data):
Two real bugs were found and fixed in this new harness before
trusting its output (see errors.log ERR-0012).
With N_BANKS = N_SLOTS, per-slot utilization stays ~98-99% REGARDLESS
of N_SLOTS (1/2/4/8) and dispatch stagger (0-8 cycles), giving REAL,
near-linear AGGREGATE throughput scaling (stagger=1, a realistic
Director dispatch gap):
N_SLOTS=1: 0.990 tiles/cycle
N_SLOTS=2: 1.979 tiles/cycle (1.999x vs N=1)
N_SLOTS=4: 3.950 tiles/cycle (3.990x vs N=1)
N_SLOTS=8: 7.869 tiles/cycle (7.949x vs N=1)
With N_BANKS=1 (matching today's single shared activation port),
utilization collapses under ANY nonzero stagger exactly as V2's real
benchmark showed (e.g. N_SLOTS=2, N_BANKS=1, stagger=1: 0.498, a
49.8% utilization loss from a single cycle of dispatch offset alone).
At stagger=0 (perfect lockstep -- all slots want the identical tile
index every cycle), N_BANKS=1 already suffices (broadcast serves
everyone from one read) -- N_BANKS only matters once slots DIVERGE in
which tile index they need, which real dispatch timing guarantees.
Intermediate bank counts (N_BANKS<N_SLOTS) show graceful, sweep-
measured degradation as stagger grows relative to N_BANKS (e.g.
N_SLOTS=8, N_BANKS=4, stagger=8: 0.498) -- real evidence that
N_BANKS should track N_SLOTS, not a smaller fixed constant.
errors: see ERR-0012 (2 bugs in this new harness).
decision: see DEC-0018.
next_action: this simulation result validates the CONCEPT (banked
activation SRAM with broadcast resolves the scaling bottleneck) but
says nothing yet about real Fmax/BRAM/LUT cost of building N_SLOTS
real banks with real broadcast/multiplex logic -- that is STEP4
(candidate selection) and STEP5/6 (real synthesis + place&route),
not yet attempted.
EXP-0019
timestamp: 2026-09-05T23:10:00Z
git_commit: 5c9ec61 (+ uncommitted NMS STEP4/5/6 work)
session: v2-NMS-STEP5-STEP6-activation-candidates
module: hardware/v2/nms/rtl/nms_activation_replicated.v,
hardware/v2/nms/rtl/nms_activation_banked.v,
hardware/v2/nms/synthesis/harness_nms_activation_replicated.v,
hardware/v2/nms/synthesis/harness_nms_activation_banked.v
configuration: DATA_WIDTH=8, P_IN=8. Two real, synthesizable candidate
RTL implementations of the shared Activation SRAM identified in
DEC-0018: Candidate A "replicated" (N_SLOTS private full-vector
copies, broadcast-write fill, zero read contention by construction)
and Candidate B "banked" (ONE logical vector striped across N_BANKS
single-port memories, real round-robin arbiter + broadcast-on-same-
address + 2-stage registered crossbar, deliberately register-to-
register per DEC-0016's own Fmax lesson). Both bit-exact verified in
real Verilator simulation first (tb_nms_activation_candidates.v,
covering pure broadcast, no-contention, forced same-bank contention,
and mixed cases, at N_BANKS=2 and 4) before any synthesis number was
trusted. Real synthesis+PnR: N_SLOTS/N_BANKS in {2,4,8}, MAX_TILES in
{16 (matching today's activation_cache.v sizing), 256 (a much deeper,
more realistic shared-vector length, per the user's own STEP6 ask to
not assume "smaller depth = less BRAM" from M3)}, plus a fixed
smaller N_BANKS=4 variant at N_SLOTS=8 (the BRAM-saving alternative
flagged in DEC-0018).
action: NMS STEP5 (real synthesis of candidates) + STEP6 (real
place&route), 7 total synthesis+PnR runs.
reason: DEC-0018 named these as the leading candidates from
simulation-only contention modeling (EXP-0018); this experiment
measures their REAL Fmax/LUT/FF/BRAM cost, since EXP-0018 said
nothing about hardware cost.
command (per config): yosys -p "read_verilog <harness+dut files>;
chparam -set N_SLOTS <n> [-set N_BANKS <n>] -set MAX_TILES <t>
<harness_module>; synth_ecp5 -json top.json -top <harness_module>"
&& nextpnr-ecp5 --45k --package CABGA381 --speed 8 --freq 80 --json
top.json --lpf-allow-unconstrained --textcfg top.config
(both candidates needed an LFSR-driven-input/XOR-checksum-output
synthesis harness, same pattern as hardware/v2/synthesis/
harness_memory_manager.v -- their raw wide ports exceed the
LFE5U-45F's TRELLIS_IO budget as bare top-level modules, confirmed
by an initial nextpnr placement failure before the harnesses existed)
result (POST-P&R MEASURED, real nextpnr-ecp5 -- component-level Fmax
in isolation, NOT yet the full-system integrated critical path):
See /tmp/exp19_table.txt content reproduced below.
Candidate A (replicated), MAX_TILES=16:
N_SLOTS=2: Fmax=613.87 MHz, DP16KD=0, COMB=42, FF=56, RAMW=4
N_SLOTS=4: Fmax=505.82 MHz, DP16KD=0, COMB=74, FF=72, RAMW=8
N_SLOTS=8: Fmax=382.56 MHz, DP16KD=0, COMB=138, FF=104, RAMW=16
Candidate A, MAX_TILES=256:
N_SLOTS=8: Fmax=130.70 MHz, DP16KD=8 (7% of chip's 108), COMB=137,
FF=136, RAMW=0 (BRAM inference only kicks in at this greater
depth -- at MAX_TILES=16 Yosys chose distributed LUT-RAM for
BOTH candidates, DP16KD=0 everywhere; the user's own M3-derived
warning against assuming "shallower depth = less BRAM" is
directly confirmed here: shallow depth here means NO BRAM at
all, not less of it).
Candidate B (banked, N_BANKS=N_SLOTS), MAX_TILES=16:
N=2/B=2: Fmax=339.79 MHz, DP16KD=0, COMB=72, FF=69, RAMW=4
N=4/B=4: Fmax=175.56 MHz, DP16KD=0, COMB=437, FF=98, RAMW=8
N=8/B=8: Fmax=111.92 MHz, DP16KD=0, COMB=3348, FF=152, RAMW=16
Candidate B, MAX_TILES=256, N=8/B=8: Fmax=106.84 MHz, DP16KD=0,
COMB=1053, FF=54, RAMW=0.
Candidate B, fixed N_BANKS=4 @ N_SLOTS=8, MAX_TILES=16: Fmax=109.61
MHz, DP16KD=0, COMB=1667, FF=102, RAMW=8 (a real, measured
~15x more COMB than Candidate A at the same N_SLOTS, for LOWER
Fmax, AND -- per EXP-0018 -- a real cycle-count regression under
contention that N_BANKS=N_SLOTS avoids; not a good trade on any
axis measured).
CENTRAL FINDING: Candidate A (replicated) strictly dominates
Candidate B (banked+broadcast+arbitration) on every measured axis at
every tested N_SLOTS -- higher Fmax (2-4x at N_SLOTS=8), far fewer
LUTs (24x fewer COMB cells at N_SLOTS=8, MAX_TILES=16), and simpler,
structurally starvation-free correctness (no arbiter at all). The
real cost of replication is BRAM that scales with N_SLOTS x vector
depth (8 DP16KD at N_SLOTS=8/MAX_TILES=256, still only 7% of the
chip's total) -- a real, honestly small price for this project's own
realistic workload sizes.
errors: none new in the candidate RTL itself this round (both verified
bit-exact in simulation first); see errors.log ERR-0011/ERR-0012 for
bugs already fixed in the STEP1/STEP3 harnesses this round built on.
decision: see DEC-0019.
next_action: STEP7 selection is effectively concluded for the
Activation SRAM sub-decision (Candidate A/replicated). Weight SRAM
(private per-slot, no arbitration needed at all per STEP2's own
analytical conclusion) still needs its own real DP16KD/width/depth/
packing sweep per the user's own explicit STEP6 ask (§6 of the NMS
spec) -- not yet attempted. Then STEP8 (full NMS integration).
EXP-0020
timestamp: 2026-09-06T01:00:00Z
git_commit: 5c9ec61 (+ uncommitted NMS work)
session: v2-NMS-STEP5-STEP6-weight-candidates
module: hardware/v2/nms/rtl/nms_weight_direct.v,
hardware/v2/nms/rtl/nms_weight_packed.v,
hardware/v2/nms/synthesis/harness_nms_weight_direct.v,
hardware/v2/nms/synthesis/harness_nms_weight_packed.v
configuration: DATA_WIDTH=8, P_IN=8. Two real, synthesizable candidate
Weight SRAM implementations, both private-per-slot (weights are
never shared, STEP2's own conclusion -- no arbitration exists in
either): Candidate W1 "direct" (one native P_IN*DATA_WIDTH=64-bit-
wide memory per slot, mirrors hardware/v2/rtl/weight_buffer.v's own
M3-era structure exactly) vs Candidate W2 "packed" (each slot's tile
storage decomposed into P_IN separate DATA_WIDTH=8-bit-wide per-lane
memories, reassembled by static concatenation). Both bit-exact
verified in real Verilator simulation first
(tb_nms_weight_candidates.v, full slot x tile fill/readback
coverage) before any synthesis number was trusted. Real
synthesis+PnR: N_SLOTS in {2,4,8} x MAX_TILES in {16,256} (12 runs),
directly following M3's own original warning (weight_buffer.v's real
DP16KD cost was flat across an 8x depth change, EXP-0004) and the
user's own explicit instruction not to assume width/depth/packing
effects on DP16KD without measuring them.
action: NMS Weight SRAM STEP5 (real synthesis) + STEP6 (real
place&route) -- the remaining half of STEP5/6 after EXP-0019's
Activation SRAM candidates.
command (per config): yosys -p "read_verilog <harness+dut files>;
chparam -set N_SLOTS <n> -set MAX_TILES <t> <harness_module>;
synth_ecp5 -json top.json -top <harness_module>" && nextpnr-ecp5
--45k --package CABGA381 --speed 8 --freq 80 --json top.json
--lpf-allow-unconstrained --textcfg top.config
result (POST-P&R MEASURED, real nextpnr-ecp5, component-level Fmax in
isolation):
MAX_TILES=16 (today's small workload sizing): direct and packed are
IDENTICAL on every resource metric at every N_SLOTS (both map to the
same distributed-LUT-RAM structure at this shallow depth, DP16KD=0
for both) -- Fmax differs only slightly and inconsistently (packed
faster at N=4/8, direct faster at N=2), not a meaningful
differentiator at this depth.
MAX_TILES=256 (realistic deep shared vector, matching EXP-0019's own
activation comparison depth): DP16KD count is IDENTICAL between
direct and packed at every N_SLOTS (2/4/8 DP16KD at N_SLOTS=2/4/8 --
exactly 1 DP16KD per slot either way, since one slot's full
256x64bit tile storage = 16384 bits = exactly one DP16KD's native
16Kbit capacity regardless of how that width is internally
partitioned). BUT packed uses MEANINGFULLY FEWER LUTs and FFs at
every N_SLOTS:
N_SLOTS=2: direct COMB=50/FF=88 vs packed COMB=41/FF=56
N_SLOTS=4: direct COMB=95/FF=104 vs packed COMB=44/FF=56
N_SLOTS=8: direct COMB=126/FF=136 vs packed COMB=67/FF=64
(packed uses ~1.9x fewer LUTs and ~2x fewer FFs than direct at
N_SLOTS=8, for the SAME real BRAM cost). Fmax is comparable, within
the noise of a single-seed placement (packed 176.77 vs direct 175.10
MHz at N=2; packed 157.98 vs direct 153.47 at N=4; packed 131.80 vs
direct 136.72 at N=8 -- packed slightly behind only at N=8, well
within normal seed-to-seed variation per the project's own DEC-0013
6-seed-sweep precedent, not re-swept here for time).
CENTRAL FINDING: decomposing each slot's wide tile storage into
narrow per-MAC-lane memories (packed) is a real, free LUT/FF win at
no BRAM cost once vector depth is deep enough to actually need real
DP16KD blocks (MAX_TILES=256) -- the wide single-memory's own
byte-lane write-enable/mux decode logic (needed to write a sub-slice
of a 64-bit word) is exactly what the packed layout avoids by
construction (each lane has its own independent, always-full-width
write port). At the shallow MAX_TILES=16 depth this project's own
current workloads actually use, the difference disappears entirely
(both map to the same LUT-RAM structure) -- packing only pays off
once real BRAM is in play.
errors: none new this round.
decision: see DEC-0020.
next_action: with both Activation SRAM (Candidate A, DEC-0019) and
Weight SRAM (Candidate W2/packed, DEC-0020) decided on real synthesis
data, STEP7 selection is complete for the memory-organization half of
the NMS. STEP8 (full NMS integration: prefetch engine, DMA, scheduler,
forwarding, NP-facing interface) is the next major remaining item.
EXP-0021
timestamp: 2026-09-06T01:50:00Z
git_commit: 5c9ec61 (+ uncommitted NMS STEP8 work)
session: v2-NMS-STEP8-integration
module: hardware/v2/nms/rtl/nms_dataflow_core.v,
hardware/v2/nms/rtl/nms_memory_manager.v,
hardware/v2/nms/rtl/nms_activation_fill_ctrl.v,
hardware/v2/nms/sim/tb_nms_dataflow_core.v
configuration: DATA_WIDTH=8, P_IN=8, ACC_WIDTH=32, ADDR_WIDTH=23,
N_SLOTS=2, N_NODES=8, MAX_DEPS=4, QUEUE_DEPTH=4, MAX_TILES=16.
action: NMS STEP8 -- first full integration of the DEC-0019/DEC-0020
decided pieces (Activation SRAM replicated, Weight SRAM packed) into
a complete Dependency Manager -> Neural Director -> N_SLOTS x
(nms_memory_manager + neural_processor) dataflow, mirroring
hardware/v2/rtl/dataflow_core.v's own scope exactly (M6/M5 reused
VERBATIM, unmodified) but replacing the M4 memory_manager.v +
activation_cache.v cluster.
reason: DEC-0019/DEC-0020 selected the memory-organization pieces on
their own (isolated) real synthesis/simulation data; this experiment
verifies they compose correctly into the SAME real end-to-end
dependency-wake-up loop V2's own M7 milestone proved, plus the
specific shared-activation and multi-tile scenarios this NEW
architecture introduces that the OLD one never needed to handle the
same way.
command (sim, Verilator): verilator --binary --timing -j 0 -Wno-fatal
--top-module tb hardware/v2/rtl/dependency_manager.v
hardware/v2/rtl/neural_director.v hardware/v2/rtl/neural_processor.v
hardware/v2/rtl/prefetch_engine.v
hardware/v2/nms/rtl/nms_activation_replicated.v
hardware/v2/nms/rtl/nms_activation_fill_ctrl.v
hardware/v2/nms/rtl/nms_weight_packed.v
hardware/v2/nms/rtl/nms_memory_manager.v
hardware/v2/nms/rtl/nms_dataflow_core.v
hardware/v2/nms/sim/tb_nms_dataflow_core.v
result (RTL SIMULATION, real Verilator, bit-exact vs hand-computed
expected values):
Two real bugs found and fixed before trusting any result -- see
errors.log ERR-0013.
7/7 tests PASS, bit-exact:
Group 1 (same DAG shape as dataflow_core.v's own M7 test): node0
(x=2,w=3,n_tiles=1) -> 48; node1 (x=1,w=1) -> 8; node2 depends on
BOTH, dispatched only after both genuinely complete -> 40.
Confirms the wake-up loop still closes correctly through the
ENTIRELY NEW memory subsystem.
Group 2 (shared x_base, THE scenario EXP-0018 modeled): node3 and
node4 dispatched together on the two available slots with the
IDENTICAL x_base but DIFFERENT (never-shared) weights -> 64 and
96 respectively, both correct. Confirms the replicated Activation
SRAM's broadcast-fill (nms_activation_fill_ctrl.v's single-tag
dedup, DEC-0019) serves BOTH concurrently-dispatched slots
correctly from ONE real word-level PSRAM fetch.
Group 3 (n_tiles=4, multi-tile -- never exercised by groups 1-2):
node5, 4 tiles with distinct per-tile X (1,2,3,4) and constant
W=1 -> expected sum 8*(1+2+3+4)=80, got 80. This is the test that
caught ERR-0013 item 2 (would have silently corrupted alternating
tiles without the fix).
SCOPE NOTE (honest limitation of this test, not the RTL): this
testbench's poke/peek tasks hardcode memory-port index N_SLOTS=2 for
the shared activation backing store, mirroring
hardware/v2/sim/tb_dataflow_core.v's own equally-fixed-at-N_SLOTS=2
scope -- an N_SLOTS=4 run of THIS SAME file fails (all results read
0) purely because the testbench itself pokes/peeks the wrong memory
index at N_SLOTS=4, not because of any real RTL scaling defect. The
actual N_SLOTS-scaling ARCHITECTURAL claim (N_BANKS=N_SLOTS keeps
utilization near-linear) was already validated separately and
correctly in EXP-0018's own dedicated, N_SLOTS-parametric harness.
Re-parametrizing THIS testbench's poke/peek tasks for a real
multi-N_SLOTS end-to-end run is flagged as follow-up work, not
attempted this round.
errors: see ERR-0013.
decision: see DEC-0021.
next_action: STEP9 (end-to-end benchmark: run nms_dataflow_core.v
through the same/similar workloads as the frozen V2 final-benchmark
campaign, with REAL Fmax from synthesis) and STEP10 (Current V2 vs
NMS comparison table) are the remaining STEPs.
EXP-0022
timestamp: 2026-09-06T03:15:00Z
git_commit: 5c9ec61 (+ uncommitted NMS STEP9/STEP10 work)
session: v2-NMS-STEP9-STEP10-real-benchmark-and-comparison
module: hardware/v2/nms/rtl/nms_neural_multiprocessor.v (new, mirrors
hardware/v2/rtl/neural_multiprocessor.v's own scope exactly:
nms_dataflow_core.v + real slot_mem_arbiter.v + real V1 PSRAM chain),
hardware/v2/nms/sim/tb_nms_dstress.v (adapted from
hardware/v2/sim/tb_benchmark_suite.v -- same golden model, same
register_node/poke_byte/peek_byte tasks, same cycle-accounting
instrumentation, module swapped to nms_neural_multiprocessor,
restricted to the D-Stress workload only).
configuration: DATA_WIDTH=8, P_IN=8, ACC_WIDTH=32, ADDR_WIDTH=23,
N_NODES=1024, MAX_DEPS=8, QUEUE_DEPTH=8, MAX_TILES=16,
PSRAM_DATA_WIDTH=16, CLK_FREQ_MHZ=80. D-Stress: 256 independent
neurons, 128 inputs each (16 tiles), all sharing ONE input activation
vector -- IDENTICAL workload V2's own final-benchmark campaign uses,
through the REAL, unmodified V1 PSRAM chain (memory_interface.v ->
psram_controller.v, real page-mode timing, real ~150us power-up
wait) via a real psram_model.v behavioral model.
action: NMS STEP9 (real end-to-end benchmark, real Fmax from synthesis)
+ STEP10 (Current V2 vs NMS comparison), the final two roadmap steps.
reason: EXP-0019/0020/0021 validated the memory-organization pieces and
their integration in isolation/small-scale; this experiment measures
the SAME real workload V2's own numbers are already logged for
(final-benchmark.md, benchmark.log EXP-0016), the only way to make an
honest apples-to-apples comparison.
command (synth+PnR, per N_SLOTS in 1/2/4/8): yosys -p "read_verilog
<all NMS+reused-V1/V2 RTL files>; chparam -set N_SLOTS <n> -set
MAX_TILES 16 nms_neural_multiprocessor; synth_ecp5 -json top.json
-top nms_neural_multiprocessor" && nextpnr-ecp5 --45k --package
CABGA381 --speed 8 --freq 80 --json top.json --lpf-allow-
unconstrained --textcfg top.config (no synthesis harness needed --
same real-PSRAM-pin methodology as neural_multiprocessor.v itself,
157 port bits, well under the LFE5U-45F's IO budget)
command (sim, Verilator, per N_SLOTS_CFG): verilator --binary --timing
-j 0 -Wno-fatal -GN_SLOTS_CFG=<n> --top-module tb
hardware/v1/rtl/memory_interface.v hardware/v1/rtl/psram_controller.v
hardware/v1/sim/psram_model.v hardware/v2/rtl/slot_mem_arbiter.v
hardware/v2/rtl/dependency_manager.v hardware/v2/rtl/neural_director.v
hardware/v2/rtl/neural_processor.v hardware/v2/rtl/prefetch_engine.v
hardware/v2/nms/rtl/nms_activation_replicated.v
hardware/v2/nms/rtl/nms_activation_fill_ctrl.v
hardware/v2/nms/rtl/nms_weight_packed.v
hardware/v2/nms/rtl/nms_memory_manager.v
hardware/v2/nms/rtl/nms_dataflow_core.v
hardware/v2/nms/rtl/nms_neural_multiprocessor.v
hardware/v2/nms/sim/tb_nms_dstress.v
result (POST-P&R MEASURED Fmax/resources; SIMULATED cycles, real
Verilator against the real V1 PSRAM chain; wall-clock/neurons-s/
MAC-s DERIVED from the two, never a theoretical frequency):
Four real bugs found and fixed before trusting any result at this
full real scale -- see errors.log ERR-0014 (all four are instances
of one root cause: a counter needing to represent the VALUE
MAX_TILES itself, one bit short of an address field's own width).
=== Real synthesis+PnR (nms_neural_multiprocessor.v, no harness) ===
N_SLOTS=1: Fmax=160.18 MHz PASS, LUT4=(not separately re-extracted),
TRELLIS_FF=2281, MULT18X18D=8, DP16KD=0
N_SLOTS=2: Fmax=93.10 MHz PASS, LUT4=1948, CCU2C=266, TRELLIS_FF=3522,
MULT18X18D=16, DP16KD=0, TRELLIS_DPR16X4=109
N_SLOTS=4: Fmax=56.62 MHz FAIL@80MHz, TRELLIS_FF=6004,
MULT18X18D=32, DP16KD=0
N_SLOTS=8: Fmax=31.43 MHz FAIL@80MHz, TRELLIS_FF=10966,
MULT18X18D=64, DP16KD=0
Real critical-path trace at N_SLOTS=4 (nextpnr's own report) starts
at a per-slot x_base_reg and runs THROUGH
nms_activation_fill_ctrl.v's own combinational priority-scan/address
logic (6.26ns logic + 11.40ns routing on the worst path) -- the
SAME class of O(N_SLOTS) unpipelined-scan Fmax cost DEC-0016 already
documented for the superseded activation_cache.v, reintroduced here
in a different module. Real, honest, NOT hidden: the replicated
Activation SRAM candidate itself (EXP-0019) does NOT have this
problem in isolation -- the shared FILL CONTROLLER deciding WHICH
tag to chase does, a genuinely different piece.
=== Real end-to-end D-Stress (256 neurons, 16 tiles, real PSRAM) ===
N_SLOTS=2: 185645 cycles, PASS 256/256 bit-exact vs golden.
N_SLOTS=4: 184764 cycles, PASS 256/256 bit-exact vs golden (real
per-slot imbalance: slots 0/1 delivered 2016 tiles each, slots 2/3
only 48/16 -- same "first-free fixed-priority dispatch" imbalance
already documented for V2 itself, ch.06 ofthe datasheet).
Cycles are FLAT across N_SLOTS=2->4 (185645 -> 184764, -0.5%) --
confirms the SAME real, architecture-independent finding as V2's own
campaign and as EXP-0017's own analytical floor: a single real
PSRAM port caps aggregate throughput regardless of on-chip
organization; NMS's banking work made the ON-CHIP side efficient,
it did not and could not remove the external bandwidth ceiling.
=== DERIVED: real wall-clock comparison (cycles / real Fmax) ===
| Config | Current V2 cycles/Fmax/wall-clock | NMS cycles/Fmax/wall-clock | NMS speedup |
|---|---|---|---|
| N=2 | 185428 / 87.72MHz / 2113.9us | 185645 / 93.10MHz / 1994.0us | 1.060x FASTER |
| N=4 | 184795 / 65.01MHz(FAIL) / 2842.6us | 184764 / 56.62MHz(FAIL) / 3263.2us | 0.871x SLOWER |
Effective MAC/s (DERIVED) @ N=2: V2 15.50M, NMS 16.43M (+6.0%).
Real resource cost @ N=2 (Yosys, matching V2's own reporting
convention): V2 LUT4=4359/CCU2C=366/FF=3924/DSP=16/BRAM=0; NMS
LUT4=1948/CCU2C=266/FF=3522/DSP=16/BRAM=0 -- NMS uses 55.3% FEWER
LUT4 and 10.2% fewer FF for the SAME DSP/BRAM cost, at HIGHER real
Fmax.
errors: see ERR-0014 (4 real bugs found and fixed this round).
decision: see DEC-0022 (final NMS vs Current-V2 recommendation).
next_action: NMS roadmap (STEP1-STEP10) is now complete. Remaining
real, honestly-flagged future work: pipeline
nms_activation_fill_ctrl.v's own priority-scan/address logic (the
concrete fix for the N_SLOTS=4/8 Fmax regression, matching the exact
precedent DEC-0016 already set for activation_cache.v); re-measure
N_SLOTS=1/8 D-Stress cycle counts for full parity with V2's own
4-point table (only N=2/4 measured this round, time-bounded); a
fixed smaller N_BANKS variant of the Activation SRAM was never
revisited after DEC-0019 selected full replication (BRAM cost was
cheap enough at this project's real workload sizes that it was
never worth reconsidering).
EXP-0023
timestamp: 2026-09-06T04:00:00Z
git_commit: 5c9ec61 (+ uncommitted NMS STEP11 work)
session: v2-NMS-STEP11-weight-prefetch
module: hardware/v2/nms/sim/tb_weight_prefetch_sweep.v (adapted from
tb_bandwidth_study.v/EXP-0017, TILE_BYTES=P_IN=8 weight-only instead
of 2*P_IN=16 X+W combined -- activation deliberately out of scope
per this STEP's own instruction, EXP-0022 already showed 99.6% hit
rate)
configuration: real, unmodified neural_processor.v, N_SLOTS in {1,2},
PREFETCH_DISTANCE in {1,2,4,8,16,32}, latency in {0,1,2,4,8,16}
cycles, bandwidth in {1,2,4,8,16,32,64,128} B/cycle (ideal_memory_
model.v, simulation-only). 576 real Verilator data points.
action: STEP11's own explicit "PREFETCH DISTANCE EXPERIMENT" --
IDEAL-MEMORY SIMULATION, run BEFORE implementing any real RTL, per
this project's own established discipline (measure, then build).
result (IDEAL-MEMORY SIMULATION, not a real hardware measurement):
At ample bandwidth (128 B/cycle, never the bottleneck for an 8-byte
weight tile), N_SLOTS=1 and N_SLOTS=2 give IDENTICAL utilization
curves (no cross-slot interference at this bandwidth -- each slot's
own prefetch depth is the only limiter). Minimum PREFETCH_DISTANCE
for >=90% utilization scales with round-trip latency:
latency 0-1 cycles : PFD=4
latency 2-4 cycles : PFD=8
latency 8 cycles : PFD=16
latency 16 cycles : PFD=32
Real, monotonic, roughly PFD~2x(latency+2) -- confirms STEP11's own
expected qualitative relationship (deeper latency needs deeper
lookahead) while providing the actual real numbers rather than
assuming them. PFD=1 (no real lookahead beyond one outstanding
request, matching the CURRENT nms_memory_manager.v's own real
behavior) caps utilization at 25% even at latency=0 -- confirms
ERR-... class finding: the current architecture's gap is NOT
insufficient lookahead distance (it already tries to fetch as far
ahead as n_tiles allows) but ZERO outstanding-request depth (only
one fetch ever in flight), which this ideal model isolates cleanly
by showing PFD=1 is bad even under a ZERO-latency, generous-
bandwidth memory.
decision: implement a real, synthesizable weight prefetch engine
supporting PREFETCH_DISTANCE up to at least 16 (covering this
project's own real PSRAM round-trip latency, to be independently
measured against the actual psram_controller.v timing before final
candidate selection).
next_action: design + implement the real RTL (weight_prefetch_engine.v
+ tile-state tracking), verify bit-exact, then re-run this SAME
question against the REAL V1 PSRAM chain (not the ideal model) to
pick the real PREFETCH_DISTANCE candidates for synthesis.
EXP-0024
timestamp: 2026-09-05T23:41:08Z
git_commit: 5c9ec61 (+ uncommitted NMS STEP11 work)
session: v2-NMS-STEP11-weight-prefetch
module: hardware/v2/nms/rtl/weight_prefetch_engine.v (real RTL,
post-ERR-0015 fix), hardware/v2/nms/rtl/nms_memory_manager_pf.v,
nms_dataflow_core_pf.v, nms_neural_multiprocessor_pf.v (new "_pf"
A/B variants, nms_memory_manager.v/nms_dataflow_core.v/
nms_neural_multiprocessor.v themselves left UNTOUCHED as the
baseline reference per this STEP's own explicit constraint),
hardware/v2/nms/sim/tb_nms_dstress_pf.v (adapted from
tb_nms_dstress.v/EXP-0022: identical D-Stress workload, golden
model, register_node/poke_byte/peek_byte tasks, bit-exact
correctness check; added a PFD_CFG parameter and NEW, testbench-only
instrumentation for weight_stall_cycles and prefetch_effectiveness
per STEP11's own exact formula: tiles consumed with zero
weight-blocking cycles beforehand / total tiles consumed).
configuration: same as EXP-0022 -- DATA_WIDTH=8, P_IN=8, ACC_WIDTH=32,
ADDR_WIDTH=23, N_NODES=1024, MAX_DEPS=8, QUEUE_DEPTH=8, MAX_TILES=16,
PSRAM_DATA_WIDTH=16, CLK_FREQ_MHZ=80. D-Stress: 256 independent
neurons, 128 inputs each (16 tiles), ONE shared input activation
vector, through the REAL, unmodified V1 PSRAM chain (real
page-mode timing, real ~150us power-up wait). PREFETCH_DISTANCE
swept over {1,2,4,8,16} for N_SLOTS in {1,2}.
command (sim, Verilator, per N_SLOTS_CFG x PFD_CFG): verilator
--binary --timing -j 0 -Wno-fatal -GN_SLOTS_CFG=<n> -GPFD_CFG=<pfd>
--top-module tb hardware/v1/rtl/memory_interface.v
hardware/v1/rtl/psram_controller.v hardware/v1/sim/psram_model.v
hardware/v2/rtl/slot_mem_arbiter.v hardware/v2/rtl/dependency_manager.v
hardware/v2/rtl/neural_director.v hardware/v2/rtl/neural_processor.v
hardware/v2/rtl/prefetch_engine.v
hardware/v2/nms/rtl/nms_activation_replicated.v
hardware/v2/nms/rtl/nms_activation_fill_ctrl.v
hardware/v2/nms/rtl/nms_weight_packed.v
hardware/v2/nms/rtl/weight_prefetch_engine.v
hardware/v2/nms/rtl/nms_memory_manager_pf.v
hardware/v2/nms/rtl/nms_dataflow_core_pf.v
hardware/v2/nms/rtl/nms_neural_multiprocessor_pf.v
hardware/v2/nms/sim/tb_nms_dstress_pf.v
command (synth+PnR, per N_SLOTS x PFD in {1,2}x{2,8}): yosys -p
"read_verilog -sv <all NMS-PF+reused-V1/V2 RTL files>; chparam -set
N_SLOTS <n> -set MAX_TILES 16 -set PREFETCH_DISTANCE <pfd>
nms_neural_multiprocessor_pf; synth_ecp5 -json top.json -top
nms_neural_multiprocessor_pf" && nextpnr-ecp5 --45k --package
CABGA381 --speed 8 --freq 80 --json top.json --lpf-allow-
unconstrained --textcfg top.config
result (POST-P&R MEASURED Fmax/resources; RTL SIMULATION cycles, real
Verilator against the real V1 PSRAM chain; wall-clock/MAC-per-cycle
DERIVED from the two; prefetch_effectiveness/weight_stall_cycles are
testbench-only instrumentation, same idiom as EXP-0022's own):
=== N_SLOTS=1 (no PSRAM-port contention) ===
PFD=1 : 181489 cycles, PSRAM util=59.3%, sustained_MAC/cycle=0.1806,
weight_stall=87.91%, prefetch_effectiveness=0.02%
PFD=2 : 162876 cycles, PSRAM util=66.4%, sustained_MAC/cycle=0.2012,
weight_stall=86.44%, prefetch_effectiveness=0.39%
PFD=4/8/16: IDENTICAL to PFD=2 in every measured metric (162876
cycles, 66.4% util, 0.2012 MAC/cycle) -- confirms the achievable
benefit plateaus completely at PFD=2 for this workload's per-job
granularity (each of the 256 neurons is its OWN separate 16-tile
job with a completely distinct, non-reusable weight vector -- the
engine can never accumulate more than ~2 tiles of real lookahead
before a 16-tile job ends and the next job's fetch stream must
start over from tile 0).
PFD=1->2 real win: -10.3% cycles (181489->162876), a genuine,
reproducible improvement from eliminating the OLD design's
per-tile-boundary control-plane restart gap (matches the design
rationale in weight_prefetch_engine.v's own header comment) --
but PFD=1 here is NOT identical to the pre-STEP11 architecture
(nms_memory_manager.v's own prefetch_engine.v/pf_busy-gated
restart), only a close, weaker-than-actual-old-design lower bound,
since even at PFD=1 this engine still streams continuously within
ONE tile's own 4 words.
=== N_SLOTS=2 (real shared PSRAM port, real slot_mem_arbiter
round-robin contention -- this project's own primary reference
configuration per EXP-0022) ===
PFD=1 : 185410 cycles, PSRAM util=90.5%, sustained_MAC/cycle=0.1767,
weight_stall=93.32%, prefetch_effectiveness=0.78%
PFD=2 : 185408 cycles (-0.001% vs PFD=1) -- same util/MAC/cycle/
stall/effectiveness to 2 decimal places
PFD=4 : 185404 cycles
PFD=8 : 185398 cycles
PFD=16: 185390 cycles
ALL FIVE PFD values are statistically indistinguishable (max spread
185390-185410, 0.011% of the total) -- REAL WEIGHT PREFETCHING
PROVIDES NO MEASURABLE BENEFIT AT N_SLOTS=2, in stark contrast to
the real ~10% win measured at N_SLOTS=1 above. Compared directly
against EXP-0022's own "Current NMS" baseline (no prefetch engine at
all, N_SLOTS=2): 185645 cycles, PSRAM util effectively identical --
this STEP's own new engine, plugged into the exact same real system,
changes the real, measured cycle count by -0.13% (185645 -> 185398
at PFD=8), i.e. NOTHING, within run-to-run noise.
=== Root cause of the N_SLOTS=1 vs N_SLOTS=2 divergence (why
deeper prefetch helps at N=1 but not at N=2) ===
At N_SLOTS=1 the shared PSRAM port belongs entirely to one slot's
own traffic; PFD=1's own real per-tile-boundary control-plane
restart gap leaves the port genuinely idle between tiles, and PFD=2
closes that gap (which is exactly what this STEP's engine was
designed to do). At N_SLOTS=2, TWO slots contend for the SAME
single physical port via slot_mem_arbiter's round-robin arbitration
-- even at PFD=1, whichever slot isn't currently being serviced
keeps the port busy on the OTHER slot's behalf, so there is no idle
gap left at any tile boundary for a deeper PFD to close: the port
is ALREADY 90.5% busy (same figure as the pre-STEP11 baseline,
EXP-0022) regardless of PFD. Prefetching can only hide LATENCY
(idle time waiting on a request that could have been issued
earlier); it structurally cannot manufacture more BANDWIDTH out of
a single already-saturated physical port. This is the SAME real
finding EXP-0022 already reported for N_SLOTS=2->4 scaling ("a
single real PSRAM port caps aggregate throughput regardless of
on-chip organization") -- STEP11 confirms it is ALSO true across
PREFETCH_DISTANCE at fixed N_SLOTS, not just across N_SLOTS at fixed
PFD.
=== Quantified bandwidth gap vs the STEP11 success target ===
Required for >=90% of theoretical MAC/cycle: N=1 needs
sustained_MAC/cycle>=7.2 (achieved 0.2012, 2.8% of target -- a real
PSRAM bandwidth ~35.8x higher than currently achieved would be
needed); N=2 needs >=14.4 (achieved 0.1767, 1.2% of target -- a real
PSRAM bandwidth ~81.5x higher would be needed). Both gaps are far
too large to be closed by any lookahead/buffering scheme -- this is
a genuine, physical, external PSRAM BANDWIDTH ceiling (the real,
single, ISSI IS66WVE4M16EBLL-70BLI x16 PSRAM chip's own real access
timing, already contended by N_SLOTS clients through one physical
port), not a latency-hiding problem STEP11's own RTL-scheduling
scope can solve.
=== Real synthesis+PnR (nms_neural_multiprocessor_pf.v) ===
N=1 PFD=2: Fmax=132.26MHz PASS, LUT4=1333, CCU2C=203, FF=2245,
MULT18X18D=8, DP16KD=0 (TRELLIS_DPR16X4=77)
N=1 PFD=8: Fmax=137.76MHz PASS, LUT4=1464, CCU2C=200, FF=2245,
MULT18X18D=8, DP16KD=0
N=2 PFD=2: Fmax=97.16MHz PASS, LUT4=1941, CCU2C=368, FF=3449,
MULT18X18D=16, DP16KD=0
N=2 PFD=8: Fmax=95.25MHz PASS, LUT4=1908, CCU2C=362, FF=3449,
MULT18X18D=16, DP16KD=0 (TRELLIS_DPR16X4=109)
vs Current NMS baseline (EXP-0022, no prefetch engine): N=1
Fmax=160.18MHz FF=2281 DSP=8; N=2 Fmax=93.10MHz LUT4=1948 CCU2C=266
FF=3522 DSP=16. The new weight_prefetch_engine.v is resource-NEUTRAL
to slightly cheaper at N=2 (LUT4 -2.1% to -0.4%, FF -2.1%, CCU2C
+36% to +38% -- CCU2C is the carry-chain-adder primitive, higher
here because the new engine's own address arithmetic uses more
adder chains than the old single-shot FSM's simpler restart logic,
but this does NOT translate into a worse LUT4/FF/Fmax outcome) and
real Fmax is actually slightly HIGHER (+2.3% to +4.4% at N=2) --
the new design does not "simply move the bottleneck from memory to
an enormous combinational controller" (the STEP11 spec's own
explicit worry): resource cost and Fmax are both a wash or a small
net win. The bottleneck genuinely is external PSRAM bandwidth.
errors: see ERR-0015 (window_limit PREFETCH_DISTANCE-truncation
deadlock at PFD>=32, MAX_TILES=16 -- found via this experiment's own
PFD=32 sweep point, fixed and regression-tested before trusting any
other result at this scale).
decision: see DEC-0023 (STEP11 final outcome: PARTIAL/NEGATIVE --
recommend PFD=2 as the smallest PREFETCH_DISTANCE that captures ALL
the real, measurable benefit available at N_SLOTS=1; do NOT adopt
the new engine as the default for N_SLOTS>=2 production
configurations, since it provides zero measured benefit there and
the STEP11 90%-utilization success criterion is not met at ANY
N_SLOTS tested).
next_action: STEP11 is now closed with an honest Outcome
B(N=1)/C(N=2) result (see DEC-0023). The real, quantified next step
-- OUTSIDE this STEP's own RTL-scheduling scope -- would be
increasing real PSRAM bandwidth itself (wider/parallel physical
memory, multiple independent PSRAM banks each with their own port,
or a genuinely different backing-store technology), not a deeper or
smarter prefetch/lookahead scheme against the SAME single physical
port.
EXP-0024 (addendum): exact external-bandwidth requirement for N_SLOTS=2
to reach 90%/95%/99% of theoretical MAC/cycle, "all else unchanged".
timestamp: 2026-09-06T00:00:00Z
trigger: user question -- quantify EXACTLY the external bandwidth
needed for N_SLOTS=2 to reach 90%, 95%, 99% of theoretical compute
throughput, holding everything else fixed. This addendum corrects
the informal "~81.5x bandwidth needed" estimate given in EXP-0024's
own main body, which implicitly assumed ALL 185398 real cycles
scale with bandwidth -- an oversimplification not supported by the
data actually measured in this same experiment.
model (DERIVED, roofline decomposition from EXP-0024's own 5 real
N_SLOTS=2 PFD data points, PFD in {1,2,4,8,16}):
total_cycles = non_memory_cycles + memory_cycles(k)
memory_cycles(k) = psram_busy_cycles_ref / k (k = bandwidth
multiplier relative to today's real, contended, single-port
achieved bandwidth)
Empirical anchor: non_memory_cycles = total_cycles - psram_busy_cycles
measured 17530 (PFD=1), 17532 (PFD=2), 17536 (PFD=4), 17544
(PFD=8), 17560 (PFD=16) -- CONSTANT to within 0.17% across the
entire real PFD sweep, direct empirical proof this component is
genuinely independent of the weight-prefetch/bandwidth mechanism
(it is real per-job dispatch + neural_processor.v's own internal
pipeline/FSM latency, NOT PSRAM-port time). Reference point used
below: PFD=8 (non_memory_cycles=17544, psram_busy_cycles=167854,
total_cycles=185398).
Workload: total_MACs = 256 neurons x 128 inputs = 32768 (fixed,
independent of k). theoretical_MAC_per_cycle(N=2) = 16.
result (DERIVED, exact):
utilization(k) = 32768 / (16 * (17544 + 167854/k))
k=1 (today) : util=1.105% (cross-check: matches the measured
1.10% processor_utilization exactly)
k=2 : util=2.018%
k=5 : util=4.007%
k=10 : util=5.966%
k=50 : util=9.799%
k=100 : util=10.654%
k=1000 : util=11.563%
k->infinity : util->11.674% (32768/(16*17544)) -- the HARD
CEILING imposed purely by the measured, bandwidth-independent
non-memory floor.
Solving utilization(k)=f for k: k = 167854 / (2048/f - 17544).
f=0.90: required total budget=2275.56 cycles < fixed floor of 17544
cycles alone -> k is NEGATIVE (167854/-15268.44) -- mathematically
the signature of an INFEASIBLE target.
f=0.95: required budget=2155.79 cycles -- same result, infeasible.
f=0.99: required budget=2068.69 cycles -- same result, infeasible.
EXACT CONCLUSION: there is NO finite external bandwidth (not even an
literally infinite one) that reaches 90%, 95%, or 99% of theoretical
MAC/cycle at N_SLOTS=2 while holding job granularity (256 separate
per-neuron jobs), neural_processor.v's own internal pipeline, and
the dependency-manager/director dispatch scheme unchanged. The
asymptotic ceiling (11.674%) is itself an order of magnitude below
even the loosest target (90%). The earlier "~81.5x bandwidth"
estimate in this experiment's main body is hereby SUPERSEDED --
it did not account for this real, measured, bandwidth-independent
floor and understated how far the system is from the target.
To reach 90%/95%/99% at N_SLOTS=2 at all, the non-memory floor
itself would ALSO have to shrink from ~68.5 cycles/neuron
(17544/256) down to roughly 8.9/8.4/8.1 cycles/neuron respectively
(2275.56/256, 2155.79/256, 2068.69/256) -- i.e. a ~7.7-8.5x
reduction in per-job control/pipeline overhead, achievable only by
changing job granularity (e.g. batching multiple neurons per
dispatched job) or neural_processor.v's own pipeline -- explicitly
OUTSIDE "everything else unchanged" and outside this STEP's scope.
classification: DERIVED (closed-form roofline model fit to 5 already-
measured REAL D-Stress data points; the model's only free parameter,
the bandwidth multiplier k, is validated at k=1 by reproducing the
measured 1.10% utilization exactly). No new RTL simulation was run
for this addendum -- the fixed-overhead invariance across all 5 real
PFD points already measured is the empirical anchor: any two of them
would have sufficed to fit the two-parameter model, and all five
agree with each other to within 0.17%.
EXP-0025
timestamp: 2026-09-06T01:00:00Z
git_commit: 5c9ec61 (+ uncommitted NMS STEP11-13 work)
session: v2-NMS-STEP13-batch-continuous-processor
module: hardware/v2/nms/rtl/nms_memory_manager_pf.v (analysis only,
no modification), hardware/v2/rtl/neural_processor.v (analysis only)
-- isolated cycle-level trace testbench built in
/tmp/nms_pf_build/step13_trace/tb_trace.v (scratch, not committed to
the repo -- pure analysis harness, superseded by the real
standalone/integrated testbenches built later in this STEP).
configuration: neural_processor.v + nms_memory_manager_pf.v (PFD=16)
driven with WEIGHT and ACTIVATION SRAM data tied to constants and
mem_ready held permanently high (zero real memory latency anywhere)
-- isolates the pure control-plane floor with NO external memory
bottleneck whatsoever, per this STEP's own explicit Step-1 mandate
("identify exactly which cycles remain when external memory
latency/bandwidth approaches zero").
command: verilator --binary --timing -j0 --top-module tb
hardware/v2/rtl/neural_processor.v
hardware/v2/nms/rtl/weight_prefetch_engine.v
hardware/v2/nms/rtl/nms_memory_manager_pf.v <scratch tb>, with
per-cycle state-transition ($display on every mm.state/np_state/
tile_idx change) tracing enabled for a clean back-to-back
steady-state job (n_tiles=16).
result (RTL SIMULATION, cycle-exact trace):
A 16-tile job with ZERO real memory latency still takes 81 cycles
(NOT 16, NOT ~20). Per-cycle trace shows tile_idx advances every
EXACTLY 4 cycles in steady state (tiles 1..15: gaps of 4,4,4,...,4,
no variance) -- NOT the "1 outstanding fetch, restart per tile"
picture EXP-0022/DEC-... discussed for the WEIGHT path specifically;
this is a DIFFERENT, previously uncounted serialization, entirely
inside nms_memory_manager_pf.v's own ST_RUN state, in the
OPERAND-PRESENTATION logic (act_rd_en/wgt_rd_en -> read_issued ->
read_ready -> operand_valid), which is written as a strictly
sequential if/else-if chain: issue read (1 cycle) -> read_issued
observed (1 cycle) -> read_ready observed, operand presented (1
cycle) -> operand consumed, tile_idx increments (1 cycle) -> ONLY
THEN does the chain re-check "start next read". Four single-cycle
states per tile, ZERO overlap between consecutive tiles' reads --
even though: (a) the local activation/weight SRAMs
(nms_activation_replicated.v/nms_weight_packed.v) have only a
1-cycle rd_en-to-data latency, and (b) neural_processor.v's own
operand_ready is HELD HIGH continuously throughout NP_WAIT_OPERANDS
(its datapath is explicitly designed, per its own header comment, to
"accept a new tile every cycle while previous tiles are still
draining through the adder tree/accumulator") -- nothing on either
side of this interface actually requires 4 cycles/tile; it is purely
an artifact of nms_memory_manager_pf.v's own un-pipelined FSM.
Total-81-cycle decomposition: 16 tiles x 4 cycles/tile (steady-state
serialization) = 64 cycles, + 17 cycles of genuine per-job overhead
(FSM entry, weight-prefetch-engine fill latency for tile 0
specifically, NP_FINISH pipeline drain [8 cycles, matches the
P_IN=8 pipeline depth exactly], NP_WRITE_RESULT/ST_WRITE_RES/ST_DONE
handshakes). 64+17=81, exact.
CROSS-CHECK against EXP-0024's real N_SLOTS=2 D-Stress measurement
(non_memory_cycles=17544, /256 neurons=68.53 cycles/neuron): this
isolated trace's 64-cycle tile-serialization component alone
accounts for 64/68.53 = 93.4% of the REAL measured non-memory floor.
Only 4.53 cycles/neuron (6.6%) remains attributable to genuine
per-job dispatch/drain overhead in the real system.
classification: RTL SIMULATION (isolated, zero-memory-latency
configuration) for the 81-cycle/64-cycle numbers; DERIVED for the
93.4%/6.6% cross-check against EXP-0024's real data.
interpretation: EXP-0024's informal "~68.5 cycles/neuron ~ dispatch
overhead" framing (and this STEP's own governing spec, which framed
the problem as primarily inter-job/per-neuron dispatch cost amenable
to "batching K neurons per job") is SUPERSEDED by this more precise
trace: the dominant real cost (93.4%) is an INTRA-job, PER-TILE
operand-delivery serialization inside nms_memory_manager_pf.v's own
ST_RUN FSM, not inter-job dispatch overhead. Batching multiple
neurons into one dispatch would only address the remaining 6.6%
(~4.5 cycles/neuron) -- it would leave the 64-cycle/neuron
tile-serialization component completely untouched, since it recurs
on EVERY tile of EVERY job/batch regardless of dispatch granularity.
decision: see DEC-0024. The primary architectural fix is a pipelined/
continuous per-TILE operand-delivery redesign of the memory manager
(read-ahead with a skid buffer, decoupling "issue next tile's SRAM
read" from "current tile consumed"), NOT primarily a neuron-batching
scheme at the job-dispatch level. neural_processor.v itself requires
NO modification -- it already supports the required continuous
1-tile/cycle acceptance; the bottleneck is entirely upstream of it.
next_action: design and implement nms_memory_manager_stream.v (new
A/B variant, nms_memory_manager_pf.v itself untouched) with a
pipelined read-ahead operand-delivery FSM targeting ~1 cycle/tile
steady state (down from 4), verify bit-exact, then re-run the
ideal-memory and real-PSRAM benchmarks to quantify the new asymptotic
utilization ceiling.
EXP-0026
timestamp: 2026-09-06T01:30:00Z
module: hardware/v2/nms/rtl/nms_memory_manager_stream.v (NEW, per
DEC-0024), tested via the same isolated zero-real-memory-latency
harness as EXP-0025.
result (RTL SIMULATION, isolated, ideal mem_ready=1 always): total
cycles for a 16-tile job dropped from 81 (nms_memory_manager_pf.v)
to 80 -- i.e. essentially UNCHANGED, NOT the ~4x reduction the fix
targets. Per-cycle trace (buf_valid/rd_ptr/rd_pending/wgt_ready_count
dumped every cycle) shows WHY: the new manager's own read-ahead
logic works exactly as designed (issue_rd_now correctly fires the
cycle immediately after each buffer slot frees, achieving genuine
back-to-back issuing whenever data is available) -- but
`can_issue_rd` is gated on `rd_ptr < wgt_ready_count`, and
wgt_ready_count itself only advances every EXACTLY 4 cycles, tied
to weight_prefetch_engine.v's own WORDS_PER_TILE=P_IN/2=4 separate
16-bit word-transactions per tile, each requiring a minimum of 1
cycle even with mem_ready held permanently high (the fastest
possible turnaround for a request/response protocol over a 16-bit
bus). The fix ELIMINATED the memory-manager's own FSM-serialization
bottleneck (confirmed: whenever wgt_ready_count/usable_act allow it,
a new read issues the very next cycle, zero added delay) but
immediately hit a SECOND, previously-MASKED bottleneck at the exact
same numeric value (4 cycles/tile) for an entirely different, more
fundamentally physical reason: the 16-bit-wide real PSRAM data bus
itself limits weight delivery to 2 bytes/cycle, and a P_IN=8-byte
weight tile requires 4 such word-transactions NO MATTER HOW FAST the
underlying memory or how well the control logic is pipelined --
this is a bus-WIDTH ceiling, not a latency or FSM-scheduling ceiling.
interpretation: at TODAY's real hardware bandwidth (fixed 16-bit
PSRAM bus), this fix provides NO net cycle-count benefit -- the two
bottlenecks happen to coincide numerically. However, they are
architecturally DIFFERENT ceilings: the OLD manager's 4-cycles/tile
was a hard FSM-serialization floor that persists regardless of
external bandwidth (as EXP-0024's own PFD sweep already showed: more
bandwidth/lookahead depth cannot fix a control-plane bug). The NEW
manager's floor is a pure bus-bandwidth ceiling that WOULD improve
if external bandwidth genuinely increased (wider bus, faster PSRAM,
multiple banks) -- i.e. this fix removes a bug that was independently
capping the system, and now leaves ONLY the physical bandwidth
ceiling EXP-0024's roofline model already identified. This must be
verified with a genuinely variable-bandwidth ideal model (not the
fixed-16-bit-word real protocol) to confirm the new design's
utilization actually SCALES with bandwidth where the old one could
not (see EXP-0027).
classification: RTL SIMULATION (isolated, zero-real-memory-latency).
decision: proceed to (1) bit-exact integration verification of
nms_memory_manager_stream.v in the full NMS dataflow core/top-level,
(2) a genuinely variable-bandwidth ideal sweep to confirm the fix
removes the OLD hard ceiling once bandwidth is no longer fixed at
today's real 16-bit-bus rate, (3) the real N_SLOTS=2 D-Stress
re-benchmark (predicted: no net change vs the "_pf" baseline at
today's real bandwidth, for the reason above -- an important,
honest, PREDICTED-null result to confirm rather than a fix to
celebrate prematurely).
EXP-0027
timestamp: 2026-09-06T02:00:00Z
module: scratch-only nms_memory_manager_stream_idealwgt.v
(/tmp/nms_pf_build/step13_trace/, NOT part of the deliverable RTL --
a test-only variant with weight_prefetch_engine.v's instance
replaced by `assign wgt_ready_count = job_active_reg ? n_tiles_reg :
0`, i.e. weight instantly, fully resident the moment a job starts).
purpose: directly test the k->infinity endpoint of STEP13's own Step6
bandwidth sweep -- with the weight-fetch-rate bottleneck (identified
in EXP-0026 as a co-located, numerically-coincidental 4-cycles/tile
ceiling tied to the real 16-bit PSRAM bus width) completely removed,
does nms_memory_manager_stream.v's OWN read-ahead pipeline actually
achieve near-1-cycle/tile steady state, or was EXP-0026's unchanged
81->80 cycle result actually evidence the FIX ITSELF doesn't work
(as opposed to being masked by a second bottleneck)?
result (RTL SIMULATION, isolated, weight-fetch bypassed): tile_idx
advances EVERY SINGLE CYCLE in steady state (cycles 44,45,46,...,58,
gap of exactly 1 for all 14 steady-state tiles) -- a clean, genuine
1 cycle/tile sustained throughput, CONFIRMED. Total job cycles: 30
(16 tiles x 1 cycle + ~14 cycles job-level entry/drain/writeback
overhead), vs 80-81 cycles for the SAME job with the real weight-
fetch engine active (4 cycles/tile). This is a genuine ~2.7x total
job speedup, and a 4x speedup in the steady-state tile-delivery rate
specifically (1 vs 4 cycles/tile) -- matching the P_IN=8 pipeline's
own maximum possible per-tile acceptance rate EXACTLY (100% of
theoretical, since neural_processor.v's own datapath is designed for
exactly 1 tile/cycle acceptance).
Cross-reference: nms_memory_manager_pf.v (the OLD, un-pipelined
design) was ALREADY measured at 4 cycles/tile in EXP-0025 even
though its OWN weight_prefetch_engine instance had ALSO already
raced ahead to full readiness (wgt_ready_count=16) well before tile
1 was needed in that same trace -- i.e. EXP-0025's 4-cycles/tile WAS
ALREADY the FSM-serialization-only ceiling, weight-fetch-rate was
NOT yet the limiter there. This confirms: OLD design's ceiling is a
hard 4-cycles/tile REGARDLESS of external bandwidth (it cannot do
better even with the exact same "weight always ready" advantage);
NEW design's ceiling, under the SAME advantage, is 1 cycle/tile --
a REAL, structural, 4x improvement in the achievable ceiling.
classification: RTL SIMULATION (isolated scratch harness, not part of
the deliverable RTL or its own testbenches).
interpretation: STEP13's Step6 question ("does the new architecture
remove the asymptotic ceiling?") is answered YES for the
control-plane/FSM-serialization component specifically: the new
design's OWN achievable ceiling is 4x higher than the old design's.
However, EXP-0026 already showed this improvement is CURRENTLY
MASKED at today's real hardware bandwidth, because
weight_prefetch_engine.v's own word-fetch rate (tied to the fixed
16-bit real PSRAM bus) is ALSO exactly 4 cycles/tile today -- a
second, independent, currently-co-dominant ceiling that this STEP's
own scope (memory-manager/dataflow redesign) does not and cannot
address (fixing it would require a wider PSRAM bus, multiple banks,
or a redesigned weight-fetch protocol able to deliver more than one
16-bit word per cycle -- explicitly outside "everything else
unchanged" and outside this STEP's own RTL-scheduling scope, same
conclusion class as EXP-0024's own bandwidth-requirement addendum).
The practical, honest conclusion: this fix is REAL, CORRECT, and
REMOVES A GENUINE ARCHITECTURAL BUG, but delivers ZERO measurable
benefit until/unless external weight-fetch bandwidth is ALSO
increased beyond today's real 16-bit-bus rate -- at which point this
fix becomes NECESSARY (without it, the old 4-cycles/tile FSM
ceiling would immediately become the new bottleneck and cap all
further bandwidth gains at 25% utilization, regardless of how much
faster the memory becomes).
decision: see DEC-0025. Adopt nms_memory_manager_stream.v (retire
reliance on nms_memory_manager_pf.v for any FUTURE hardware revision
that increases real PSRAM bandwidth) since it is a strict
improvement with no measured downside at today's bandwidth (bit-
exact, same resource/Fmax class, EXP-0028) and REQUIRED groundwork
for any future bandwidth increase to actually pay off.
EXP-0028
timestamp: 2026-09-06T02:15:00Z
module: hardware/v2/nms/rtl/nms_memory_manager_stream.v,
nms_dataflow_core_stream.v, nms_neural_multiprocessor_stream.v
(real synthesis + P&R), hardware/v2/nms/sim/tb_nms_dstress_stream.v
(real D-Stress bit-exactness + benchmark).
command (sim): verilator --binary --timing -j0 -GN_SLOTS_CFG=2
-GPFD_CFG=8 --top-module tb <full real dependency set incl.
hardware/v1/rtl/memory_interface.v, psram_controller.v,
hardware/v1/sim/psram_model.v> hardware/v2/nms/sim/tb_nms_dstress_stream.v
command (synth+PnR, N_SLOTS in {1,2}, PFD=8): yosys -p "read_verilog
-sv <all NMS-stream+reused RTL>; chparam -set N_SLOTS <n> -set
MAX_TILES 16 -set PREFETCH_DISTANCE 8
nms_neural_multiprocessor_stream; synth_ecp5 ..." && nextpnr-ecp5
--45k --package CABGA381 --speed 8 --freq 80 ...
result (POST-P&R MEASURED Fmax/resources; RTL SIMULATION cycles, real
Verilator against the real V1 PSRAM chain):
D-Stress (N_SLOTS=2, PFD=8): PASS 256/256 neurons bit-exact vs
golden model. total_cycles=185270 (vs 185398 for "_pf" at the same
config, EXP-0024 -- a -0.07% difference, i.e. NO measurable net
change, exactly as predicted by EXP-0026/0027's own analysis: the
fix is masked by the co-dominant weight-fetch-rate ceiling at
today's real bandwidth). sustained_MAC/cycle=0.1769 (vs 0.1767),
weight_stall=94.42% (vs 93.32% -- slightly HIGHER, likely because
the new "weight_blocking" instrumentation definition itself changed
slightly, see tb_nms_dstress_stream.v's own comment -- not a real
regression, a metric-definition artifact of removing read_issued/
read_ready from the blocking condition).
Synthesis: N=1 PFD=8: Fmax=142.92MHz PASS (vs "_pf"'s 137.76MHz,
+3.7%), LUT4=1433 (vs 1464, -2.1%), CCU2C=206 (vs 200, +3.0%),
FF=2250 (vs 2245, +0.2%), DSP=8, BRAM=0.
N=2 PFD=8: Fmax=92.57MHz PASS (vs "_pf"'s 95.25MHz, -2.8%, still
comfortably above the 80MHz target), LUT4=2014 (vs 1908, +5.6%),
CCU2C=371 (vs 362, +2.5%), FF=3459 (vs 3449, +0.3%), DSP=16, BRAM=0.
All changes are small (within +/-6%), consistent with the modest
added logic (rd_ptr register + comparator, skid-buffer control) --
the fix does NOT "move the bottleneck to an enormous combinational
controller" (STEP11's own explicit worry, still holding here).
classification: POST-P&R MEASURED (Fmax/resources), RTL SIMULATION
(cycles, real V1 PSRAM chain), bit-exact PASS.
decision: see DEC-0025.
EXP-0029
timestamp: 2026-09-06T03:00:00Z
git_commit: 5c9ec61 (+ uncommitted NMS STEP11-14 work)
session: v2-NMS-STEP14-partB-activation-timing
module: hardware/v2/nms/rtl/nms_activation_fill_ctrl.v (analysis only,
no modification this entry -- real post-P&R critical path report
mined from STEP13's own N_SLOTS=4 synthesis run,
/tmp/nms_stream_synth/n4_pfd8/pnr.log, nms_neural_multiprocessor_stream.v).
command: nextpnr-ecp5 --45k --package CABGA381 --speed 8 --freq 80
--json top.json --lpf-allow-unconstrained --textcfg top.config
(already run in STEP13/EXP-0028; this entry re-analyzes its own
full critical-path report rather than re-running P&R).
result (POST-P&R MEASURED, real critical-path trace, RTL line numbers
from nextpnr's own "Defined in:" annotations -- not assumed):
Fmax=55.22 MHz (FAIL @80MHz), critical path total 18.11 ns
(6.25 ns logic + 11.85 ns routing), exact path:
SOURCE: u_dataflow_core.u_act_fill.resident_tag[11] (register Q)
-> COMBINATIONAL, chained, NO register in between:
(1) max_n_tiles computation, nms_activation_fill_ctrl.v:92
(`if (n_tiles_flat[i*16+:16] > max_n_tiles) max_n_tiles =
...` inside the N_SLOTS-wide always@* for-loop, lines
90-95) -- synthesized as a long CCU2C carry-chain (16-bit
magnitude comparison, chained across N_SLOTS=4 iterations)
(2) resident_count < max_n_tiles comparison,
nms_activation_fill_ctrl.v:165 (the ST_IDLE case's own
refill/continue-fetch condition) -- ANOTHER 16-bit
magnitude-comparison carry chain, feeding DIRECTLY off (1)
in the SAME cycle, no pipeline register between them
(3) into pf_start's own next-state logic
-> DESTINATION: u_act_fill.pf_addr's own clock-enable (CE) pin.
Two full 16-bit magnitude comparisons (line 92 AND line 165) sit in
ONE combinational cone across ONE clock edge, with poor physical
locality (many short logic hops, 0.85-0.97ns routing EACH between
scattered CCU2C cells -- 11.85ns of the 18.11ns total is routing,
suggesting the long carry chain is not compactly placed).
interpretation: this CONFIRMS, with an exact RTL-line-level and
post-P&R-measured trace (not assumed), the failure mode DEC-0016/
EXP-0022 already predicted analytically ("O(N_SLOTS) unpipelined
combinational scan feeding directly into a control decision") --
but now precisely localized to TWO specific, back-to-back,
un-pipelined 16-bit comparisons (max_n_tiles's own computation, and
its immediate use in the refill/continue decision), not the
priority-encoder (`desired_valid`/`desired_x_base`, lines 77-86)
that was the FIRST suspect -- that logic does NOT appear anywhere in
this critical path at all. The real cost is the COMPARISON logic
(lines 92 and 165), not the priority-scan itself.
classification: POST-P&R MEASURED (critical path from real P&R run),
DERIVED (RTL-line attribution from nextpnr's own "Defined in"
annotations, cross-checked against the actual source file).
decision: see DEC-0026. Minimum fix: register max_n_tiles ONE cycle
before its use in the resident_count comparison, breaking the two
chained 16-bit comparisons into separate clock cycles. This is a
refill-DECISION path only (evaluated once per tile-fill-trigger
boundary, not on every real-time-critical per-tile-consumption
cycle already decoupled by STEP13's own streaming fix) -- adding one
cycle of latency here is functionally free for steady-state
throughput.
next_action: implement nms_activation_fill_ctrl_v2.v (pipelined
max_n_tiles), re-synthesize N=4, confirm Fmax>=80MHz and bit-exact
correctness preserved, confirm no new serialization introduced
(steady-state per-tile cycle count unchanged from STEP13's own
streaming-manager result).
EXP-0030
timestamp: 2026-09-06T03:30:00Z
module: hardware/v2/nms/rtl/nms_activation_fill_ctrl_v2.v (1-stage
fix, insufficient alone), nms_activation_fill_ctrl_v3.v (2-stage
fix, FINAL), nms_dataflow_core_actfix2.v,
nms_neural_multiprocessor_actfix2.v, nms_activation_fill_ctrl.v
itself UNTOUCHED.
command (synth+PnR, N=4, iterating on the fix): yosys/nextpnr-ecp5,
same real command pattern as EXP-0028, chparam N_SLOTS=4
MAX_TILES=16 PREFETCH_DISTANCE=8.
command (sim, bit-exact + benchmark, N=2 and N=4): verilator, same
D-Stress full-real-PSRAM harness as EXP-0028
(tb_nms_dstress_actfix2.v).
result (POST-P&R MEASURED + RTL SIMULATION bit-exact):
v2 (max_n_tiles registered once, before its use in the
resident_count comparison): Fmax=72.78MHz -- REAL improvement
(+31.8% over the 55.22MHz baseline) but STILL FAILS 80MHz. New
critical path traced (same technique as EXP-0029): now entirely
within max_n_tiles's OWN computation (nms_activation_fill_ctrl_v2.v
line 92) -- an N_SLOTS-wide SEQUENTIALLY-CHAINED running-max fold,
each iteration mixing a 23-bit tag-equality check with a 16-bit
magnitude comparison, feeding max_n_tiles_reg's own D input.
13.74ns total (5.40ns logic + 8.34ns routing).
v3 (SECOND pipeline stage: per-slot tag-equality + masking
registered ONE cycle FIRST -- independent per-slot work, no
N_SLOTS-dependent chain -- THEN the max-fold operates alone on the
already-registered, already-masked per-slot values): Fmax=106.81MHz
-- PASSES 80MHz with real margin (+93.4% over the original 55.22MHz
baseline, +46.8% over the v2-only fix). Resource cost at N=4:
LUT4=2776 (vs 2937 baseline, -5.5%), CCU2C=705 (unchanged),
FF=5957 (vs 5877, +1.4%, expected from the 2 added pipeline stages),
DSP=32 (unchanged).
Bit-exact verification (tb_nms_dstress_actfix2.v, real V1 PSRAM
chain, D-Stress workload): N_SLOTS=4 PASS 256/256 neurons bit-exact,
total_cycles=184771 (statistically identical to N=2's own
185270-185398 range from EXP-0028 -- confirms the SAME single-
shared-PSRAM-port ceiling already documented, unaffected by this
timing fix, exactly as expected: this fix addresses FMAX, not
memory bandwidth). N_SLOTS=2 regression check: PASS 256/256,
sustained_MAC/cycle=0.1769, IDENTICAL to EXP-0028's own pure-
streaming (no actfix) result -- confirms ZERO regression, NO new
serialization introduced by the 3-cycle total added latency to the
(rare, tile-refill-boundary-only) activation-refill decision path,
satisfying STEP14's own explicit B4 requirement.
Per-slot tile delivery imbalance observed at N=4 (slot0/1: 2016
tiles each, slot2/3: 48/16 tiles) -- the SAME "first-free fixed-
priority dispatch" imbalance already documented in EXP-0022 for
N_SLOTS=4, unrelated to and unaffected by this timing fix.
classification: POST-P&R MEASURED (Fmax/resources), RTL SIMULATION
bit-exact (cycles, real V1 PSRAM chain).
decision: see DEC-0027 (adopt nms_activation_fill_ctrl_v3.v as the
new reference activation-fill controller for N_SLOTS>=4
configurations).
EXP-0031
timestamp: 2026-09-06T03:40:00Z
module: nms_neural_multiprocessor_actfix2.v @ N_SLOTS=8 (exploratory,
per STEP14's own explicit "N=8 does not need to pass all targets"
scope).
command: same real synth+PnR command as EXP-0030, chparam N_SLOTS=8.
result (POST-P&R MEASURED): DSP=64/72 (89% of budget, FEASIBLE),
LUT4=4653 (well within the ~44k available on the LFE5U-45F,
FEASIBLE), FF=10855 (FEASIBLE), CCU2C=1367. Fmax=52.25MHz, FAILS
80MHz (regressed back down from N=4's 106.81MHz).
interpretation: the v3 activation-fill-controller fix (EXP-0030)
pipelines the per-slot tag-equality/masking stage (N_SLOTS-
independent depth) but its SECOND stage -- the max_n_tiles
sequential fold itself -- is STILL an O(N_SLOTS)-deep chained
comparison (unchanged from before, just now isolated in its own
cycle). At N_SLOTS=4 this was short enough to clear 80MHz; at
N_SLOTS=8 the fold is twice as deep and becomes the dominant cost
again, reproducing the same class of Fmax regression. This is
expected and consistent -- the v3 fix shifted the crossover point,
it did not eliminate the underlying O(N_SLOTS) dependency.
Limiting resource for N=8: Fmax/timing (routing+logic depth of the
fold), NOT DSP/LUT/FF/BRAM -- all of which have ample headroom.
classification: POST-P&R MEASURED.
decision: N=8 is resource-feasible (DSP/LUT/FF all comfortably within
budget) but NOT timing-feasible with the current 2-stage fix. A
genuine balanced-tree reduction (or additional pipeline stages
scaling with log2(N_SLOTS) rather than a flat 2-stage split) would
be required to reach 80MHz at N=8 -- NOT undertaken this round
(STEP14's own explicit scope: N=8 is exploratory, quantify the
limit, do not necessarily fix it). Flagged as concrete future work
with a precise, evidence-based mechanism (not a vague "needs more
optimization").
EXP-0032
timestamp: 2026-09-06T04:00:00Z
module: hardware/v2/nms/rtl/weight_prefetch_engine_wide.v (NEW,
parameterized MEM_DATA_WIDTH, simulation-only/exploratory),
nms_memory_manager_stream_wide.v (NEW, streaming manager + wide
engine, separate wide logical weight port from the real 16-bit
result-writeback port), nms_weight_packed.v (unchanged, real
production SRAM), hardware/v2/nms/sim/tb_weight_prefetch_wide.v
(bit-exact correctness, parametrized MEM_DATA_WIDTH).
configuration: MEM_DATA_WIDTH in {16,32,64,128}, P_IN=8, DATA_WIDTH=8
fixed (TILE_BITS=64 always). PFD=4 for correctness sweep.
command (bit-exact, per width): verilator --binary --timing -j0
-GMEM_DATA_WIDTH=<w> -GPFD=4 --top-module tb
weight_prefetch_engine_wide.v nms_weight_packed.v
tb_weight_prefetch_wide.v
command (ideal-memory cycles/tile, isolated, zero real latency, per
width): same pattern as EXP-0025/26/27's own isolated trace
testbench, mem_ready tied permanently high on the wide logical port.
result (RTL SIMULATION bit-exact + isolated ideal-memory cycles/tile):
Bit-exact: ALL 4 widths PASS (9/9 tests, 0 errors each), including
under injected extra memory latency (EXTRA_WAIT=4). One real bug
found and fixed during development: the initial address-stepping
arithmetic used WORDS_PER_TILE*BYTES_PER_WORD as the inter-tile
byte stride, which is WRONG whenever MEM_DATA_WIDTH > TILE_BITS
(the 128-bit case, WORDS_PER_TILE=1 but BYTES_PER_WORD=16 while the
tile itself is only 8 bytes) -- this double-counts the unused
surplus bits of a too-wide transaction as real address space and
skips over the next tile's actual data in the packed backing store
(344 real FAILs observed before the fix, all "got = 2x expected"
starting exactly at tile index 1). Fixed by defining TILE_BYTES =
TILE_BITS/8 (the tile's own natural, MEM_DATA_WIDTH-independent
size) as the canonical inter-tile stride. Post-fix: 128-bit also
PASSES 9/9 bit-exact.
Ideal-memory cycles/tile (isolated, single slot, zero real memory
latency, PFD=16 so lookahead never gates): a 16-tile job's total
cycles and PER-TILE STEADY-STATE gap (confirmed via exact per-cycle
tile_idx-transition tracing at MEM_DATA_WIDTH=64):
16-bit: total=80 cycles (steady-state 4 cycles/tile, matches
EXP-0025/26's own real-engine result exactly -- same WORDS_PER_
TILE=4)
32-bit: total=48 cycles (steady-state 2 cycles/tile)
64-bit: total=32 cycles (steady-state EXACTLY 1 cycle/tile,
confirmed cycle-by-cycle: tile_idx advances at 48,49,50,...,62,
a perfect 1-cycle gap for all 14 steady-state tiles)
128-bit: total=32 cycles (steady-state 1 cycle/tile -- IDENTICAL
to 64-bit, ZERO further benefit, exactly as predicted: WORDS_
PER_TILE=ceil(64/128)=1, same as WORDS_PER_TILE=ceil(64/64)=1 --
a bus wider than one full tile cannot deliver more than one
tile per transaction in this single-tile-per-request design).
All four widths match cycles/tile = WORDS_PER_TILE =
ceil(TILE_BITS/MEM_DATA_WIDTH) EXACTLY (4, 2, 1, 1) -- confirming
the architectural prediction with zero surprises.
KEY FINDING: MEM_DATA_WIDTH=64 (exactly P_IN*DATA_WIDTH) is the
precise architectural point at which the weight-fetch steady-state
rate (1 cycle/tile) exactly matches nms_memory_manager_stream.v's
own control-plane ceiling (1 cycle/tile, EXP-0027) -- NEITHER side
limits the other at this width. This is the answer to STEP14's own
key A3 question ("at what weight-path width does the processor stop
being fundamentally starved by weight delivery?"): 64 bits.
classification: RTL SIMULATION bit-exact (A4), RTL SIMULATION isolated
ideal-memory (A3, zero real latency -- classified IDEAL MEMORY per
this project's own convention, not POST-P&R/real-PSRAM).
decision: see DEC-0028 (A5 logical-vs-physical distinction) and the
STEP14 combined summary for the full implication.
EXP-0033 (roofline reconstruction)
timestamp: 2026-09-06T04:30:00Z
purpose: rebuild the EXP-0024 roofline model (T(k)=17.544+167.854/k)
using STEP13/14's own precise, decomposed understanding of where
every cycle goes -- per STEP14's own explicit instruction NOT to
reuse the old model blindly.
components identified (all real, RTL-traced, not assumed):
T_control (memory-manager operand-delivery serialization): WAS 3 of
4 cycles/tile (EXP-0025); FIXED by nms_memory_manager_stream.v
(STEP13) -- now ~0 (1 cycle/tile achieved whenever weight is not
the limiter, EXP-0027).
T_weight (weight-fetch rate, real 16-bit physical PSRAM bus):
STILL 4 cycles/tile on real hardware (EXP-0026/28/30) -- a
physical bus-WIDTH floor, not a scheduling floor. Proven
(EXP-0032, ideal/simulation-only) to drop to 1 cycle/tile at a
64-bit LOGICAL width, but this requires a matching PHYSICAL
bandwidth increase to realize on real hardware (DEC-0028) -- not
available on the real ISSI x16 PSRAM this project targets.
T_activation (activation-fill-controller Fmax): does NOT affect
cycle count at all (confirmed: N=2 cycles identical before/after
the Part B fix, EXP-0030) -- it only gates the clock frequency the
design can run at (55.22->106.81MHz @ N=4), a WALL-CLOCK factor,
not a CYCLE-COUNT factor.
T_startup + T_drain (per-job, non-weight, non-control overhead: NP's
own 8-stage pipeline drain, job entry, result write-back
handshake): ~16 cycles/job, MEASURED IDENTICAL at both 16-bit and
64-bit weight width (EXP-0032: 80-64=16, 32-16=16) -- confirms
this component is genuinely independent of weight-path width, a
separate, smaller, already-minimal residual.
T_external_memory (real PSRAM port contention across N_SLOTS,
real slot_mem_arbiter): the TRUE dominant real bottleneck --
confirmed by N_SLOTS=2/4/8 (actfix2) all producing STATISTICALLY
IDENTICAL real total_cycles (185270/184771/184771, within 0.3%)
despite theoretical MAC/cycle scaling 16/32/64 -- the single real
physical PSRAM port caps AGGREGATE throughput regardless of
on-chip slot count, exactly as EXP-0022/0024 already established,
now confirmed to persist THROUGH both STEP13 and STEP14's own
fixes (neither touches the physical port itself).
new decomposition (per-job, n_tiles=16, real hardware, N_SLOTS=1):
T(n_tiles) = T_startup_drain + n_tiles * T_weight
= 16 + n_tiles * 4 [cycles, REAL 16-bit bus]
(T_control and T_activation no longer contribute measurable cycle
cost on real hardware -- both are fully resolved as SEPARATE axes:
T_control by STEP13, T_activation's Fmax by STEP14 Part B.)
asymptotic utilization (real hardware, unchanged from EXP-0024):
U_inf @ N=2 = 32768 / (16 * 17544) = 11.674% -- IDENTICAL to
EXP-0024's own number. NOT because nothing was fixed, but because
the DOMINANT component of that 17544-cycle floor (T_weight, ~64 of
every 68.5 cycles/neuron, EXP-0025's own cross-check) is a PHYSICAL
bus-width constraint that neither STEP13 nor STEP14's own RTL fixes
could touch -- both real fixes targeted SMALLER, genuinely-separate
components (T_control: fixed, was already small at 6.6% of the
floor; T_activation: Fmax only, zero cycle-count effect).
DERIVED, hypothetical (NOT real hardware -- assumes a future 64-bit-
wide PHYSICAL PSRAM interface AND, unrealistically, zero real
port-contention across N_SLOTS=2, an idealized upper bound):
U_64bit_ideal @ N=2 = 32768/(16*4096) = 50.0%. This is the CEILING
ON THE CEILING -- even with the weight-bus-width problem fully
solved, real N_SLOTS>=2 port contention (T_external_memory, NOT
measured at 64-bit since no real 64-bit PSRAM exists to test) would
likely bring this DOWN further; 50% is an optimistic upper bound,
not a promise.
classification: DERIVED (roofline reconstruction from real, already-
measured EXP-0025/26/27/28/30/32 data).
answer to STEP14's own key roofline question ("does the new
architecture remove the previous 11.674% asymptotic ceiling?"):
NO, not on real hardware today -- the ceiling is numerically
unchanged, because its dominant cause (T_weight, physical bus width)
is untouched by any RTL-level fix available within this project's
own scope. YES, in principle, once external physical bandwidth is
increased (EXP-0027/32 both prove the RTL-level ceiling -- 1
cycle/tile, both for control-plane and for weight-fetch given
sufficient bus width -- has ALREADY been achieved architecturally;
only the physical PSRAM interface itself remains as the blocker).
EXP-0034
timestamp: 2026-09-06T05:00:00Z
git_commit: 5c9ec61 (+ uncommitted NMS STEP11-15 work)
session: v2-NMS-STEP15-physical-memory-bandwidth
module: hardware/v2/rtl/weight_prefetch_engine.v (real, 16-bit,
unmodified) driven against the REAL, complete V1 chain --
hardware/v1/rtl/memory_interface.v, hardware/v1/rtl/
psram_controller.v, hardware/v1/sim/psram_model.v -- isolated,
single slot, zero cross-slot contention. Scratch testbench (not
committed to the repo): tb_real_weight_baseline.v.
command: verilator --binary --timing -j0 --top-module tb
memory_interface.v psram_controller.v psram_model.v
weight_prefetch_engine.v <scratch tb>; MAX_TILES=256,
PREFETCH_DISTANCE=300 (>MAX_TILES, window never gates fetching --
isolates the PURE physical fetch rate), CLK_FREQ_MHZ=80 (matches
every other real-PSRAM benchmark in this project), real ~150us
power-up wait observed before measurement begins.
purpose: STEP15 Part B's own explicit requirement -- "use the
existing real memory engine as the reference point... do not simply
multiply bandwidth... validate the cycle-level transaction model."
result (REAL-PSRAM SIM, RTL SIMULATION against the real, unmodified
V1 controller): 256-tile job, total=4377 cycles, 17.0977 cycles/
tile average -- NOT the ~4 cycles/tile figure used as a scratch
simplification in STEP13/14's own isolated "zero real latency"
traces (EXP-0025/26/27, which used a plain 1-cycle-turnaround
scratch memory model deliberately chosen to isolate CONTROL-PLANE
behavior, NOT real PSRAM timing).
Root-caused via direct state-transition tracing of the real
psram_controller.v: contrary to a naive reading of its own
ACCESS_CYCLES=6/PAGE_CYCLES=2 constants (@80MHz, from tAA=70ns/
tAPA=20ns), the controller's own STATE_PAGE_OPEN state ADDS a
further, real, measured 2 cycles per transaction (both page hits
AND misses) beyond ACCESS_CYCLES/PAGE_CYCLES themselves -- so the
REAL per-word cost is 4 cycles (page hit: 2 PAGE_CYCLES + 2
STATE_PAGE_OPEN) or 8 cycles (page miss: 6 ACCESS_CYCLES + 2
STATE_PAGE_OPEN), not 2/6 as the raw constants alone would suggest.
Real page size confirmed = 16 words = 32 BYTES (address bits above
A[3] must match for a hit, per the controller's own comment and
code). For weight fetch (P_IN=8 bytes/tile, sequential access),
this gives exactly 1 page-miss every 16 words (4 tiles): average
= (15x4 + 1x8)/16 = 4.25 cycles/word x 4 words/tile = 17.0 cycles/
tile -- matches the real measurement (17.0977) almost exactly (tiny
residual from the very first tile's own cold-start transient over a
256-tile average).
classification: REAL-PSRAM SIM (RTL simulation against the real,
unmodified V1 PSRAM chain) -- the authoritative PHY_WIDTH=16
baseline for STEP15.
decision: this REAL baseline (17.0 cycles/tile, single-slot,
uncontended) supersedes the STEP13/14 scratch estimate (~4 cycles/
tile) as the reference point for ANY future real-hardware-timing
claim about the weight-fetch path -- the STEP13/14 number remains
valid for what it was actually measuring (control-plane-only
behavior under an idealized memory), but must not be read as "the
real PSRAM's own best-case rate", which is 17.0 cycles/tile.
next_action: EXP-0035 (DERIVED, RTL-validated page-mode-aware model
generalized to PHY_WIDTH in {32,64,128}, calibrated against this
real 16-bit measurement).
EXP-0035
timestamp: 2026-09-06T05:15:00Z
module: scratch sim_wide_mem_pagemode.v (NEW, generalized page-mode-
aware physical memory model, PHY_WIDTH-parametrized) driving
weight_prefetch_engine_wide.v (STEP14, unmodified). Scratch tb (not
committed): tb_wide_pagemode_sweep.v.
configuration: PHY_WIDTH in {16,32,64,128}, MAX_TILES=512,
PREFETCH_DISTANCE=600 (unconstrained fetch, isolates pure physical
rate), PAGE_BYTES=32 (matches the REAL controller's own confirmed
page size, EXP-0034), per-transaction cost calibrated to reproduce
EXP-0034's own real 16-bit measurement exactly (hit=4 cycles/
transfer, miss=8 cycles/transfer, after correcting a +2-cycle
systematic offset in an early draft of the scratch model itself,
found by comparing against EXP-0034's own real number rather than
trusting the scratch model's parameters at face value).
result (RTL SIMULATION against a DERIVED, EXP-0034-calibrated model --
NOT measured against real wider silicon, since none exists):
16-bit: 17.002 cycles/tile (matches EXP-0034's real 17.0977 to
within a fraction of a percent -- confirms calibration)
32-bit: 9.002 cycles/tile
64-bit: 5.002 cycles/tile
128-bit: 5.002 cycles/tile -- IDENTICAL to 64-bit, a genuine
PLATEAU, not the "slight regression" an early hand/analytical
model predicted.
Real, RTL-verified explanation for the 128-bit plateau (not
assumed): weight_prefetch_engine_wide.v's own inter-tile ADDRESS
STRIDE is fixed at TILE_BYTES=8 bytes REGARDLESS of PHY_WIDTH (a
correctness requirement fixed in STEP14/EXP-0032, since the backing
store is packed at natural tile density) -- so at 128-bit
(16 bytes/transfer > 8-byte tile), consecutive REQUESTS still only
advance by 8 bytes even though each transfer nominally fetches 16.
This means the REAL hit/miss PATTERN (1 miss every 4 requests, since
32-byte page / 8-byte stride = 4) is IDENTICAL for 64-bit and
128-bit -- both issue exactly 1 transfer/tile at the SAME 8-byte
address cadence, so both see the SAME miss rate. 128-bit therefore
neither helps (no multi-tile bursting is implemented) nor hurts (no
extra page-boundary penalty beyond what 64-bit already pays) --
a genuine, RTL-confirmed plateau. An initial analytical (Python)
model, before this RTL cross-check, incorrectly predicted a 128-bit
REGRESSION (6.0 cycles/tile) by assuming transfers_per_page =
PAGE_BYTES/BYTES_PER_TRANSFER -- wrong whenever a transfer is wider
than the tile's own natural stride. The RTL simulation caught and
corrected this analytical error.
classification: RTL SIMULATION, DERIVED (calibrated model, not
measured against real wider silicon -- explicitly flagged: this
assumes the underlying array timing/page size are physical
properties of the memory technology, invariant to externally
exposed data width -- a reasonable but UNVERIFIED assumption for any
real wider part or parallel-bank implementation).
decision: 64-bit is confirmed as the practical ceiling for a
single-tile-per-request design (no measured or plausible benefit
beyond it); 128-bit is neither harmful nor helpful under this
model -- see DEC-0029 for the full roofline/recommendation
built from this data.
EXP-0036 (DERIVED full-system projection, N=2/4/8)
timestamp: 2026-09-06T05:30:00Z
purpose: scale EXP-0034/35's real single-slot, uncontended weight-
fetch measurements up to the FULL real multi-slot D-Stress system
(real slot_mem_arbiter.v contention, real activation/result-
writeback traffic sharing the same port) -- WITHOUT re-synthesizing
a full nms_dataflow_core_wide/nms_neural_multiprocessor_wide
variant at each PHY_WIDTH (a substantial additional engineering
effort not undertaken this round; explicitly flagged as a
limitation below).
method: calibrate a single "real-system degradation factor" from the
ALREADY-MEASURED real N=4 D-Stress result (EXP-0030, actfix2,
16-bit: 184771 total cycles / 4096 tiles = 45.11 cycles/tile) versus
THIS STEP's own real single-slot ideal-page-mode measurement
(EXP-0034: 17.0 cycles/tile) -- factor = 45.11/17.0 = 2.6535.
Applied this SAME factor to EXP-0035's 32/64/128-bit single-slot
numbers to project the corresponding real multi-slot result,
under the EXPLICIT, LABELED ASSUMPTION that arbitration/contention/
activation/writeback overhead scales PROPORTIONALLY with the
weight-fetch component rather than staying fixed or growing as a
LARGER fraction of a now-shorter transaction (a real, unresolved
uncertainty -- see caveat below).
result (DERIVED, N=4, total workload 4096 tiles fixed):
16-bit: 184771 cycles (= real measured, EXP-0030, exact anchor)
32-bit: ~97820 cycles (DERIVED) -- 1.889x speedup for 2x nominal
physical bandwidth
64-bit: ~54344 cycles (DERIVED) -- a further 1.80x speedup for
another 2x nominal bandwidth (3.40x cumulative vs 16-bit)
128-bit: ~54344 cycles (DERIVED) -- IDENTICAL to 64-bit (matches
EXP-0035's own single-slot plateau finding)
sustained MAC/cycle (N=4, theoretical=32): 0.1773 / 0.335 / 0.603 /
0.603 -- utilization 0.55% / 1.05% / 1.88% / 1.88% of theoretical.
Real N=2 (EXP-0028, 16-bit: 185270/4096=45.23 cyc/tile) and real N=8
(EXP-0030-class run, 16-bit: 184771/4096=45.11 cyc/tile) are BOTH
statistically identical to N=4's own 45.11 -- confirming (again)
that N_SLOTS does not change the port-bound ceiling, so this SAME
DERIVED projection applies equally to N=2/4/8 within the 16-128 bit
range explored (the workload remains memory-bound throughout; no
crossover to compute-bound is reached at any width tested).
explicit caveat (NOT resolved this round): the calibration assumes
the 2.6535x degradation factor is INVARIANT to PHY_WIDTH. This is
UNVERIFIED. A real risk exists that per-transaction arbitration/
grant overhead (a likely small, FIXED number of cycles per
transaction switch, independent of transfer width) would represent
a LARGER proportion of each transaction as PHY_WIDTH grows (since
each transaction itself becomes shorter) -- meaning the TRUE
degradation factor could be WORSE (higher) at 32/64/128-bit than at
16-bit, making this projection OPTIMISTIC. Confirming or refuting
this would require the full new synthesis+multi-slot-simulation
campaign flagged as future work (see DEC-0029).
classification: DERIVED (calibrated projection from real measured
anchors, NOT independently re-measured at 32/64/128-bit in the full
multi-slot system).
EXP-0037
timestamp: 2026-09-06T06:00:00Z
git_commit: 5c9ec61 (+ uncommitted NMS STEP11-15 work)
session: v2-NMS-STEP15-32bit-validation
module: hardware/v2/nms/rtl/psram_controller_dual32.v (NEW, real
dual-chip 32-bit physical memory interface), hardware/v2/nms/sim/
tb_psram_dual32.v (NEW, bit-exact + timing regression).
architecture_decision: "duplicated controller instances, shared
address/control, duplicated data path" -- SELECTED over "one
widened controller" (rejected: psram_controller.v's own psram_dq is
a single inout bus per instance, cannot represent two separate
physical chips) and "interleaved controllers" (rejected: solves
capacity, not per-transfer width). Two full, real, BYTE-FOR-BYTE
UNMODIFIED psram_controller.v instances, fed IDENTICAL clk/rst/
mem_req/mem_wr/mem_addr every cycle (broadcast) -- structurally,
cycle-exact synchronized by construction (both instances are the
same RTL executing the same real timing FSM against the same
inputs), not by any added synchronization logic. Real, synthesizable
cross-check added (lane_sync_error, latches if ready0!==ready1 --
never expected to fire; confirmed never fires in every test run).
command (bit-exact + timing, isolated): verilator --binary --timing
-j0 --top-module tb hardware/v1/rtl/psram_controller.v
hardware/v1/sim/psram_model.v hardware/v2/nms/rtl/
psram_controller_dual32.v hardware/v2/nms/rtl/
weight_prefetch_engine_wide.v (STEP14, UNMODIFIED, MEM_DATA_WIDTH=32)
hardware/v2/nms/rtl/nms_weight_packed.v tb_psram_dual32.v
result (REAL-PSRAM SIM, RTL SIMULATION bit-exact): after fixing THREE
real bugs found via direct simulation (not assumed):
BUG 1 -- address space mismatch: weight_prefetch_engine_wide.v's own
mem_addr is a BYTE address (its established STEP14 convention);
the real psram_controller.v instances each require a per-chip WORD
address (2 bytes/word). The wrapper's first draft fed the BYTE
address directly to both instances, unshifted -- every access
landed ~4x further out than intended. Fixed: mem_addr[ADDR_WIDTH-
1:2] (a genuine >>2 conversion, 4 bytes/32-bit-word) fed to both
instances internally; the module's own EXTERNAL contract stays a
byte address (so it plugs into weight_prefetch_engine_wide.v
without modifying that already-validated module).
BUG 2 -- mem_ready timing misalignment: the wrapper's first draft
REGISTERED mem_ready (`mem_ready <= ready0`) while mem_rdata
remained combinational -- a real one-cycle skew between the ready
pulse and the data it should qualify, causing the caller to sample
stale/settling data. Fixed: mem_ready is now a plain continuous
assignment (`assign mem_ready = ready0`), matching the real,
single-chip psram_controller.v's own timing exactly, which this
wrapper must preserve by design.
BUG 3 -- testbench DEPTH too small: psram_model.v instances declared
with DEPTH=16384 words, but the real test base address (0x60000
bytes -> word index 98304 at 32-bit indexing) exceeds that bound --
silent out-of-bounds pokes/reads, same bug CLASS already documented
elsewhere in this project's own history (sim_byte_mem's own too-
small DEPTH, tb_nms_dstress.v's own header comment). Fixed: DEPTH
raised to 131072.
Post-fix: ALL TESTS PASSED, 6/6, 0 errors -- n_tiles in
{0,1,2,15,16,511} (edge cases incl. the project's own mandatory
"counter-width bug at value 16" class, and MAX_TILES-1=511),
back-to-back jobs, lane_sync_error=0 throughout every run (both
physical chips stayed cycle-exact synchronized, confirming the
"shared control, no added sync logic" architecture is sound).
Real cycles/tile (single slot, uncontended, unconstrained lookahead,
512-tile job): 8.5488 -- NOT exactly the ~9.0 the prior STEP15
DERIVED/scratch model predicted. TRACED (not silently adjusted):
the prior model assumed a fixed 32-BYTE page regardless of
PHY_WIDTH (an explicitly-flagged, UNVERIFIED assumption at the
time). The REAL 2-parallel-16-bit-chip implementation's own
per-chip page granularity (16 of EACH CHIP's OWN word-addresses,
matching the real psram_controller.v's unmodified page-hit logic)
maps to a LARGER effective byte range in the COMBINED 32-bit address
space than a hypothetical native-32-bit single chip would have,
because chip_word_addr advances 1:1 with 32-bit-word/tile-pair
transactions (2 words/tile at 32-bit) rather than with raw bytes --
giving an EFFECTIVE page of 16 combined-32-bit-word transactions
(64 bytes), not 8 (32 bytes) as the earlier model assumed. Recomputed
with the REAL page depth (16 transfers/page): avg_cycles/transfer =
(15x4+1x8)/16=4.25, x2 words/tile=8.5 -- matches the measured 8.5488
almost exactly. The REAL 2-chip architecture is measurably BETTER
than the earlier abstract model predicted, not worse or equal --
a genuine, positive, and fully explained discovery.
classification: REAL-PSRAM SIM (RTL simulation against two real,
unmodified psram_controller.v instances) -- the authoritative,
concrete PHY_WIDTH=32 single-slot baseline, superseding EXP-0035's
own DERIVED/calibrated scratch-model number for this specific
metric (EXP-0035 remains valid for what it measured -- a GENERIC
page-mode-aware model exploring the abstract PHY_WIDTH sweep before
any concrete architecture was chosen).
decision: psram_controller_dual32.v (post-fix) is the validated,
bit-exact, real dual-chip 32-bit physical memory interface. Proceed
to full N=4 system integration (EXP-0038).
EXP-0038
timestamp: 2026-09-06T06:15:00Z
module: hardware/v2/nms/rtl/nms_dataflow_core_dual32.v (NEW),
nms_neural_multiprocessor_dual32.v (NEW), rtl/slot_mem_arbiter_wide.v
(NEW, DATA_WIDTH-parametrized copy of the real, unmodified
slot_mem_arbiter.v), hardware/v2/nms/sim/tb_nms_dstress_dual32.v
(NEW). Full real system: N_SLOTS instances of
nms_memory_manager_stream_wide.v (STEP14, MEM_DATA_WIDTH=32) ->
slot_mem_arbiter_wide.v -> psram_controller_dual32.v (2 real
physical chips, weight fetch ONLY) running ALONGSIDE the ORIGINAL,
UNTOUCHED slot_mem_arbiter.v -> memory_interface.v ->
psram_controller.v (1 real physical chip, activation-fetch + result
write-back ONLY, unchanged from every prior STEP).
command: verilator --binary --timing -j0 -GN_SLOTS_CFG=<n>
-GPFD_CFG=8 --top-module tb <full real dependency set incl. BOTH
real PSRAM chains, 3 total physical psram_model.v instances>
tb_nms_dstress_dual32.v. D-Stress workload (256 neurons, 16 tiles
each), identical to every prior benchmark in this project.
result (POST-P&R pending; RTL SIMULATION bit-exact, real V1 timing
chain(s) throughout):
N_SLOTS=4 (PRIMARY reference, DEC's own validated N=4 config):
total_cycles=74038, PASS 256/256 neurons bit-exact vs golden.
sustained_MAC/cycle=0.4426 (vs 0.1773 @16-bit baseline, EXP-0030).
Original 16-bit port utilization: 4.4% (3222/74038) -- activation+
write-back traffic ALONE, now essentially idle, confirms weight
traffic (previously dominant on the shared port) is now entirely
on the separate dual-chip path.
REAL SPEEDUP vs 16-bit baseline (184771 cycles, EXP-0030):
184771/74038 = 2.496x.
N_SLOTS=2 (sensitivity): total_cycles=75676, PASS 256/256 bit-exact.
sustained_MAC/cycle=0.4330. REAL SPEEDUP vs 16-bit baseline
(185270-185645 cycles range): ~2.449-2.454x.
N=2 and N=4 give statistically similar total cycles (75676 vs
74038, within 2.2%) -- CONFIRMS (again, now for the real 32-bit
architecture too) that N_SLOTS does not change the port-bound
ceiling; the same real, physical weight-fetch port remains the
aggregate bottleneck regardless of slot count.
IMPORTANT: this REAL, independently-measured speedup (2.45-2.50x)
SUBSTANTIALLY EXCEEDS the STEP15 (prior round)'s own DERIVED
projection (1.89x, EXP-0036). Investigated, not silently accepted:
the DERIVED projection calibrated a single "degradation factor"
(2.65x) from the OLD, single-shared-port 16-bit system, where
weight, activation, and result-write-back traffic all contended for
the SAME physical port -- and implicitly assumed that SAME
degradation factor would persist after widening. The ACTUAL,
concrete architecture built and validated here gives weight fetch
its OWN, physically SEPARATE port (via the new dual-chip interface)
-- REMOVING cross-traffic-type contention entirely, not merely
widening the shared bus. This is a real, structural, additional
benefit the single-degradation-factor projection could not capture
by construction, and explains the entire gap between 1.89x
(projected) and 2.496x (measured).
classification: RTL SIMULATION (real V1 PSRAM timing chains, full
real system, bit-exact). Synthesis/P&R pending (EXP-0039).
decision: the real, measured 2.496x (N=4) speedup is adopted as the
authoritative end-to-end throughput result, SUPERSEDING EXP-0036's
own DERIVED 1.89x projection for this specific comparison (N=4,
16-bit vs 32-bit dual-chip). EXP-0036's own methodology/caveat
remains a valid, honest account of what it assumed and did not
measure -- this entry documents why reality exceeded it.
EXP-0039
timestamp: 2026-09-06T06:45:00Z
module: nms_neural_multiprocessor_dual32.v, real full synthesis+P&R
for the ACTUAL target: LFE5U-45F-8CABGA381.
command (synth): yosys -p "read_verilog -sv <full real dependency set
incl. slot_mem_arbiter.v (unmodified) + slot_mem_arbiter_wide.v
(NEW) + psram_controller_dual32.v (NEW) + nms_dataflow_core_dual32.v
+ nms_neural_multiprocessor_dual32.v>; chparam -set N_SLOTS 4 -set
MAX_TILES 16 -set PREFETCH_DISTANCE 8
nms_neural_multiprocessor_dual32; synth_ecp5 ..."
command (P&R): nextpnr-ecp5 --45k --package CABGA381 --speed 8
--freq 80 --json top.json --lpf-allow-unconstrained --textcfg
top.config
result (POST-SYNTH + POST-P&R MEASURED):
FIRST ATTEMPT (separate psram0_*/psram1_* address+control, 90 pins
for the weight interface alone): P&R FAILED -- "Unable to place
cell 'psram0_a[0]$tr_io', no BELs remaining to implement cell type
TRELLIS_IO". Real, exact I/O budget discovered (not assumed): this
package provides 245 total TRELLIS_IO; the EXISTING design (real
registration interface + the original single-chip 16-bit PSRAM
path) already commits 157 of them (confirmed from a prior
successful STEP14 build's own nextpnr utilisation report), leaving
88 free -- 2 pins short of the 90 the first-draft dual-chip
interface needed.
FIX (real, valid PCB technique, not a synthesis trick): chip0 and
chip1's own address/control (CE#/OE#/WE#/LB#/UB#/ZZ#) outputs are,
by construction, byte-for-byte identical every cycle (EXP-0037's own
synchronization argument) -- shared them to ONE set of top-level
pins (psram01_a/ce_n/oe_n/we_n/lb_n/ub_n/zz_n), keeping only DQ
(genuinely independent, bidirectional per-chip data) separate.
Reduces the weight-interface pin requirement from 90 to 61
(23+6+16+16), matching this STEP's own original architectural
estimate exactly.
SECOND ATTEMPT (pin-shared): P&R SUCCEEDED.
TRELLIS_IO: 218/245 (88.9%) -- fits, with only 27 spare pins
remaining (a real, tight constraint worth flagging for board
planning -- see the STEP15 report's own I/O section).
Real Fmax (final, post-route-optimization value -- nextpnr reports
an earlier, lower preliminary estimate first (95.99MHz) and a
later, final, HIGHER value after further optimization passes,
same pattern as every prior synthesis run in this project):
**110.28 MHz**, PASS at 80MHz target -- HIGHER than the STEP14
baseline's own 106.81MHz (+3.2%), not lower as might have been
assumed for a design with MORE real logic (arbiter + 2 extra
controller instances).
Resources: LUT4=2893 (vs 2776 @ STEP14 baseline, +4.2%),
CCU2C=721 (vs 705, +2.3%), FF=6273 (vs 5957, +5.3%), DSP=32
(unchanged), DP16KD=0 (unchanged). All modest, expected increases
from the added weight-path arbitration + duplicated real
controller logic -- no disproportionate blowup.
Bit-exact regression (tb_nms_dstress_dual32.v) re-confirmed
UNCHANGED (74038 cycles, PASS 256/256) after the pin-sharing
refactor, as expected (pure port-list/wiring change at the pad
level, zero functional difference).
classification: POST-SYNTH (resources), POST-P&R MEASURED (Fmax,
real I/O placement feasibility) -- the ACTUAL target device
(LFE5U-45F-8CABGA381), not a reduced/generic target.
decision: the pin-shared dual32 architecture (nms_neural_
multiprocessor_dual32.v, final version) is VALIDATED at the
synthesis+P&R level: real Fmax 110.28MHz (PASS, actually exceeding
the 106.81MHz baseline), real bit-exact correctness preserved,
real I/O feasibility confirmed (218/245 TRELLIS_IO, fits with 27
pins of headroom remaining). See DEC-0030 for the full STEP15
executive conclusion.
EXP-0040
timestamp: 2026-09-06T06:08:18Z
step: STEP16 Phase 1-3 -- AS4C4M16SA-6TIN SDR SDRAM controller,
isolated correctness regression.
classification: RTL SIMULATION (Verilator 5.050), real timing-checked
behavioral model (sdram_model.v), real target parameters
(CLK_FREQ_MHZ=166, BURST_LEN=4, ADDR_WIDTH=22).
phase1_findings (repository analysis, before writing any RTL): the
existing psram_controller.v's mem_req/mem_wr/mem_addr/mem_wdata/
mem_rdata/mem_ready protocol was reused unmodified as the external
interface convention for the new sdram_controller.v, generalized to
a burst-oriented transaction (one req = one full BURST_LEN-word
transfer) since that matches the real per-tile weight access
granularity better than a word-at-a-time protocol. P_IN*DATA_WIDTH=
64 bits = 4 x16-bit words/tile -- an exact natural match to
BURST_LEN=4, identified before any RTL was written.
architecture: sdram_controller.v implements real power-up (200us wait,
PRECHARGE ALL, 8x AUTO REFRESH, LOAD MODE REGISTER), periodic
AUTO REFRESH taking priority over pending req in S_IDLE, and ALWAYS
uses auto-precharge (A10=1) on every READ/WRITE -- an explicit
correctness-first design choice (no per-row open-state tracking, one
code path regardless of address history), trading page-hit
performance for structural simplicity per the governing spec's own
stated priority order (correctness > reliability > timing >
performance > ...).
bugs_found_and_fixed: three real, reproducible bugs found via this
regression (not by inspection alone) -- see ERR-0016 (A10
auto-precharge bit misplaced at a[8] instead of a[10] in
sdram_controller.v, causing every bank to stay open forever), ERR-0017
(sdram_model.v silently dropped every write burst's first word --
one-cycle-late capture relative to real SDR SDRAM's command-concurrent
first-word timing), ERR-0018 (sdram_model.v read path had a matching
one-cycle-late pipe insertion PLUS a redundant registered output
stage, compounding to a 2-cycle-late read-data corruption). All three
root-caused via cycle-exact hand tracing of the FSM against the
model's own real timing-violation messages and the observed
got-vs-expected data-shift patterns, not by adjusting expected values
to match observed output.
result: after all three fixes, tb_sdram_controller.v (BURST_LEN=4,
CLK_FREQ_MHZ=166) -- 460/460 tests PASS, 0 errors. Covers scenario A
(write->read single), B (16 sequential addresses), E (row change,
same bank), F (bank change, all 4 banks), I (address limits: row
0/4095, bank 0/3), H (32 pseudo-random addresses), and G (400
back-to-back transactions spanning >1 real tREFI interval, confirming
correct AUTO REFRESH interleaving with zero data loss/corruption).
Measured cycles/transaction: 11 cycles per BURST_LEN=4 read-or-write
(bit-exact write+read round trip verified via check_word, each
individual transaction taking 11 cycles: ACTIVATE wait (T_RCD=3) +
CAS_LATENCY(3) + burst(4) + PRECHARGE(T_RP=3), consistent with the
real timing parameters at 166MHz).
note: a benign AUTO REFRESH spacing WARNING (2606 vs tREFI=2594 cycles,
0.5% over) was observed once during Test G -- traced to the
controller correctly finishing an in-flight transaction before
servicing a pending refresh (a real, expected consequence of a
single-outstanding-refresh design, not data corruption) -- explicitly
NOT silently dismissed, flagged here for the record and for
consideration in the Phase 7 comparison/risk section.
next: BURST_LEN=1 and BURST_LEN=8 regressions (Phase 3 completion),
then Phase 4's real cycle/throughput measurement sweep at
100/133/166MHz.
EXP-0041
timestamp: 2026-09-06T06:08:18Z
step: STEP16 Phase 3 completion (all BURST_LEN) + Phase 4 real
cycle/throughput measurement sweep.
classification: RTL SIMULATION (Verilator 5.050), real timing-checked
behavioral model, 9 real builds/runs (CLK_FREQ_MHZ in {100,133,166}
x BURST_LEN in {1,4,8}), each the full tb_sdram_controller.v Phase 3
suite (A/B/E/F/H/I/G, 460 checks per run).
result: 9/9 configurations PASS 460/460, 0 errors (after fixing
ERR-0019, which this exact sweep exposed). Phase 3 is now fully
closed for all three required BURST_LEN values at the real 166MHz
target frequency, and additionally cross-validated at 100/133MHz.
measured_cycles_per_transaction (real RTL simulation, not estimated --
one full ACTIVATE->CAS->burst->PRECHARGE round trip, steady state):
| CLK_FREQ_MHZ | BURST_LEN=1 | BURST_LEN=4 | BURST_LEN=8 |
|---|---|---|---|
| 100 | 7 cyc | 10 cyc | 14 cyc |
| 133 | 8 cyc | 11 cyc | 15 cyc |
| 166 | 8 cyc | 11 cyc | 15 cyc |
(100->133/166 step reflects T_RCD/T_RP's own real ns_to_cycles
re-derivation: 2 cycles @100MHz vs 3 cycles @133/166MHz for the same
18ns requirement -- a real, re-derived-per-frequency timing
parameter, not a fixed/hardcoded value, per the module's own header
comment. CAS_LATENCY is fixed at 3 cycles across all three
frequencies, matching the AS4C4M16SA-6TIN's own fixed CL=3 spec
rating -- no attempt was made to model a lower CAS_LATENCY the real
part could technically use at 100/133MHz, since -6 speed grade parts
are commonly operated at a single fixed CL setting in practice and
the governing spec did not ask for a CL sweep.)
derived_bandwidth (DERIVED from the measured cycle counts above --
bytes_per_txn = BURST_LEN*2; time_ns = cycles*(1000/CLK_FREQ_MHZ);
MB/s = bytes_per_txn / time_ns * 1000, decimal MB=1e6 bytes,
matching this project's own STEP15 convention):
| CLK_FREQ_MHZ | BURST_LEN | nominal BW (2B x F) | measured single-txn BW | %util |
|---|---|---|---|---|
| 100 | 1 | 200.0 MB/s | 28.57 MB/s | 14.3% |
| 100 | 4 | 200.0 MB/s | 80.00 MB/s | 40.0% |
| 100 | 8 | 200.0 MB/s | 114.29 MB/s | 57.1% |
| 133 | 1 | 266.0 MB/s | 33.25 MB/s | 12.5% |
| 133 | 4 | 266.0 MB/s | 96.72 MB/s | 36.4% |
| 133 | 8 | 266.0 MB/s | 141.86 MB/s | 53.3% |
| 166 | 1 | 332.0 MB/s | 41.50 MB/s | 12.5% |
| 166 | 4 | 332.0 MB/s | 120.72 MB/s | 36.4% |
| 166 | 8 | 332.0 MB/s | 177.06 MB/s | 53.3% |
This is ISOLATED single-transaction bandwidth (one burst, back to
back with no other traffic) -- NOT yet the real N=4 arbitrated
system bandwidth (that requires Phase 5's real datapath integration,
logged separately). Larger BURST_LEN amortizes the fixed
ACTIVATE+CAS+PRECHARGE overhead over more data words, raising
%util -- exactly the "auto-precharge always closes the row" design
trade-off the module's own header comment predicted, now confirmed
with real numbers rather than assumed.
BURST_LEN=4 @ 166MHz (the natural per-tile granularity identified in
Phase 1, P_IN*DATA_WIDTH/16=4 words/tile) is the configuration
carried forward into Phase 5: 11 cycles/tile, 120.72 MB/s isolated
bandwidth, 36.4% of the x16 bus's own 332MB/s nominal ceiling.
overhead_breakdown (BURST_LEN=4 @ 166MHz, real, not estimated): of the
11 total cycles/transaction, 3 are ACTIVATE-to-CAS wait (T_RCD), 3
are CAS latency, 4 are the actual burst data cycles, and the
PRECHARGE wait (T_RP=3 cycles) overlaps the NEXT transaction's own
ACTIVATE-wait window rather than adding fully serially (confirmed by
the measured 11 cycles being less than the naive
T_RCD+CAS_LATENCY+BURST_LEN+T_RP=3+3+4+3=13 sum) -- i.e. only 4/11
cycles (36.4%) are real data transfer, matching the %util figure
above exactly (as it must, by construction).
next: Phase 5 -- real FPGA-Neural datapath integration (weight fetch
pattern, N=2/N=4 bit-exact, real arbitrated bandwidth) using
BURST_LEN=4 @ 166MHz as the carried-forward configuration.
EXP-0042
timestamp: 2026-09-06T06:08:18Z
step: STEP16 Phase 5 -- real FPGA-Neural datapath integration (real
weight access pattern, real D-Stress workload, N=2 and N=4).
classification: RTL SIMULATION (Verilator 5.050), full real system:
nms_neural_multiprocessor_sdram.v (forked from the validated dual32
baseline, ONLY the wide weight-fetch backend replaced) ->
nms_dataflow_core_sdram.v (MEM_DATA_WIDTH=64, WORDS_PER_TILE=1,
natural one-burst-per-tile match) -> slot_mem_arbiter_wide.v (reused
UNCHANGED at DATA_WIDTH=64) -> sdram_weight_backend.v -> the real,
isolated-and-validated sdram_controller.v (BURST_LEN=4) -> the real
timing-checked sdram_model.v. Activation+result-writeback path
(memory_interface.v -> psram_controller.v, single 16-bit chip) is
BYTE-FOR-BYTE UNCHANGED from the dual32 baseline -- per the governing
spec's own "do not create an artificial benchmark" instruction, only
the piece under test (weight-fetch physical memory) differs. Same
clock (CLK_FREQ_MHZ=80, CLK_PERIOD=12.5ns) as the dual32 baseline's
own functional-simulation testbench, for a direct cycle-count-based
apples-to-apples comparison (STEP15's own reported 2.496x speedup was
itself computed at this same 80MHz functional clock, not the P&R
Fmax -- matched here deliberately).
workload: D-Stress (256 independent neurons, 128 inputs each, MAX_TILES
=16 tiles/neuron, dense-layer shape), the SAME real workload/golden-
model/bit-exact-verification methodology as tb_nms_dstress_dual32.v.
bugs_found_and_fixed: ONE real, reproducible full-system deadlock at
N_SLOTS_CFG=2 (see ERR-0020) -- N=4 passed on the first real run, but
N=2 hung at 228/256 neurons, root-caused via hierarchical debug
tracing to a gap in ERR-0019's own fix (only covered req arriving
during S_IDLE, not during any other busy state) and fixed by
latching req unconditionally every cycle regardless of controller
state. Re-confirmed the full Phase 3 regression (9/9 configs, 460/460
each) still passes unchanged after this fix.
result (N=4, N_SLOTS_CFG=4, PFD_CFG=8):
- PASS: all 256 neurons bit-exact vs the golden software model.
- total_cycles = 49430 (vs dual32 baseline's own 74038, vs the
original 16-bit baseline's own 184771)
- tiles_delivered = 4096 (256 neurons x 16 tiles, matches exactly)
- cycles/tile = 12.07 (DERIVED: total_cycles/tiles_delivered)
- sustained MAC/cycle = 0.6629 (real tiles*P_IN/total_cycles)
- compute utilization = 0.6629/32 = 2.072% of the N=4 theoretical
32 MAC/cycle ceiling (vs dual32's own reported 1.383% -- HIGHER,
i.e. measurably LESS memory-bound, consistent with fewer total
cycles for identical real work)
- effective bandwidth (DERIVED: tiles*8 bytes / (total_cycles x
12.5ns), decimal MB=1e6 convention matching STEP15's own): 32768
bytes / 617875ns = 53.04 MB/s (vs dual32's own reported 35.41 MB/s
N=4 effective bandwidth)
- speedup vs ORIGINAL 16-bit baseline: 184771/49430 = 3.738x
- speedup vs dual32 32-bit baseline: 74038/49430 = 1.498x
result (N=2, N_SLOTS_CFG=2, PFD_CFG=8):
- PASS (after ERR-0020's fix): all 256 neurons bit-exact.
- total_cycles = 52161 (vs dual32's own 75676, vs original 185270)
- tiles_delivered = 4096, cycles/tile = 12.73
- sustained MAC/cycle = 0.6282, compute utilization = 0.6282/16 =
3.926% of the N=2 theoretical 16 MAC/cycle ceiling
- effective bandwidth: 32768 bytes / 652012.5ns = 50.26 MB/s
- speedup vs original 16-bit baseline: 185270/52161 = 3.552x
- speedup vs dual32 baseline: 75676/52161 = 1.451x
note: N=2 and N=4 give similar cycle counts (52161 vs 49430, within
5.5%) -- same N_SLOTS-insensitivity to the port-bound ceiling STEP15
itself already found for the dual32 architecture, now confirmed for
the single-chip SDRAM architecture too (weight-fetch bandwidth, not
slot count, remains the limiting resource in both architectures).
shared (16-bit, activation+writeback) PSRAM port utilization stayed
low in both runs (6.5% at N=4, 6.2% at N=2), confirming this path
remains a non-bottleneck exactly as STEP15 established -- unaffected
by the weight-fetch backend swap, as expected since it is unchanged.
next: Phase 6 -- real synthesis (Yosys) + real place & route
(nextpnr-ecp5) for the actual LFE5U-45F-8CABGA381 target, measuring
Fmax/LUT/FF/EBR/DSP/I-O and verifying real package I/O feasibility.
EXP-0043
timestamp: 2026-09-06T06:08:18Z
step: STEP16 Phase 6 -- real synthesis (Yosys) + real place & route
(nextpnr-ecp5) for the actual target LFE5U-45F-8CABGA381, full
nms_neural_multiprocessor_sdram.v system (dependency_manager +
neural_director + N_SLOTS x (memory_manager + neural_processor) +
activation replicated/fill_ctrl + weight_packed + the real SDRAM
weight-fetch backend + the real single-chip 16-bit PSRAM activation/
writeback path), matching the exact real hierarchy validated in
Phase 5 (EXP-0042).
classification: POST-SYNTH (Yosys 0.68+, synth_ecp5) + POST-P&R
(nextpnr-ecp5 0.11.1-19-g8dbcee5c, --45k --package CABGA381,
--lpf-allow-unconstrained -- i.e. free real-package I/O placement,
no hand-built board-specific LPF, same methodology this project's own
prior nms_multiproc synthesis logs used and the same limitation
STEP15's own dual32 report explicitly flagged: "bank-by-bank
assignment...flagged as the concrete next step" -- not repeated here
as a NEW gap, inherited unchanged from the established baseline
methodology).
methodology_note: synthesized and P&R'd at BOTH N_SLOTS=2 and N_SLOTS=4
(via `hierarchy -chparam N_SLOTS <n>`) since the STEP15 report itself
did not specify which N_SLOTS its own P&R figures came from, and this
round's own N=4 TRELLIS_FF count (6215) turned out to sit within ~1%
of the dual32 report's own cited FF figure (6273) -- strongly
suggesting DEC-0030's own P&R was ALSO an N=4 configuration, so N=4
is treated as the primary comparison point below (matching the
governing STEP16 spec's own "N=4 as primary configuration" framing),
with N=2 reported alongside for completeness.
resources (N=4, real nextpnr-ecp5 post-P&R device utilisation, not
yosys pre-map estimates):
TRELLIS_IO=194/245 (79%), TRELLIS_FF=6215/43848 (14%),
TRELLIS_COMB=5516/43848 (12%), MULT18X18D=32/72 (44%),
DP16KD(EBR)=0/108 (0%), TRELLIS_RAMW=173/5481 (3%).
resources (N=2): TRELLIS_IO=194/245 (79%, IDENTICAL to N=4 -- I/O count
is fixed by the external port list, independent of N_SLOTS, as
expected), TRELLIS_FF=3724/43848 (8%), TRELLIS_COMB=3783/43848 (8%),
MULT18X18D=16/72 (22%), DP16KD=0, TRELLIS_RAMW=109/5481 (2%).
io_comparison_vs_dual32_baseline: 194/245 (79.2%) for THIS design vs
the dual32 baseline's own reported 218/245 (88.9%) -- a real,
measured 24-pin SAVINGS, matching this design's own real single-chip
SDRAM weight interface (2 BA + 12 A + 6 control (CKE/CS#/RAS#/CAS#/
WE#) + 2 DQM + 16 DQ = 38 pins... actual measured delta is 24 pins,
consistent with a single-chip interface replacing dual32's own
61-pin two-chip interface) -- confirms the design's own physical I/O
feasibility on the real package with MORE headroom than the already-
validated dual32 baseline, not less.
timing (real nextpnr-ecp5 Fmax, best-of-3-seeds at N=4, single seed at
N=2, all at the SAME 80MHz operating point the Phase 5 cycle-count
benchmark itself assumed): N=4: seed1=80.15MHz, seed2=74.88MHz (FAIL
at 80MHz target for that specific seed), seed3=81.55MHz -- BEST
achieved (and the representative figure carried forward, matching
this project's own established "final, best achieved" reporting
convention): 81.55 MHz, PASS at 80MHz. N=2 (single seed): 94.32 MHz,
PASS at 80MHz. BOTH configurations close real timing at the 80MHz
operating point the Phase 5 benchmark used -- but BOTH sit clearly
BELOW the dual32 baseline's own reported 110.28MHz. The critical
path in EVERY run traced entirely to dependency_manager.v's own
reg_ready/reg_valid/node_state combinational registration-handshake
chain -- a module completely UNCHANGED from the dual32 baseline, NOT
any part of the new SDRAM controller/backend logic itself. The exact
cause of the Fmax gap vs the dual32 baseline's own reported number is
NOT fully explained by this round's own investigation (seed variance
alone spans 74.9-81.6MHz at N=4, real but insufficient to close a
~29MHz gap to 110.28MHz) -- reported honestly as an open, unresolved
discrepancy rather than a fabricated explanation, per the governing
spec's own "if something cannot be measured, state so explicitly"
instruction.
correctness: no gate-level/post-P&R re-simulation was performed (timing
closure and RTL bit-exact correctness were validated as SEPARATE,
non-overlapping checks -- the same methodology the dual32 baseline's
own STEP15 validation used).
EXP-0044
timestamp: 2026-09-06T10:22:22Z
git_commit: 5c9ec618d354c6933cc6fe26820a62216ca7f2a7 (working tree dirty:
hardware/v2/nms/, hardware/v2/reports/ untracked -- STEP16/17 own new
files, not yet committed per "never commit unless asked")
step: STEP17 Part A -- real N=4 timing-closure investigation. Tools:
Yosys 0.68+post (git c12172fbae8), nextpnr-ecp5 0.11.1-19-g8dbcee5c.
classification: POST-SYNTHESIS + POST-P&R, real target
LFE5U-45F-8CABGA381 (--45k --package CABGA381
--lpf-allow-unconstrained, same methodology as STEP16/EXP-0043).
commands: `yosys -q -l yosys.log synth.ys` (synth_ecp5 -top
nms_neural_multiprocessor_sdram / nms_neural_multiprocessor_dual32,
hierarchy -chparam N_SLOTS <2|4>) then `nextpnr-ecp5 --json top.json
--45k --package CABGA381 --freq 80 --seed <1|2|3>
--lpf-allow-unconstrained --textcfg ... --log ...`.
method: re-synthesized BOTH the SDRAM design (already built in STEP16)
AND the dual-PSRAM baseline (freshly re-synthesized this round, since
STEP16's own report lacked path-level detail for a fair comparison)
at BOTH N_SLOTS=2 and N_SLOTS=4, each across 3 nextpnr random seeds,
to separate real seed-to-seed variance from genuine architectural
difference. Full results: step17_timing_seeds.csv.
result:
SDRAM N=2: 102.29/94.32/103.99 MHz (best 103.99)
SDRAM N=4: 80.15/74.88(FAIL)/81.55 MHz (best 81.55)
Dual-PSRAM N=2: 100.48/103.52/101.37 MHz (best 103.52)
Dual-PSRAM N=4: 84.68/77.72(FAIL)/87.26 MHz (best 87.26)
KEY FINDING: dual-PSRAM's own STEP15/DEC-0030 reported Fmax
(110.28MHz) does NOT reproduce with this toolchain/seed/methodology
for the SAME, unmodified dual32 RTL at N=4 -- best achieved here is
87.26MHz. This means the STEP16 report's own "110.28 -> 81.55MHz,
~26% gap" comparison was NOT apples-to-apples; the real,
consistent-methodology gap is ~7% (81.55 vs 87.26MHz).
critical_path_analysis: SDRAM N=4 (seed3, 81.55MHz) critical path is
entirely inside dependency_manager.v's own first_ready_idx priority-
encoder scan (lines 100-112) feeding directly into four wide array
reads (node_x_base/node_w_base/node_n_tiles/node_result_addr, lines
183-187) -- 3.01ns logic + 9.26ns routing (75% routing-dominated).
Dual-PSRAM N=4 (seed3, 87.26MHz) critical path is instead inside
nms_memory_manager_stream_wide.v's own buf_valid/issue_rd_now read-
issue combinational chain -- 3.41ns logic + 8.05ns routing. BOTH
paths sit in modules completely UNCHANGED between the two
architectures. Interpretation: the N=4 Fmax ceiling is primarily an
N-SCALING effect of shared control/arbitration logic fan-out
(confirmed by both architectures' large N=2->N=4 Fmax drop: SDRAM
-21.6%, dual-PSRAM -15.7%), with SDRAM's own added logic providing a
smaller secondary placement-congestion effect on top of the shared
bottleneck (SDRAM's own drop is somewhat larger than dual-PSRAM's).
Neither architecture's critical path involves its own external-
memory controller (sdram_controller.v / psram_controller.v) at all.
decision: no RTL change is warranted purely for Fmax -- N=4 already
meets the governing spec's own hard minimum (>=80MHz) on the
unmodified, STEP16-validated RTL (81.55MHz best-of-3-seeds). See
ERR-0021 for a real, reverted attempt at a minimal fix.
EXP-0045
timestamp: 2026-09-06T10:22:22Z
git_commit: 5c9ec618d354c6933cc6fe26820a62216ca7f2a7
step: STEP17 Parts B/C -- real cycle-decomposition and SDRAM
effectiveness measurement for the N=4 (and N=2) D-Stress benchmark.
classification: INTEGRATED BENCHMARK (Verilator 5.050, the real,
STEP16-validated nms_neural_multiprocessor_sdram.v system, unchanged
RTL, real D-Stress workload, bit-exact vs golden), plus DERIVED
percentages/ratios computed from those real counts. New testbench-
only instrumentation added to tb_nms_dstress_sdram.v (no RTL
touched): per-cycle active-slot-count histogram, per-cycle useful-
tile-delivery accumulation, startup/drain cycle boundaries, and real
signal tracing on u_sdram_backend.u_sdram_ctrl (req/ready/wr/busy/
state) for transaction counts, busy-cycle fraction, refresh-event
count, and req-to-ready latency (min/max/avg).
commands: `verilator --binary --timing -GN_SLOTS_CFG=<2|4> -GPFD_CFG=8
--top-module tb -Mdir <dir> sim/tb_nms_dstress_sdram.v <rtl deps>`,
then run the resulting Vtb binary.
result (N=4, 49430 total cycles, 4096 tiles, 256/256 bit-exact PASS):
slot-cycle budget = 4*49430 = 197720. useful_mac_cycles=4078 (2.06%).
weight_stall_cycles=179756 (90.91%, pre-existing STEP11
instrumentation, reused unchanged). per-slot idle (sum)=1772 (0.90%).
unaccounted residual=12114 (6.13%) -- NOT further subdivided this
round, explicitly disclosed rather than guessed (plausibly
activation-wait + pipeline/tile-boundary bubbles + dispatch
overhead, per the governing spec's own "if a category cannot be
separated reliably, state that explicitly" instruction). startup=56
cycles, drain=32 cycles (both <0.2% of total, negligible).
Active-slot-count histogram: 0 active 0.03%, 1 active 0.03%, 2 active
0.66%, 3 active 2.07%, 4 active 97.22% -- nearly always "active"
(mm.state!=IDLE) despite only 2.06% of slot-cycles being USEFUL
(delivering a tile), confirming "active" (FSM not idle) and "useful"
(real MAC progress) are very different things here.
SDRAM controller (single physical chip, all weight traffic): 4096
read transactions, 0 writes (as designed, weight fetch is read-
only), busy 49392/49430 cycles (99.92%), 40 real AUTO REFRESH
commands issued, request latency min=10/max=16/avg=10.06 cycles
(matching the isolated Phase-4/6 single-transaction cost of 10
cycles almost exactly -- confirms near-zero extra arbitration queuing
on average). Sustained bandwidth (DERIVED): 53.04 MB/s of 160.0 MB/s
nominal (33.2% utilization).
result (N=2, 52161 total cycles): useful_mac=4080/104322 (3.91%),
weight_stall=86464 (82.88%), per-slot idle=1072 (1.03%), unaccounted
residual=12706 (12.18%). SDRAM busy 49404/52161 (94.71%), 42 refresh
events, same 10.06-cycle avg latency. Sustained bandwidth: 50.26MB/s
(31.4% of nominal).
interpretation: system is MEMORY-BANDWIDTH-BOUND at both N=2 and N=4
(controller busy 94.71%/99.92%, latency at its own fixed minimum,
not latency-bound; negligible extra arbitration queuing, not
primarily arbitration-bound). Compute utilization (sustained/
theoretical peak) is 3.93% at N=2, 2.07% at N=4 -- DROPS at N=4
because total cycles barely improve (52161->49430, -5.5%) while
theoretical peak DOUBLES (16->32 MAC/cycle) -- the architecture
cannot yet convert added compute parallelism into proportional
throughput because the shared SDRAM port is already the binding
constraint, confirming STEP15's own prior finding (N_SLOTS does not
change the port-bound ceiling) now holds for the SDRAM architecture
too, with real, freshly-measured numbers.
next: roofline update (Part D) and final report -- see
hardware/v2/reports/step17_n4_timing_throughput.md,
step17_cycle_decomposition.csv, step17_sdram_effectiveness.csv.
EXP-0046
timestamp: 2026-09-06T10:45:21Z
git_commit: 5c9ec618d354c6933cc6fe26820a62216ca7f2a7
step: STEP18 Part C -- SDRAM weight-packing experiment (2 tiles/real
transaction via BURST_LEN=8). First draft (1-entry cache): real
regression, see ERR-0022. This entry covers the ACCEPTED, fixed
version (N_ENTRIES=4).
classification: RTL SIMULATION (isolated unit regression,
tb_sdram_weight_backend_pack128.v, 20/20 PASS, real sdram_model.v)
+ INTEGRATED BENCHMARK (full N=2/N=4 D-Stress via tb_nms_dstress_
sdram_pack128.v, real nms_neural_multiprocessor_sdram_pack128.v).
method: new module sdram_weight_backend_pack128.v presents the
IDENTICAL external 64-bit mem_req/mem_addr/mem_rdata/mem_ready
contract as STEP16's own sdram_weight_backend.v (weight_prefetch_
engine_wide.v and neural_processor.v UNCHANGED) but internally uses
sdram_controller.v with BURST_LEN=8 (already protocol-validated in
STEP16 Phase 3, 460/460 tests, reused unmodified) and a 4-entry
fully-associative address-tagged cache holding the "other half" of
each real 128-bit fetch, round-robin-allocated (safe under any
sizing: an evicted-too-early entry only costs an extra real fetch,
never incorrect data, since a cache MISS always falls back to a
real, address-exact fetch).
result: isolated regression 20/20 PASS (sequential access: real fetch
16 cycles then cache-hit 1 cycle, alternating; non-sequential/odd-
first access: correct fallback; address-limit pattern: correct).
Full D-Stress: N=4 44,935 cycles (-9.1% vs STEP16/17's own 49,430
baseline), N=2 47,399 cycles (-9.1% vs 52,161 baseline), BOTH
256/256 bit-exact vs golden. Sustained bandwidth (DERIVED): N=4
58.34 MB/s (36.5% of 160MB/s nominal, up from 33.2%); N=2 55.30 MB/s
(34.6%, up from 31.4%). Sustained MAC/cycle: N=4 0.7292 (+10.0% vs
0.6629), N=2 0.6913 (+10.0% vs 0.6282).
synthesis/pnr: Yosys 0.68+post + nextpnr-ecp5 0.11.1-19-g8dbcee5c,
--45k --package CABGA381 --lpf-allow-unconstrained, N=4: TRELLIS_IO
194/245 (unchanged), TRELLIS_FF 6483 (+4.3% vs 6215), TRELLIS_COMB
6106 (+10.7% vs 5516), MULT18X18D 32 (unchanged), DP16KD 0
(unchanged). Fmax best-of-3-seeds: 78.89(FAIL)/80.15/81.47 MHz ->
best 81.47MHz, PASS at 80MHz, essentially unchanged vs the STEP17
baseline's own 81.55MHz. N=2 (single seed only): 86.04MHz, PASS.
decision: ACCEPT as the new N=4 V2 weight-fetch backend. All STEP18
decision criteria met (bit-exact, no deadlock/timeout/dropped jobs,
protocol correct, Fmax>=80MHz, cycles improve, sustained MAC/cycle
improves, memory efficiency improves, no processor serialization).
See DEC-0033.
EXP-0047
timestamp: 2026-09-06T10:45:21Z
git_commit: 5c9ec618d354c6933cc6fe26820a62216ca7f2a7
step: STEP18 Parts A/B/D/E/F/G/I -- reframing and targeted extension
of existing STEP16/17 measurements into the required THEORETICAL ->
CONTROLLER MAX -> REALISTIC SUSTAINABLE bandwidth ladder, plus a few
new structural findings not previously stated explicitly.
classification: DERIVED (reframing of already-classified STEP16/17
data) + RTL SIMULATION (new: isolated pack128 controller-max
measurement, single requester, back-to-back: 17 cycles/16 bytes =
~114.3 MB/s @80MHz, vs the baseline's own 64.0 MB/s isolated max).
key_findings:
(1) Part B's own working hypothesis ("multiple transactions per P8
tile") is REFUTED by direct inspection: MEM_DATA_WIDTH=64 in
nms_dataflow_core_sdram.v (STEP16) already makes WORDS_PER_TILE=1,
and sdram_req_count=4096 exactly equals tiles_delivered=4096
(STEP17 EXP-0045) -- one tile already costs exactly one transaction.
(2) The 10-cycle (BURST_LEN=4) / 16-cycle (BURST_LEN=8) transaction
cost is dominated by FIXED row-open/row-close overhead (always-
precharge design, STEP16): 6 of 10 cycles (60%) at BURST_LEN=4 are
overhead, independent of burst length or physical bus width -- a
32/64-bit physical bus with the same always-precharge design would
show the identical overhead RATIO, just fewer transactions for the
same total bytes.
(3) Row locality (Patterns E/F) provides ZERO benefit BY
CONSTRUCTION -- confirmed structurally from sdram_controller.v's own
FSM (no row ever stays open across transactions, no conditional path
exists that could make same-row vs different-row access differ) --
not re-benchmarked, since the RTL itself rules out any difference.
(4) Refresh (Pattern G) costs ~0.8% of total cycles (40 events x~10
cycles / 49430 total, STEP17 data) -- not a meaningful factor.
(5) Activation traffic (Part G) is confirmed <=7.2% of total cycles
in every configuration measured (STEP15/16/17/18) -- weight traffic
dominates external memory activity by a wide margin.
(6) N2/N4 scaling (Part I): packing improves N=2 and N=4 by an
IDENTICAL 9.1% -- it is a pure memory-side win independent of slot
count, and does not change the underlying N2-vs-N4 relative gap
(5.2% before and after), confirming the shared SDRAM port remains
the binding resource for both configurations.
decision: no new isolated SDRAM pattern tests were built for Patterns
A-D/G (already covered by STEP16 Phase 3/4 and STEP17's own
instrumentation) or E/F (structurally ruled out, not requiring
simulation) -- reusing prior real measurements is preferred over
re-deriving identical numbers, per the project's own "don't repeat
work that already produced a real, classified answer" practice.
EXP-0048
timestamp: 2026-09-06T11:26:46Z
git_commit: 5c9ec618d354c6933cc6fe26820a62216ca7f2a7 (pre-STEP19 commit;
this experiment's own changes are staged for the STEP19 freeze commit)
step: STEP19 -- SINGLE external SDRAM hardware freeze. Removed the V1
PSRAM dependency (memory_interface.v + psram_controller.v) from the
V2 physical path entirely. Weights, activations, AND results now all
share ONE physical AS4C4M16SA-6TIN SDRAM chip through ONE real
sdram_controller.v instance (BURST_LEN=8), via a new sdram_unified_
backend.v presenting two logical ports (W: 64-bit weight fetch,
reusing the STEP18 pack128 cache unchanged; AR: 16-bit byte-
maskable, activation-fill + result-writeback, replacing the real V1
psram_controller.v exactly).
classification: RTL SIMULATION (new isolated unit test,
tb_sdram_unified_backend.v, 40/40 PASS after ERR-0023's fix) +
INTEGRATED BENCHMARK (real D-Stress via tb_nms_dstress_sdram_
unified.v) + POST-SYNTHESIS + POST-P&R (real LFE5U-45F-8CABGA381
target, Yosys 0.68+post + nextpnr-ecp5 0.11.1-19-g8dbcee5c,
--lpf-allow-unconstrained, same methodology as STEP16-18).
key enabling mechanism: extended sdram_controller.v with a real,
tested per-burst-word DQM write-mask input (`wmask`, 2 bits/word),
exercised via a new Test J in tb_sdram_controller.v (byte-masked
write, verified neighboring bytes/words in the SAME real 128-bit
SDRAM block are untouched) -- confirmed PASS across all 9 existing
frequency/burst configurations (461/461 each) plus the new test, zero
regression. This lets a single result BYTE be written inside a
shared 128-bit burst transaction with NO read-modify-write at all
(the real SDRAM chip itself leaves DQM-masked bytes unchanged, by
JEDEC definition) -- the key fact that made single-SDRAM unification
practical without a larger controller rewrite.
result: full N=4 D-Stress: 49,771 cycles, 256/256 bit-exact vs golden
(vs the STEP18 dual-memory baseline's own 44,935 cycles -- a real,
disclosed +10.8% cycle-count cost from now sharing physical
bandwidth between weight/activation/result traffic that previously
had a separate, independent PSRAM chip). N=2: 49,788 cycles,
256/256 bit-exact (essentially IDENTICAL to N=4 now -- 49788 vs
49771 -- confirming the single shared SDRAM is now even MORE
strongly the binding resource than before). Real sdram_wr_count=256
(exactly one write per neuron result, confirms correct write
granularity). 40 real AUTO REFRESH events interleaved correctly
during both runs, zero corruption.
resources (Yosys+nextpnr, N=4): TRELLIS_IO 149/245 (DOWN from the
dual-memory baseline's own 194/245 -- a real 45-pin reduction,
EXACTLY matching the real PSRAM interface's own pin count removed,
confirms the design is complete and consistent), TRELLIS_FF 6425
(vs 6483, slightly FEWER despite the new arbitration logic, since an
entire redundant V1 controller's own real logic was removed),
TRELLIS_COMB 6023, MULT18X18D 32, DP16KD 0 (all essentially
unchanged or improved).
timing (8 P&R seeds, N=4, real POST-P&R Fmax): 66.97/74.00/74.45/
74.92/79.23/79.53/79.80/81.84 MHz -- only 1/8 seeds PASS at 80MHz.
Classification: MARGINAL per the governing spec's own rule (some
seeds >=80MHz, most do not) -- reported honestly, NOT masked by
citing only the best seed. This is a REAL, measured regression vs
the STEP18 dual-memory baseline's own 5/8 pass rate at N=4. Critical-
path tracing on the best seed (81.84MHz) confirms the bottleneck is
STILL dependency_manager.v's own first_ready_idx/reg_ready chain --
the SAME pre-existing, shared-architecture bottleneck STEP17 already
identified, NOT a new path introduced by sdram_unified_backend.v
itself. Interpretation: consolidating all traffic onto one physical
SDRAM adds overall die logic/routing pressure that further squeezes
an ALREADY-marginal, pre-existing placement-sensitive bottleneck --
a real, disclosed cost of the single-SDRAM architecture, not a new
defect in the new RTL.
decision: ACCEPT the single-SDRAM architecture as the STEP19 hardware
freeze reference DESPITE the worse timing margin, per the governing
spec's own explicit, binding instruction ("una SDRAM, anche se
richiede un Memory Manager piu intelligente" / do not solve a
performance problem by adding a second memory) -- functional
correctness (bit-exact, no deadlock, real sustained refresh
operation) is fully achieved, and the timing regression is reported
as a real, unresolved CRITICAL item for follow-up (see DEC-0034),
not hidden or worked around by reverting to two chips.
EXP-0049 -- Phase 0 baseline for the new N=8-timing/85F-retarget/
SDRAM-bank sweep brief (2026-09-15)
config: fpga_neural_v2_top (real board-level top), N_SLOTS=4, RTL
bit-identical to DEC-0042's frozen state (no RTL changes)
action: real Yosys synthesis + fresh 8-seed nextpnr-ecp5 P&R, real
physical pins (constraints/v2_board_top.lpf), real PLL-derived 64MHz
internal clock domain
result: 8/8 PASS at 64MHz. Fmax worst=81.20MHz, mean=91.05MHz (full
per-seed numbers and utilization in synthesis.log/timing.log).
Resources: LUT4 6905/43848 (15%), DFF 6527/43848 (14%), MULT18X18D
32/72 (44%), DP16KD 0/108 (0%).
decision: adopted as the operative Phase-0 BASELINE row (see
timing.log for the disclosed, unresolved discrepancy vs DEC-0042's
own historical numbers, and STEP19-era experiments.log precedent
showing N=4 Fmax as high as 81.84MHz on a related pre-fix config --
this range is not without precedent in this project's own history).
N_SLOTS=8 baseline deferred by explicit user request after the wrong
synthesis target (obsolete nms_neural_multiprocessor_sdram_unified.v
wrapper, see ERR-0031) caused a 2h42m non-converging P&R run;
N_SLOTS=8 to be re-attempted against fpga_neural_v2_top with an
agreed time budget.
next_action: Phase 1 (N=8 timing fix) blocked on N_SLOTS=8 baseline;
in the meantime this N_SLOTS=4 baseline is committed to branch v21.
EXP-0051 -- Two-physical-SDRAM-bank experiment (SIMULATION ONLY): does
splitting weight-fetch (W) and activation/result (AR) traffic onto two
independent physical SDRAM channels remove the memory-bound thrashing
EXP-0049/0050 measured on the real board-level top? (2026-09-16)
DATE: 2026-09-16
CONTEXT: per decisions.log's own "next recommended step" note after
EXP-0049/0050 closed Phase 0 (real board-top baseline, N=4 worst
82.43MHz/N=8 worst 77.36MHz, both 8/8 PASS @ 64MHz -- see decisions.log
for that specific number set, gathered in a prior pass of this same
session) -- before spending effort on Phase 2 (85F retarget, N=16),
verify whether the system is genuinely external-memory-bandwidth-bound
(as tb_nms_dstress_sdram_unified.v's own instrumentation already
strongly suggested: SDRAM controller port busy ~81.6% of all D-Stress
cycles at BOTH N_SLOTS=4 (49927 cycles) and N_SLOTS=8 (49909 cycles) --
see decisions.log/errors.log) by testing the "Fase 3" 1-vs-2-SDRAM-bank
question from the brief's own original scope, at N=4/N=8, ahead of
schedule. NOTE: STEP19/EXP-0048's own governing spec explicitly
mandated a SINGLE physical SDRAM for the real board ("una SDRAM, anche
se richiede un Memory Manager piu intelligente") -- this experiment
does NOT propose reopening that decision for the real hardware/v2
board (constraints/v2_board_top.lpf is untouched, still wires exactly
one physical chip); it is scoped, per this session's own current
brief, as SIMULATION-ONLY architecture exploration to inform whether a
future board revision or a different RTL fix direction is worth
pursuing at all.
TOOLCHAIN (recorded per timing.log's own process recommendation after
the EXP-0049/0050 Yosys-version discrepancy investigation): this
session's OSS CAD Suite install at ~/tools_cache/oss-cad-suite/ reports
`yosys -V` = "Yosys 0.69+59 (git sha1 d85872386-dirty)" -- the IDENTICAL
commit hash already recorded for the EXP-0049/0050 session, confirming
NO toolchain drift since that investigation closed (this experiment
uses Verilator only, no synthesis/P&R was run). `verilator --version` =
"Verilator 5.053 devel rev v5.052-85-g270c528af (mod)".
METHOD: forked nms_neural_multiprocessor_sdram_unified.v (STEP19) into
a new module, `nms_neural_multiprocessor_sdram_dualbank.v` --
u_dataflow_core/u_arbiter/u_arbiter_wide all byte-for-byte unchanged;
the single sdram_unified_backend.v instance is replaced by TWO
instances of that SAME, unmodified module: u_sdram_backend_w (W port
only, ar_req tied to 0) and u_sdram_backend_ar (AR port only, w_req
tied to 0), each with its own sdram_controller.v and its own physical
SDRAM pins. Safety of the permanent tie-off verified by inspection: an
always-0 ar_req/w_req means S_AR_RD_WAIT/S_AR_WR_WAIT (resp. S_W_WAIT)
are simply never entered -- no dead-state risk. Forked
tb_nms_dstress_sdram_unified.v into `tb_nms_dstress_sdram_dualbank.v`
(new files, both under hardware/v2/nms/{rtl,sim}/) -- identical D-Stress
workload/golden-model/bit-exact verification; only the backdoor
poke_byte/peek_byte targets change (weight pokes -> u_sdram_w.mem,
activation/result pokes -> u_sdram_ar.mem, a split that already existed
in the original testbench's own naming convention even when both
pointed at the same array) and instrumentation now reports each bank's
own sdram_controller.v busy%/req/ready/refresh counts separately, plus
an "either bank busy" figure directly comparable to the single-bank
sdram_busy_pct.
command: `verilator --binary --timing -j 0 -Wno-fatal -GN_SLOTS_CFG=<4|8>
-GPFD_CFG=8 --top-module tb -Mdir <dir> sim/tb_nms_dstress_sdram_
dualbank.v sim/sdram_model.v <rtl deps>`, then run the resulting Vtb
binary. Baseline (single-bank) re-run first for direct comparison,
same command against the unmodified tb_nms_dstress_sdram_unified.v --
reproduced EXACTLY: N=4 49927 cycles/81.56% busy, N=8 49909
cycles/81.62% busy, both 256/256 bit-exact -- confirms this session's
toolchain/methodology matches the numbers already on record before
trusting the new dual-bank numbers below.
RESULT (dual-bank, both 256/256 neurons bit-exact vs golden, data_ready
PASS, zero functional regression):
N=4: total_cycles=45724 (vs single-bank 49927, a real but MODEST
8.4% reduction). BANK W (weight-fetch): busy=35112/45724 (76.79%).
BANK AR (activation+result): busy=5375/45724 (11.76%). EITHER-bank-
busy=36391/45724 (79.59%) -- barely different from the single-bank
figure of 81.56%.
N=8: total_cycles=44980 (vs single-bank 49909, 9.9% reduction). BANK
W: busy=35057/44980 (77.94%). BANK AR: busy=5368/44980 (11.93%).
EITHER-bank-busy=35976/44980 (79.98%) -- again barely different
from the single-bank 81.62%.
Both configs: BANK W's own req/ready counts are near-identical
across N=4 and N=8 (2163 vs 2160) -- confirms weight-fetch traffic
volume itself does not grow much with N_SLOTS (same total tiles
processed either way), yet Bank W alone still saturates at ~77-78%
busy EVEN with a fully dedicated physical channel and zero AR
cross-traffic.
INTERPRETATION (SURPRISING, disclosed honestly -- this is NOT the
dramatic "thrashing disappears with 2 banks" result the hypothesis's
naive framing might have predicted): the memory-bound hypothesis is
CONFIRMED at the system level (~80% memory-port busy either way) but
REFINED in a way that changes the recommended next step. Splitting
traffic by CLASS (W vs AR) barely moves total_cycles (8-10%) because
the AR path was never the dominant contention source in the first
place (STEP17/EXP-0045 already showed AR at <=7.2% of all external-
memory activity, confirmed again here: Bank AR sits at ~12% busy even
with its own fully dedicated channel and zero contention). The real
ceiling is BANK W's OWN throughput -- i.e. how fast a SINGLE weight-
fetch channel (BURST_LEN=8, one sdram_controller.v transaction in
flight at a time, W_ENTRIES=4 "other half" cache) can deliver 64-bit
weight words to however many slots are requesting them -- not
arbitration contention between logically-different traffic classes on
one shared bus. Giving AR its own physical bank was, in effect, solving
a problem that was not the binding one.
decision: do NOT recommend a 2-physical-bank (W/AR split) board
revision on this evidence alone -- the ~8-10% cycle-count gain does
not obviously justify the doubled physical SDRAM pin count (74 vs 37
pins) for Phase 2's LFE5U-85F retarget, given the real bottleneck
visibly sits inside the weight-fetch channel itself, not in cross-
class contention. This does NOT close the memory-bandwidth question
-- it REDIRECTS it: the next diagnostic worth running before Phase 2
is characterizing what specifically caps Bank W's own ~77-78%
ceiling (single-transaction-in-flight controller design? W_ENTRIES=4
cache depth/hit rate under N=8 contention? BURST_LEN=8 granularity
vs per-tile fetch size?) and whether splitting WEIGHT traffic itself
across two banks (e.g. by slot-group, not by traffic class) would
fare differently -- that specific variant was NOT tested here and is
a real, disclosed gap, not assumed to also fail.
next_action: report this refined finding to the user before choosing
between (a) a slot-group-split weight-bank experiment as a follow-up
to this same Fase-3 investigation, (b) a Bank-W-internal-only
optimization pass (cache depth, burst size, pipelining), or (c)
proceeding directly to Phase 2 (85F retarget + N=16) with the
memory-bandwidth ceiling accepted as a known, disclosed limitation
rather than something Phase 3 can cheaply remove. New files (not yet
used by the real board top, additive only): hardware/v2/nms/rtl/
nms_neural_multiprocessor_sdram_dualbank.v, hardware/v2/nms/sim/
tb_nms_dstress_sdram_dualbank.v.
EXP-0052 -- Bank-interleaved pipelining for the W (weight-fetch) SDRAM
channel: the mechanism works in isolation (verified) but the real
D-Stress integration gain is negligible, because the CALLER never
issues a second request early enough to trigger it (2026-09-16)
DATE: 2026-09-16
CONTEXT: follow-up to EXP-0051, which found the weight-fetch (W)
channel itself (not W/AR cross-traffic) as the real ~77-78%-busy
ceiling, and identified the per-transaction fixed cost (measured ~16
cycles: 1 issue + 2 T_RCD + 4 CAS_LATENCY-wait + 7 BURST_LEN=8 read +
2 T_RP) as the lever to attack, since the real transaction COUNT is
already close to optimal (2163 measured vs 2048 theoretical minimum
for the D-Stress workload, ~5.6% overhead). This session chose to
pursue this via a fork given the real correctness-risk history of this
exact FSM area (ERR-0019/0020/0023, all req-latching races).
TOOLCHAIN: unchanged from EXP-0051 (Yosys 0.69+59 d85872386-dirty,
Verilator 5.053) -- this experiment is Verilator-only, no synthesis/
P&R.
METHOD (two-phase, isolated-correctness-first per this project's own
established discipline):
Phase A: `sdram_controller_pipelined.v` (new, forked from
sdram_controller.v) remaps the addr->{bank,row,col} decomposition
from high-order bits (today: bank always 0 for this project's
compact weight region, since bank comes from the TOP address bits)
to LOW-order bits placed just above the burst-alignment zero bits --
so consecutive burst-aligned weight fetches (each BURST_LEN=8 words
apart) now naturally rotate across the SDRAM's own 4 internal banks
instead of all landing on bank 0. Added a depth-1 "shadow" slot: while
the current transaction is in CAS_WAIT/BURST/PRECHARGE_WAIT (command
bus otherwise idle), a newly-arriving request for a DIFFERENT bank
has its ACTIVATE issued immediately, overlapping that bank's own
T_RCD wait with the current transaction's tail. Same-bank requests,
and refresh, are unaffected (S_IDLE priority: open shadow > refresh >
new request, so AUTO REFRESH can never fire with a row left open).
New isolated testbench `tb_sdram_controller_pipelined.v`: 38/38 PASS,
bit-exact across all 4 banks. Real, INDEPENDENTLY RE-VERIFIED result
for back-to-back different-bank transactions: 30 cycles total vs a
32-cycle serial baseline for the same pair -- exactly 2 cycles saved
(= T_RCD), NOT a multiple-x speedup. This matches the theoretical
ceiling worked out BEFORE measuring: CAS_LATENCY+BURST_LEN (11 of the
16 cycles) are serial on the SHARED data bus regardless of bank, and
no amount of bank interleaving can hide that -- only the T_RCD+T_RP
portion (5 of 16 cycles) is bank-local and therefore hideable, and
only T_RCD (2 cycles) was actually recovered here since the OTHER
bank's T_RP tail still has to clear before its OWN next reuse. Same-
bank consecutive case: unchanged, no regression. Refresh-during-
interleaving case (Test 4): AUTO REFRESH spacing rose from 634 to 657
cycles under sustained back-to-back different-bank stress (vs
tREFI=626 target) -- a real, disclosed +3.6%, already within the
margin this project's OWN unmodified controller already tolerates
under the same synthetic stress pattern, not a new violation.
One bug found and fixed, in the NEW TESTBENCH ONLY (not the RTL):
calling wait_ready() twice in a row double-consumed the same `ready`
pulse -- fixed by advancing one extra @(posedge clk) between calls.
Phase B (integration, gated on Phase A passing): forked
`sdram_unified_backend_pipelined.v` (swaps in the pipelined
controller, W_ENTRIES cache and W/AR arbitration untouched) and
`nms_neural_multiprocessor_sdram_pipelined.v`, plus a new
`tb_nms_dstress_sdram_pipelined.v` (same D-Stress workload/golden
model; backdoor peek/poke rewritten to go through a `sdram_model.v`
backdoor_read/write helper keyed on the SAME decomposition the new
controller uses, instead of the old flat-address assumption, so
bit-exact verification stays valid under the new addr->bank mapping
-- this was flagged in advance as the one correctness trap in this
whole exercise, and was handled by construction rather than by
parallel, error-prone reimplementation).
RESULT (INDEPENDENTLY RE-BUILT AND RE-RUN by this session directly, not
just taken from the sub-task's own report -- both PASS 256/256 bit-
exact + data_ready PASS in both configs):
N=4: total_cycles=49760 (vs single-bank baseline 49927, EXP-0049 --
only -0.33%). N=8: total_cycles=49755 (vs baseline 49909 --
-0.31%). Both essentially within noise of the unmodified single-
bank system, nowhere near either Phase A's own measured 2-cycle-
per-different-bank-pair saving scaled up, or EXP-0051's dual-bank
-8/-10%.
ROOT CAUSE of the gap between Phase A (works) and Phase B (doesn't
help): `slot_mem_arbiter_wide.v` -> `sdram_unified_backend.v`'s own W
port is a synchronous one-request-at-a-time interface -- the caller
waits for `w_ready` before ever asserting the next `w_req`. Phase A's
interleaving mechanism can ONLY help if a request for a DIFFERENT bank
is already pending WHILE the current transaction is still mid-flight
(CAS_WAIT/BURST/PRECHARGE) -- a condition the current arbiter/backend
call convention almost never creates, since nothing is ever dispatched
early. The mechanism itself is real and correctly verified in Phase A
(directly, artificially stimulated); the SYSTEM around it, as it exists
today, essentially never exercises it.
decision: do NOT integrate sdram_controller_pipelined.v into the
production path on this evidence -- the real, measured, system-level
gain (~0.3%) does not justify carrying a second, more complex
controller variant with its own (even if currently well-verified)
correctness surface. The isolated Phase A result remains genuinely
useful and is KEPT as an additive, uncommitted-to-production file:
it proves the mechanism works and quantifies its real ceiling (2
cycles/pair, not more), which is exactly the number needed to decide
whether a FUTURE arbiter/backend rewrite (teaching the W port to
dispatch its NEXT request BEFORT the current one's `ready`, i.e. a
real pipelined/multi-outstanding-request interface, not just the
memory-side FSM) would be worth attempting -- that rewrite is a
materially larger, riskier change (touches the arbiter's own request/
grant protocol, not just the memory-side FSM) and was explicitly kept
out of scope for this experiment.
next_action: report to the user; do not pursue the arbiter/backend
pipelined-dispatch rewrite without an explicit go-ahead, given its
larger scope and the modest (2 cycles/pair, capped) ceiling this
experiment just measured -- the slot-group weight-split ("aspettiamo"
item from EXP-0051) remains the other, still-open, ORTHOGONAL lever
(it does not depend on this pipelining work at all and would stack
with it if the arbiter rewrite is ever done). New files (additive
only, none touch the real board top or existing production RTL):
hardware/v2/nms/rtl/sdram_controller_pipelined.v,
hardware/v2/nms/rtl/sdram_unified_backend_pipelined.v,
hardware/v2/nms/rtl/nms_neural_multiprocessor_sdram_pipelined.v,
hardware/v2/nms/sim/tb_sdram_controller_pipelined.v,
hardware/v2/nms/sim/tb_nms_dstress_sdram_pipelined.v.
EXP-0053 -- SDRAM clock-domain-crossing bridge: decouple the physical
SDRAM clock from the 64MHz compute domain (2026-09-16)
DATE: 2026-09-16
CONTEXT: user asked for the inverse roofline calculation (given N-core
compute demand, what memory bandwidth would be needed) after EXP-0052
closed with only 0.3% real gain; derived requirement ~1GB/s/slot at
64MHz, current single 16-bit SDR SDRAM chip (Alliance AS4C32M16SB-7,
143MHz max) delivers ~90-130MB/s at the board's real 64MHz clk_sys.
Found via real ecppll -i 16 -o 64 --clkout1 <target> sweep (OSS CAD
Suite, now installed at ~/tools_cache/oss-cad-suite, added to PATH via
~/.bashrc this session) that the board's existing PLL VCO is fixed at
576MHz by the ALREADY-VERIFIED 64MHz CLKOP config -- the only clean
integer VCO/N divisors near the chip's ceiling are 576/4=144MHz (0.8%
OVER the real 143MHz max, rejected) and 576/5=115.2MHz (~19% real
margin). 115.2MHz chosen as the fast SDRAM clock, derivable from the
SAME PLL with a second CLKOS output, zero new board components.
METHOD: sdram_cdc_bridge.v -- toggle+last-seen two-flop-synchronizer
handshake (slow-domain caller interface, fast-domain sdram_
controller.v instance), safe because this project's own req/busy/
ready protocol never has more than one transaction outstanding (see
module header for the full quasi-static-bus argument). Isolated tb
(tb_sdram_cdc_bridge.v): 64MHz vs 115.2MHz, deliberately non-integer
ratio, no lucky alignment.
RESULT (Phase A, isolated): 137/137 tests, 0 errors, including
back-to-back stress. REAL measured total-cycle speedup over 40
transactions: 1.095x -- NOT the naive 1.8x clock-ratio estimate. Root
cause: the CDC handshake's own synchronizer round-trip (~4-6
slow-cycle-equivalent per transaction) is a FIXED tax that eats most
of the benefit when the underlying transaction is short (~13 cycles at
BURST_LEN=8). Ad-hoc check at BURST_LEN=32 showed speedup rising to
1.45x (fixed tax amortized over more useful cycles) but also exposed a
real, disclosed, pre-existing controller limitation (see EXP-0054).
decision: correctness verified; real-system integration deferred to
EXP-0055 (composed with EXP-0054). New files (additive only):
hardware/v2/nms/rtl/sdram_cdc_bridge.v,
hardware/v2/nms/sim/tb_sdram_cdc_bridge.v.
EXP-0054 -- open-row (page-hit/keep-row-open) SDRAM controller policy
(2026-09-16)
DATE: 2026-09-16
CONTEXT: investigating why BURST_LEN=32 broke (EXP-0053's ad-hoc
check) led to the real root cause: sdram_controller.v's own mrs_value
function only encodes JEDEC burst-length 1/2/4/8 -- any other value
silently falls through to burst-length code 3'b111 ("full page"),
a real, disclosed, unimplemented-elsewhere scope limit, not a bug to
fix. This redirected the effort toward the controller's OWN header,
which already named the real next lever: "ALWAYS uses auto-precharge
... NOT the fastest possible design (no page-hit/keep-row-open
optimization)". weight_prefetch_engine_wide.v (confirmed via grep,
NOT dead/exploratory code as its own stale header claims -- real
production traffic, instantiated by nms_dataflow_core_sdram.v,
PREFETCH_DISTANCE=8) issues strictly sequential per-job tile addresses
that mostly stay within one SDRAM row (1024 cols/row = 256 tile-blocks
at BURST_LEN=8) -- closing/reopening that row on every single tile
(today's fixed policy) pays tRP+tRCD twice per transaction for no
reason when the next transaction hits the same row anyway.
METHOD: sdram_controller_openrow.v, forked from sdram_controller.v.
Never auto-precharges; tracks the single currently-open bank+row
(same one-transaction-in-flight scope as the original); on the next
request: ROW HIT (same bank+row) skips ACTIVATE entirely (saves
tRCD); ROW MISS with a row open issues an explicit PRECHARGE first,
same total cost as today's auto-precharge, just paid on-demand. Two
real correctness hazards this policy introduces vs the original
(both fixed, not assumed safe):
(1) JEDEC AUTO REFRESH requires all banks precharged first -- the
original design's own comment ("no row is ever left open...")
no longer holds; fixed via a new S_PRE_THEN_REF_WAIT state.
(2) tWR (write recovery, 2 CLK, real datasheet value) was folded
into the original's always-paid post-write precharge wait --
now paid alone via a new S_WRITE_RECOVERY_WAIT state.
DISCLOSED, NOT independently verified: read-to-read/read-to-write
same-row turnaround has no extra wait beyond the existing 1-cycle
S_IDLE minimum (standard JEDEC page-mode reasoning) -- sdram_model.v
does NOT itself assert tCCD/tRTW/tWTR (confirmed by inspection), so
this relies on DATA-correctness checks (tb_sdram_controller_openrow.v
TEST 4) rather than an independent timing oracle.
RESULT (Phase A, isolated, vs sdram_controller.v baseline, same
sdram_model.v-checked correctness harness): 154/154 tests, 0 errors,
0 protocol VIOLATIONs -- including refresh-while-row-open (the one
real new hazard) across 80 write/read pairs spanning real tREFI.
REAL measured speedup, 32 sequential same-row tile reads (the actual
weight_prefetch_engine_wide.v access pattern): 1.141x.
decision: correctness verified; real-system integration in EXP-0055.
New files (additive only):
hardware/v2/nms/rtl/sdram_controller_openrow.v,
hardware/v2/nms/sim/tb_sdram_controller_openrow.v.
EXP-0055 -- Phase B integration: EXP-0053 (CDC) + EXP-0054 (open-row),
isolated combination AND real D-Stress system, N=4/N=8 -- combined
result is WORSE than baseline; open-row ALONE is a real, disclosed win
(2026-09-16)
DATE: 2026-09-16
CONTEXT: per user direction ("procediamo"/"implementiamo queste"),
integrate both mechanisms and measure the real combined effect on the
actual D-Stress benchmark, following this project's own established
Phase A (isolated) -> Phase B (integration) discipline.
METHOD: sdram_cdc_bridge_openrow.v (EXP-0053's CDC composed with
EXP-0054's page-hit controller as the fast-domain DUT -- CDC handshake
itself unchanged, treats the controller as a black box). Isolated tb
(tb_sdram_cdc_bridge_openrow.v): 149/149 tests, 0 errors, 0
VIOLATIONs. REAL measured combined speedup, same 32-tile same-row
sequential pattern: 1.158x -- LOWER than the naive product of the two
isolated numbers (1.095 x 1.141 = 1.25), a real, disclosed, non-linear
interaction (the CDC's fixed tax becomes a proportionally BIGGER
fraction of an already-shorter open-row transaction), not assumed.
Phase B (full system, forked exactly as EXP-0052's own minimal-diff
pattern: sdram_unified_backend_combined.v + nms_neural_multiprocessor_
sdram_combined.v + tb_nms_dstress_sdram_combined.v, real D-Stress
workload, 256/256 bit-exact + data_ready PASS in every configuration
below):
baseline (today, real): N=4: 49927 cyc N=8: 49909 cyc
CDC alone (no open-row): N=4: 54096 cyc (+8.35%, WORSE)
open-row alone (no CDC): N=4: 47445 cyc N=8: 47468 cyc
(-4.97% / -4.89%, REAL GAIN)
combined (CDC + open-row): N=4: 51931 cyc N=8: 51943 cyc
(+4.02% / +4.10%, still WORSE)
ROOT CAUSE of the combined regression: the CDC bridge's synchronizer
round-trip is a FIXED tax paid on EVERY transaction, hit or miss,
regardless of benefit -- unlike EXP-0052's pipelining mechanism (which
simply reverts to baseline-equivalent cost when its condition doesn't
trigger), this tax is not "free when unused". The real D-Stress
traffic is NOT purely sequential same-row (sdram_unified_backend.v's
own 2-way W/AR priority arbitration interleaves weight-fetch and
activation/result traffic, which live in different address regions --
see its own header, "W granted priority when both pending, AR never
starved" -- meaning the physical channel alternates row context far
more often than the open-row mechanism's own isolated same-row-sweep
test exercised). Open-row's real per-transaction saving (real, ~5%,
confirmed at both N=4 and N=8) is not enough to offset the CDC's own
per-transaction cost once row hits become less frequent under real
interleaved traffic.
DECISION: do NOT adopt the CDC clock-domain-crossing approach (EXP-
0053) -- measured net negative in the real system despite passing
isolated correctness and even showing a real isolated speedup on its
own synthetic same-row test. ADOPT-CANDIDATE: sdram_controller_
openrow.v (EXP-0054) alone, without any clock change -- real,
consistent ~5% D-Stress cycle-count improvement at both N=4 and N=8,
zero new clock domains, zero CDC correctness surface, single-variable
change. Not yet promoted to production (that would mean swapping
sdram_controller.v itself in the real board top, fpga_neural_v2_top.v
-- an explicit go-ahead item, not assumed here). This ~5% is
consistent with, and stacks multiplicatively with, EXP-0051's
dual-bank ~9% (different mechanism, same physical-floor-efficiency
class) if both are ever combined -- not measured together in this
session, an open item for a future experiment, not claimed here.
next_action: report combined finding to the user (CDC bridge measured
net-negative despite being individually correct and individually
faster in isolation -- do not pursue further without new evidence);
open-row is the one real, disclosed win from this whole EXP-0053/54/55
line and is the candidate worth promoting toward production if the
user wants that next. New files (additive only, none touch the real
board top or existing production RTL):
hardware/v2/nms/rtl/sdram_cdc_bridge_openrow.v,
hardware/v2/nms/rtl/sdram_unified_backend_combined.v,
hardware/v2/nms/rtl/nms_neural_multiprocessor_sdram_combined.v,
hardware/v2/nms/rtl/sdram_unified_backend_openrow.v,
hardware/v2/nms/rtl/nms_neural_multiprocessor_sdram_openrow.v,
hardware/v2/nms/rtl/sdram_unified_backend_cdc.v,
hardware/v2/nms/rtl/nms_neural_multiprocessor_sdram_cdc.v,
hardware/v2/nms/sim/tb_sdram_cdc_bridge_openrow.v,
hardware/v2/nms/sim/tb_nms_dstress_sdram_combined.v,
hardware/v2/nms/sim/tb_nms_dstress_sdram_openrow.v,
hardware/v2/nms/sim/tb_nms_dstress_sdram_cdc.v.
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