prosolis
29eb78a599
FINDINGS 29 priced a literal-span mode at 4*(50 + 4L*9.08) cycles and labelled
the whole section DERIVED. Session 8 step 0 was to measure it before optimising
over the mode set it implies. Two variants in blit.s, one stream per span length
from prep_spans.py, timed by span.lua, driven by span.sh in ~25 s:
v5, handed (x, npix) and left to work the copy out: 97.9/span + 10.459/px
v6, handed an address and a jump displacement: 43.7/span + 9.152/px
29 assumed 50.0/span + 9.080/px
So 29's arithmetic was right about a format nobody had written. The difference
is not tuning: v5 spends ~122 cycles a span computing a destination, dividing
npix into bursts and handling a 0..15 remainder, all of which the encoder knows
at build time. v6's record is {u32 absolute GVRAM address, u16 jump
displacement} into an unrolled chain of 24-pixel copy units -- no loop, no
remainder, no arithmetic -- and it fits 11 span lengths to 0.3%.
Three things that measurement showed and derivation could not:
- The per-pixel cost is a function of REGISTER PRESSURE. FINDINGS 24's 9.08
was a fixed blit with 12 registers free; v5 can spare 8 and pays 10.46; v6
gets 12 back only because the encoder holds the state.
- Short spans die in the remainder path -- a 12-pixel span costs MORE than a
16-pixel one -- and the fix is padding, not avoidance.
- Odd-x alignment is free (259.0 vs 261.8 cycles/span), as a 16-bit bus
implies but nobody had checked.
Re-priced against the unchanged mode maps, sasi: median 74.4% -> 52.0% (29 said
43.0), misses 37 -> 10/120 (29 said 8), 448.0 KB/s. Break-even moved from runs
of 2 blocks to runs of 4. 29.4 survives: a scene cut needs x >= 0.196 of the
frame as spans and the bus allows x <= 0.373, so it fits at 12fps.
All 23 timing configs are also checked pixel-exact, so none of this was timed
against a decoder that quietly skipped work.
FINDINGS 30. Next: lever B, the cost-aware mode decision.
Claude-Session: https://claude.ai/code/session_01194oWYW8DQXK1SZ2DnChW6
96 lines
5.1 KiB
Markdown
96 lines
5.1 KiB
Markdown
# Dragon's Lair — Sharp X68000 port
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Porting Dragon's Lair to a stock X68000 (68000 @ 10MHz, 2MB, SASI/SCSI).
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This is fundamentally a **video codec problem**, not a game-logic problem: the
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game logic is a scene table with branching input windows; the difficulty is
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pushing ~22 minutes of Don Bluth animation through a 10MHz 68000.
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**Green-light check:** `./tools/bench/check.sh` (~3 min, needs the Blu-ray
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mounted) re-runs both display regression tests, the rate-control drift test, the
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display-path coherency counterexample and a 120-frame 68000 decode, then prints
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`ALL GREEN`.
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## Read first
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- **`docs/FINDINGS.md`** — measured hardware facts, content statistics, codec
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decision, and a section on measurement traps that produced three separate
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false results. Read §4 before trusting any pipeline number.
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- **`docs/STATUS.md`** — current state, working setup, blockers, next steps.
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**Start here.** It also lists what has been explicitly abandoned, so old ideas
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do not get re-proposed.
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- **`docs/BENCHMARK.md`** — how to measure the storage subsystem, and why a
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bandwidth figure out of MAME would be meaningless.
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- **`docs/HARDWARE.md`** — X68000 GVRAM/CRTC reference.
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## Layout
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```
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docs/ findings, status, hardware reference
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tools/analysis/ measurement scripts, numbered in the order they were written
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(01/02 marked BROKEN deliberately, kept as regression refs).
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Run from the repo root — they import from tools/encoder/.
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07 finds the hottest sustained window in a stream; 08 renders
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source | decoded | block-mode map as .webm; 09 is the
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rate-control drift gate (FINDINGS 26/27) and is part of
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check.sh -- it exits non-zero if the encoder ever again
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reports a reconstruction no decoder would produce.
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10 is a COUNTEREXAMPLE, and exits non-zero by design: it
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demonstrates that the two-display-path plan of FINDINGS
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24.5/25.6 corrupts 70 of 120 frames (FINDINGS 28.1).
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11 scores a container against the MEASURED per-mode block
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costs without needing MAME; 12 prices the literal-span mode of
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FINDINGS 30 against those same mode maps, and prints whether a
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scene cut still fits at 12fps.
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tools/bench/ MAME Lua injection harness + 68000 benchmark sources.
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`check.sh` re-runs both display regression tests (~40 s).
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`blit.s`/`blit.lua` time the full-frame GVRAM blit on the
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68000 itself (FINDINGS 24) — not part of check.sh, because
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wall timings would make the green-light check host-sensitive.
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`span.sh` (prep_spans.py + span.lua + blit.s v5/v6) measures
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the literal-span mode the same way (FINDINGS 30, ~25 s); it
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also asserts all 23 timing configs drew a pixel-exact frame.
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`crtc_mode.lua` is the single source of truth for CRTC R00-R08
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and R20 — do not write CRTC values anywhere else.
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`prep_dlx.py`/`decode.lua`/`verify_decode.py` load, time and
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verify `src/player/decode.s`; the verify pass is in check.sh.
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tools/vasm/ vasm m68k assembler (built from source)
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tools/encoder/ hybrid VQ encoder + DLX1 container writer (working).
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dlx.py is the reference DECODER -- ground truth for the 68000.
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src/player/ decode.s: the 68000 DLX1 decoder. Pixel-exact, and 31% of
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frames over the 12fps CPU budget. See FINDINGS 28.
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assets/ extracted frames/audio (gitignored)
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```
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## Encoder
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```
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python3 tools/encoder/extract.py 00020 /tmp/fr 12 crop
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python3 tools/encoder/encode.py /tmp/fr out.dlx --profile sasi --preview p.png
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```
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Two quality profiles ship from one codec and one decoder — `sasi` (110 KB/s) and
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`scsi` (280 KB/s) are two points on the same rate-distortion curve. Both are
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**ceilings**: lam is bisected per frame under a leaky bucket, so the profile's
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`lam` is a quality floor rather than a setting (`--fixed-lam` opts out). The codec is
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a Cinepak-style hybrid: each 4x4 block is coded as SKIP, one 4x4 codeword, four
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2x2 codewords, or RAW literal pixels, chosen per block by rate-distortion.
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The RAW escape means `lam=0` is pixel-exact against the palettised frame, so the
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quality knob spans lossless to heavily-compressed without changing the bitstream.
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Profiles are derived from a bandwidth figure, not chosen by eye:
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```
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python3 tools/encoder/profile_gen.py --bw-mbps 4 --name scsi
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```
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> **On reading `docs/FINDINGS.md`:** it is append-only and several later sections
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> overturn earlier ones. Superseded sections carry a blockquote at the top
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> pointing to the correction — heed those, especially 18 (reversed by 21).
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Source media (`DRAGONS_LAIR.iso`) and ROMs are gitignored — supply your own.
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**Not every large stream is game footage.** `00216` is the feature with a
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burned-in commentary picture-in-picture and `00215` is the commentary itself —
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the two largest files on the disc. The clean 9.4-minute animation is **`00223`**.
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See FINDINGS 25.1 before running any size-ranked survey.
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