Measure the finer chain tail: 84/120 becomes 18/120, and the derivation was right by cancellation
blit.s gains v7 -- v6's 24-pixel movem chain plus a second chain whose unit is
one `move.l (a0)+,(a2)+`. Measured over 13 span lengths by span.sh, every config
pixel-exact:
cycles = 66.0 per span + 9.143 per COARSE pixel + 9.978 per FINE pixel
fitting all 13 to within 0.2%. v5 and v6 re-measure to FINDINGS 30 exactly, so
the harness has not drifted underneath the new variant.
Rescored against the same scsi window and the same additive model, v7 takes
84/120 frames over budget to 18/120 -- exactly what FINDINGS 39.4 derived, and
that agreement is two cancelling errors: the derivation's 2-register movem tail
is 29% too dear per pixel, and its "nothing per span" for the second chain entry
is 22.3 clocks too cheap. The plain post-incrementing move.l is the right tail
instruction, and it makes the padding quantum 2 pixels, which a run of 4x4
blocks pads to exactly zero.
The DMAC stays dropped on a measurement now rather than an argument: v7 takes
back 37 of the 43 frames the array chain would, with no reserved channel and no
timing neither emulator here can verify. Break-even against all-V1 moves from
L=4 blocks to L=2.
The fine displacement is carried mid-stream rather than in the span record, so
the decoder holds nothing across the copy and keeps all 12 payload registers --
which is the whole reason the coarse unit is 24 pixels.
span.sh is now -seconds_to_run 200 (30 s wall, 36 configs) and takes its
expected snapshot count from the generated metadata instead of a literal 23.
Claude-Session: https://claude.ai/code/session_01194oWYW8DQXK1SZ2DnChW6
This commit is contained in:
prosolis
+112
-4
@@ -1,7 +1,109 @@
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# Status & next-session handoff — end of session 10 (2026-08-23)
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# Status & next-session handoff — end of session 11 (2026-08-23)
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## Where this stands
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Session 11 measured the one item session 10 left at the top of the list, and it
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paid: **`blit.s` v7, the literal span with a fine tail, is MEASURED and takes
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the `scsi` window from 84/120 frames over budget to 18/120.** FINDINGS 40.
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```
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v7: cycles = 66.0 per span + 9.143 per COARSE pixel + 9.978 per FINE pixel
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13 span lengths, all fitted to within 0.2%, all pixel-exact
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```
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| | frames over budget, 120-frame `scsi` window |
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|---|---:|
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| today (no spans) | 84/120 |
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| v6 span as built | 55/120 |
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| **v7, measured** | **18/120** |
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| DMAC chain (datasheet) | 12/120 |
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**The DMAC stays dropped, and now on a measurement rather than an argument.**
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v7 takes back 37 of the 43 frames the DMAC chain would, with no reserved
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channel, no two-region container, and no transfer timing neither emulator here
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can verify. FINDINGS 39.1 still holds if that ever changes: a chain array entry
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and a v6/v7 span record are the same six bytes.
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**Break-even against all-V1 moves from L=4 blocks to L=2.** 39.4 predicted L=3.
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**18/120 is exactly what 39.4 derived, and that is a coincidence of two
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cancelling errors** — worth knowing before the next derived figure gets trusted
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for landing on its measurement. 39.4 assumed a 2-register `movem` tail at 14.0
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clocks/pixel (the real tail is 9.978, 29% cheaper) and assumed the second chain
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entry costs nothing per span (it costs 22.3 clocks). The two nearly cancel over
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this window. FINDINGS 40.2.
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**The tail instruction the derivation should have picked is `move.l (a0)+,(a2)+`.**
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A 2-register `movem` pays two instruction words and a `lea` to move what two
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post-incrementing `move.l`s move: 14 bus cycles against 10 for the same 4
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pixels. Taking the plain instruction also makes the padding quantum **2 pixels**
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instead of 4 — and a span is a run of 4x4 blocks, so **its padding is exactly
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zero**. FINDINGS 40.3.
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**The fine displacement lives in the STREAM, not in the span record**, after the
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coarse pixels and before the fine ones: the coarse chain falls out into
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`move.w (a0)+,d0 / jmp`, where `d0` is dead payload and `a0` already points at
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it. That is what lets v7 keep all 12 payload registers, which is the entire
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reason v6's unit is 24 pixels. The container costs 2 more bytes a span.
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FINDINGS 40.4.
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**One process note.** `span.sh` ran 13 minutes producing an empty log and zero
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snapshots; the same command with a shorter `-seconds_to_run` did the identical
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work in 30 s, and the wedge never reproduced. The cause is unidentified. What
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resolved it was not chasing the hang but **shrinking the stimulus**:
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`tmp/spans_meta.lua` holds byte offsets into a blob `prep_spans.py` writes once,
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so deleting lines from the metadata runs any subset in seconds against the same
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stream file. Keep that trick. `span.sh` is now `-seconds_to_run 200` (30 s wall
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for all 36 configs) and takes its expected snapshot count from the metadata
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instead of a literal 23. FINDINGS 40.6.
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Green light: `./tools/bench/check.sh` **ALL GREEN** at the end of this session.
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## NEXT SESSION, in order
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0. **Green light first.** `./tools/bench/check.sh` (~5 min, Blu-ray mounted).
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Verified green at end of session 11.
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1. **Build v7 into `src/player/decode.s`.** This is now the largest thing
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standing between the measured decoder and the budget: 84/120 to 18/120, the
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format is fully specified (FINDINGS 30.2, costs 40.1), the executor is
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written and pixel-exact in `blit.s`, and the encoder side is
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`prep_spans.py`'s v7 emitter. The container is `{u32 absolute GVRAM address,
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u16 coarse displacement}` per span plus one `u16` fine displacement carried
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mid-stream — see FINDINGS 40.4 before changing that layout, the register
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pressure is the reason for it.
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2. **Then re-run `14_dmac_chain.py` and `13_cpu_ratectl.py` against a container
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the encoder actually emits with spans in it.** Every span figure so far is
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scored against mode maps chosen without spans available, which FINDINGS 39.3
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flags as a lower bound on what a span-aware encoder would find.
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3. **Make sure the player actually gets DMA** — unchanged from session 10, and
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still not an optimisation. Benchmark `x68000 -exp1 cz6bs1`, **never
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`x68ksupr`**; MAME's internal SCSI has no DMA glue (`// TODO: duplicate DMA
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glue from CZ-6BS1`) and would measure a PIO fallback the real machine does
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not have.
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4. **Re-decide the framerate.** 10 fps absorbs the DMA steal on current
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estimates. Still the user's call, and now cheaper to defer: v7 buys back
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enough of the budget that 12 fps is no longer obviously out of reach.
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5. **Re-run the ring-buffer simulation at the surviving rate** and confirm the
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488 KB/s figure's provenance (FINDINGS 29.5/30.7, still open).
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**Do not start by hand-optimising `decode.s`.** Unchanged and still true: the
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hand-derived timings agree with the measurements to 0.5% on V1 and 1% on RAW
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(FINDINGS 28.4), 34 confirms the model on a second container, and the cycles to
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be won are in the budget, not the loop.
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**Always `stdbuf -oL` a MAME job that prints progress — and do not trust it.**
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Session 11 added the case where even that is not enough (40.6). If a run is not
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producing observable output, shrink the stimulus rather than waiting.
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---
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## What session 10 settled
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Session 10 cross-checked the whole cycle model against a second emulator, then
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found that the model was denominated in the wrong currency.
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@@ -77,10 +179,16 @@ at **3.3%**, and it runs against us. Two incidental results worth keeping: MAME
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rather than two tables; and FINDINGS 28.8's "V4 costs more than RAW" reproduces
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independently. FINDINGS 37.
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Everything below this line is from session 9 and still stands unless a session-10
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section above says otherwise.
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---
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## NEXT SESSION, in order
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## What session 10 put on the list
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Kept for the reasoning, not the priorities: items 0 and 1 are done (FINDINGS 40)
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and the rest are carried forward, reordered, in the session-11 list at the top.
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Everything below this point is from session 9 or earlier and still stands unless
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a later section above says otherwise.
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### session 10's list
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0. **Green light first.** `./tools/bench/check.sh` (~5 min, Blu-ray mounted).
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Verified green at end of session 9. The gate container is now
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