Dragon-s-Lair-X68k/docs/STATUS.md

Status & next-session handoff — end of session 5 (2026-08-23)

Start here: is the tree still green?

./tools/bench/check.sh

~40 s, needs the Blu-ray mounted. Re-runs both display regression tests from source media and prints ALL GREEN. Verified green at end of session 4. If it fails, fix that before doing anything else — everything downstream assumes the display path is pixel-exact.

Decisions locked

decision	value	why
Target CPU	68000 @ 10MHz (stock)	hardest honest constraint
Display mode	256 colors, 256x192 in 256x256 CRTC mode	every mode is 1 word-access/pixel, so 256c is free vs 16c
Double buffer	none — page 1 sacrificed	enables movem.l 24px bursts; delta coding needs a RAM reference frame anyway
Codec	hybrid VQ: SKIP / V1 4x4 / V4 four-2x2 / RAW, per-block rate-distortion	flat 4x4 VQ was measured and rejected — see FINDINGS 9-10
Quality modes	two: sasi and scsi (USER DECISION, session 2)	one codec, one decoder, one bitstream; only lam differs
Framerate	12 fps, explicit decimation	source has zero duplicate frames; no free "twos" win
Emulator	MAME 0.277 x68000	accurate enough that measured cycles mean something
SNES project reuse	MIT — cleared	data/events/ scene graph is reusable with attribution

The SASI/SCSI question is RESOLVED

Session 1 left "which machine do we target" open. The user's answer: ship both, as two quality profiles. This is now implemented rather than hypothetical — the bitrate ceiling is a build parameter in tools/encoder/ratectl.py:

profile	target	lam	quality (00020 / 00146)	machine
sasi	110 KB/s	60	36.9 / 29.6 dB	stock 10MHz ACE/EXPERT
scsi	280 KB/s	10	39.4 / 32.3 dB	Super/XVI, or CZ-6BS1 board

Sized against the user's working figure of 4 Mbps = 488 KB/s sustained, on SD-backed SCSI (BlueSCSI / SCSI2SD) — so that rate is a bus-limited constant, not an average over seek latency.

Both profiles fit with room. Ring-buffer simulation on the real per-frame sizes gives zero required prefill for every scene at both profiles: the fill delivers 40.69 KB per frame time and only one measured frame (42.10 KB) exceeds that, recovered by the next. A 256 KB buffer carries ~1 s of stall tolerance, far more than an SD-backed seek needs. FINDINGS 21.

An earlier warning here said scsi did not fit because a frame peaked at 96.4% of the pipe. That compared instantaneous demand to a sustained rate as if they had to match frame-by-frame; with a buffer the test is cumulative, and it passes.

scsi is now within 0.5 dB of the palette ceiling on 00020. These were initially set at 45 / 75 KB/s, which was 12% / 7% bus utilisation — read off the RD curve rather than derived from the hardware. See FINDINGS 17.

Codebooks are k=256 with 1-byte indices in both profiles. k=1024 was measured and rejected — see FINDINGS 14, it was a false-good result from a rate model that undercharged the index. Do not ship past lam~800; FINDINGS 15 has the cliff.

Because of the RAW escape mode, lam=0 is pixel-exact against the palettised frame (measured 0.00 dB loss). The profiles are two points on one continuous rate-distortion curve, not two codecs.

---

What session 3 settled

1. The display path works and is verified end to end. First real frame on an emulated X68000 screen: docs/images/x68k_first_frame_compare.png. Full write-up in FINDINGS 22. Everything before this session was Python-side or a headless -video none run, which cannot snapshot at all.
2. The render is pixel-exact, not merely close. With monitor contrast at 15, all 256 palette entries render exactly as GGGGGRRRRRBBBBBI + pal6bit predicts. That exactness is the regression test — see tools/bench/verify_frame.py, which exits non-zero if it ever drifts.
3. Three hardware facts that were previously assumed are now confirmed from MAME 0.277 source, not folklore: the palette word format, the 1024-byte GVRAM line stride, and the 256-colour page aliasing in HARDWARE.md. All three were already written down correctly; they are now cited.
4. A new quality ceiling was measured — the 15-bit+I palette alone costs 38.88 dB. Superseded by session 4: that figure assumed the shared LSB I is always 1. Chosen per entry, the ceiling is 40.81 dB. FINDINGS 23.3.
5. Two shell traps that wedged session 2's background jobs are documented in the working-setup section below. They cost ~1.5 h of wall clock and a wedged CPU core, and one of them was hit again this session.

What session 5 settled

1. 68000 code drew a frame, and the blit was measured. tools/bench/blit.s
   • blit.lua. The snapshot passes verify_frame256.py unchanged — pixel-exact in the real 256x256 mode. FINDINGS 23.5 is closed: no longer "proven from Lua only".
2. The 38% full-frame blit estimate is dead. It is 53.6%. And that is a zero-wait-state floor — MAME models no GVRAM wait states, so real hardware is worse. FINDINGS 24. Every variant was hand-derived from the MC68000 timing tables before being measured and the two agree to 0.006-0.43%, so this is not another MAME artefact.
3. Reading the source frame is exactly half the blit cost (V1 53.6% vs a write-only floor V3 of 27.1%). That is what makes the architecture question below live.
4. The decoder architecture now hinges on one unmeasured number. Writing codewords straight into GVRAM costs 76.6% of the frame budget for a full frame (V4 — the 1024-byte stride kills the movem.l burst), but scales with the non-SKIP block fraction and needs no RAM reference frame at all, because the previous frame is already in GVRAM. Compose-then-blit is a flat 53.6%. They cross at 70% of blocks changed. FINDINGS 24.5.

---

What session 4 settled

1. A real 256x256 CRTC mode exists and is verified. crtc_mode.lua, derived from x68k_crtc.cpp's divisor ladder rather than recalled — the derivation is self-checking (368 = 1104/3 exactly, so the horizontal registers divide by three with no remainder). Snapshot is native 256x512, active area pixel-exact, letterbox true black. FINDINGS 23. The x=512 wrap of FINDINGS 22.5 is gone.
2. The palette ceiling was wrong by 2 dB, in our favour. The shared LSB I must be chosen per palette entry, not hardcoded to 1. Doing so lifts the display ceiling from 38.85 to 40.81 dB and is the only way to get true black at all (pal6bit(1) = 4). 102 of 256 entries want I = 0. This supersedes FINDINGS 22.4 and gives scsi ~2 dB more headroom than believed. The encoder does not do this yet — see the encoder-gaps list.
3. Letterboxing costs one palette entry. 255 colours + a reserved black at index 0, with I = 0 on it. prep_frame.py --reserve-black. FINDINGS 23.4.
4. MAME's graphics double-scan is phase-shifted one raster line — pairs are (1,2),(3,4),..., not (0,1), because get_gfx_pixel halves the absolute scanline and vbegin = 41 is odd. Cost a false failure. FINDINGS 23.2.

---

What session 2 settled

1. The critical-path question is answered. "Does VQ soften Bluth's linework unacceptably?" — flat 4x4 VQ: yes, badly. The hybrid (SKIP/V1/V4/RAW): no. Verified by eye, not just PSNR. See docs/FINDINGS.md 9-11 and the two images in docs/images/. Both profiles use k=256; see item 2b.
2. Session 1's 12fps bitrate was wrong (183 KB/s claimed, 340 KB/s measured). Halving the framerate does not halve the bitrate. FINDINGS 8. 2b. A fourth false-good result was produced and caught this session — k=1024 codebooks looked like a +2.4 dB free win because the rate model charged 1 byte for a 10-bit index. FINDINGS 14. The k=256 configuration ships.
3. The 256-colour palettised frame is the real quality ceiling and it looks excellent. Judge the codec against that, not against 1080p.
4. Encoder exists and produces a real bitstream: tools/encoder/.

---

Encoder — working

python3 tools/encoder/extract.py 00020 /tmp/fr_00020 12 crop
python3 tools/encoder/encode.py  /tmp/fr_00020 out.dlx --profile sasi --preview p.png

file	role
extract.py	.m2ts -> 256x192 PNGs, 12fps, spatial-only denoise
vq.py	palette, blockify, hand-rolled k-means (no sklearn on this box), PSNR
vq_hybrid.py	the codec: 4 block modes + lagrangian mode decision
ratectl.py	SASI/SCSI profiles, leaky-bucket rate control
encode.py	CLI + DLX1 container writer

DLX1 container layout is documented in the encode.py docstring. All multi-byte fields are big-endian so the 68000 reads them with a plain move.

Known encoder gaps

• Rate control is written but not yet wired into encode.py — the CLI uses a fixed lam from the profile. ratectl.encode_rate_controlled() exists and builds a lam-ladder per frame; it needs hooking up and validating.
• Payload is deliberately NOT entropy-coded — deflate decode does not fit in the 68000's frame budget (FINDINGS 17.2). Do not "optimise" this later.
• Palette packing is not implemented in the encoder. It still emits 24-bit palettes; the X68000 word packing happens Lua-side. Whatever writes real palette words must pick I per entry by minimum squared error (FINDINGS 23.3, worth 1.96 dB) and reserve index 0 as black with I = 0 (FINDINGS 23.4).
• Codebooks are per-scene and rebuilt from scratch; no inter-scene reuse.
• _paint is a Python per-block loop — fine for prototyping, slow for a full disc encode. Vectorise before the 224-stream run.

---

Working setup (unchanged from session 1, re-verified)

MAME ROMs — ~/mame/roms/x68000.zip. Must pass -bios ipl10.

mame x68000 -bios ipl10 -video none -sound none -nothrottle -seconds_to_run 3

Assembler — tools/vasm/vasmm68k_mot -Fbin -o out.bin in.s

Blu-ray — udisksctl loop-setup -r -f DRAGONS_LAIR.iso -> /media/reala-misaki/BDROM (still mounted as of end of session 2).

MAME Lua harness — tools/bench/*.lua, working. Three gotchas (retain the notifier subscription in a global; the stack register is SP not A7; autoboot_script fires at PC=0 before boot) are documented in FINDINGS.

Two shell traps, both hit again this session:

• piping MAME (or any long job) through grep block-buffers — write to a file.
• pkill -f <pattern> matches your own shell and kills it (exit 144). Use pkill -x or kill by PID.
• until ! pgrep -f foo.py; do sleep; done watcher loops never exit. The watching shell's own command line contains the string foo.py, so pgrep -f matches the watcher itself and the loop spins forever. Session 2 left 11 of these wedged for over an hour. Wait on the PID (while kill -0 $PID) or on a sentinel file the job touches when it finishes -- never on a -f name match.
• timeout N mame ... does not kill MAME. MAME catches SIGTERM and, with an autoboot script blocked waiting on a flag that never arrives, never reaches its shutdown path. timeout without -k then waits forever while MAME burns a full core at -nothrottle. Always timeout -k 5 N.

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Disk throughput benchmark — still blocked, no longer gating

IOCS _B_READ returns -1 uniformly. Full diagnosis and the four untested hypotheses are in session 1's notes (git history of this file, commit 65112b9); the ordered plan for retrying is in docs/BENCHMARK.md.

Status changed twice this session — read this rather than the git history. It was briefly promoted to critical-path while the working bandwidth figure was misread as 4 MB/s. With the correct figure (4 Mbps = 488 KB/s) and the ring-buffer simulation showing zero required prefill for both profiles (FINDINGS 21), the design no longer hangs on it. Pixel-exact on SCSI is not available at 4 Mbps — it needs 92-97% of the pipe — so there is no longer a "measure it and maybe ship transparent" decision waiting.

What the benchmark is still worth doing for:

• Confirming the 4 Mbps figure. It is user-supplied and its provenance is not recorded. Every profile hangs off it.
• Confirming DMA is actually used. If transfers fall back to PIO the CPU cost rises far above the ~12-15% cycle-steal estimate and CPU becomes the binding constraint. This is the worst plausible outcome and the cheapest to check — do it first.

Do not try to get the bandwidth number out of MAME. Its SCSI/SASI devices are functional models, not timing-accurate; a KB/s figure from MAME measures the emulator's scheduler. docs/BENCHMARK.md covers the three-tier approach (MAME validates the path, derivation bounds it, real hardware settles it).

Display path — VERIFIED (session 3), in a real mode (session 4), by 68000 code (session 5).

The first real frame is on screen: docs/images/x68k_first_frame_compare.png.

Session 5 closed the gap this paragraph used to describe. GVRAM is now filled by 68000 instructions and the result is still pixel-exact, and the blit cost is measured rather than estimated: 53.6% of a 12fps frame, not 38% (FINDINGS 24). The remaining caveat is different and narrower: MAME models no GVRAM wait states, so 53.6% is a floor and real hardware is worse. Full write-up in FINDINGS 22. Harness: tools/bench/show_frame.lua + tools/bench/prep_frame.py.

Three facts the player MUST honour, none of which were guessable:

what	where	value
Un-hide the graphics layer	CRTC R20 $E80028	clear bit 11 ("G-VRAM set to buffer"); IPL leaves 0x0B16
Colour setup (256c)	CRTC R20 bits 9-8	0x0100
Monitor contrast	$E8E001 bits 3-0	IPL leaves 14; write 15 or everything renders 7% dark

The R20 = 0x0116 value quoted here in session 3 is the 768-wide IPL timing with the gate cleared. The shipping value is R20 = 0x0110 — see the mode table in tools/bench/crtc_mode.lua, which is now the single source of truth for all of R00-R08 and R20.

Bit 11 is the one that cost the most time: GVRAM writes land and read back correctly while the layer is invisible, so the video controller looks guilty and is not. Contrast 0 blanks the screen — free fade-to-black for transitions.

Palette format is now confirmed from MAME source, not assumed: GGGGGRRRRRBBBBBI (G 15:11, R 10:6, B 5:1, shared LSB I), expanded as pal6bit((field<<1)|I). With contrast at 15 the render is pixel-exact.

Ceiling: the 15-bit+I palette costs 40.81 dB against the 24-bit palettised source, once I is chosen per entry (FINDINGS 23.3 — session 3's 38.88 dB assumed I = 1). Still the same order as the scsi profile's own codec error (39.4 dB), so scsi remains near display-transparent, with ~2 dB more headroom than session 3 thought.

Snapshot recipe that works (-video none CANNOT snapshot):

SDL_VIDEODRIVER=dummy mame x68000 -bios ipl10 -video soft -window \
  -sound none -nothrottle -plugins -autoboot_script <script>.lua \
  -snapshot_directory ./snap -snapview native -seconds_to_run 6

-snapview native drops MAME's LED artwork and gives a clean 768x512 screen.

Next steps, in priority order

1. Measure the non-SKIP block fraction. (new top priority, session 5) FINDINGS 24.5: compose-in-RAM-then-blit costs a flat 53.6% of the frame budget; decode-direct-to-GVRAM costs 76.6% x (fraction of blocks that are not SKIP) and needs no RAM reference frame. They cross at 70%. Which side of 70% the content sits on decides which decoder inner loop to write, so this must come before writing one. It needs no new machinery — the mode decision in vq_hybrid.py already computes it per frame and simply never reports it. Add the histogram (SKIP / V1 / V4 / RAW counts per frame) to encode.py output and run it over the clips already extracted. Report the distribution, not the mean: a scene-cut frame is ~100% non-SKIP and a held frame near 0%, and the mean of those two is a number describing no actual frame.

2. 68000 decoder skeleton, with the inner loop chosen by (1). Parse DLX1, expand codebooks, blit per block mode. The display path is verified by 68000 code now (FINDINGS 24) and the harness pattern is tools/bench/blit.s + blit.lua, which already loads code, masks interrupts, times a loop against a flag, and snapshots the result for verify_frame256.py. Copy that. Assembler: tools/vasm/vasmm68k_mot -Fbin -o out.bin in.s.

2a. Re-budget everything against 53.6%, not 38%. Several downstream figures were derived from the old estimate. The blit alone now eats over half the frame at 12fps in the compose-then-blit design, before any decode, and MAME models no GVRAM wait states so that is a floor. This may reopen questions that were closed against the 38% number — check FINDINGS 17.2's entropy-coding rejection, which was argued as "54% LZ4 with no room beside a 38% blit". The conclusion gets stronger, not weaker, but the arithmetic should be restated.

3. Full-disc survey. Only 4 clips of 1.2-1.7 s out of 224 streams have been measured, and 00146 already runs 23% hotter than 00020. A sustained action sequence is the one thing that could still break the bitrate. Classify menu vs content first (FINDINGS 13) or the averages are diluted by static menus. Vectorise _paint before this run — it is a Python per-block loop. Pairs naturally with (1): the same run produces both numbers.

4. Wire rate control into encode.py. No longer a blocker (FINDINGS 21), but it is what gives a deterministic ceiling over content not yet measured, which was the original reason for choosing VQ. Insurance, not a fix. Pairs with (1).

5. Confirm DMA vs PIO in MAME (see the benchmark section above) — cheap, and the only thing that could still move CPU into the binding position.

6. Resolve the framing question (FINDINGS 12: crop vs squash vs wide). Needs an eyeball against arcade reference, not a measurement.

7. Import the scene graph. SNES project data/events/ (MIT, cleared), cross-checked against DirkSimple (zlib) which transcribed the same data independently — diff them to catch transcription errors before committing any of it to 68000 tables.

8. ADPCM audio. MSM6258, 15.6kHz mono, 7.8 KB/s — already budgeted in ratectl.py, not yet extracted or encoded.

Explicitly abandoned — do not re-propose

• Entropy-code the payload. Deflate decode is ~216% of the frame budget on a 68000; LZ4 is ~54% with no room beside a 38% blit (FINDINGS 17.2). All bitrates are raw payload. This also demotes the "247 KB/s lossless" figure in FINDINGS 8 to a compression upper bound, not a shippable design.
• k=1024 codebooks. False-good result from a rate model that charged 1 byte for a 10-bit index (FINDINGS 14). k=256 wins at every matched bitrate.
• Flat 4x4 VQ. Rejected by eye (FINDINGS 9).

Not yet started

• Any 68000 player code. src/player/ is still empty. 68000 code has now drawn a frame, but it lives in tools/bench/blit.s as a benchmark, not in a player: it does no bitstream parsing, no mode dispatch, no codebook expansion.
• ADPCM audio extraction/encoding
• Disk image packaging
• Game logic (scene branching, input windows, death clips)

Reproducing the 256x256 mode result (session 4)

python3 tools/encoder/extract.py 00020 tmp/fr_00020 12 crop
python3 tools/bench/prep_frame.py tmp/fr_00020 tmp/frame256.bin 0 --reserve-black
mkdir -p tmp/snap256 && cd tmp && SDL_VIDEODRIVER=dummy timeout -k 5 90 mame x68000 \
  -bios ipl10 -video soft -window -sound none -nothrottle -plugins \
  -autoboot_script ../tools/bench/show_frame256.lua \
  -snapshot_directory ./snap256 -snapview native -seconds_to_run 6
cd .. && python3 tools/bench/verify_frame256.py

Exits non-zero on any drift. Expected: 256x512 native, double-scan exact, active 256x192 pixel-exact, letterbox true black, ceiling 40.81 dB.

Reproducing the display result

python3 tools/encoder/extract.py 00020 tmp/fr_00020 12 crop
python3 tools/bench/prep_frame.py tmp/fr_00020 tmp/frame.bin 0
mkdir -p tmp/snap_verify && cd tmp && SDL_VIDEODRIVER=dummy mame x68000 -bios ipl10 \
  -video soft -window -sound none -nothrottle -plugins \
  -autoboot_script ../tools/bench/show_frame.lua \
  -snapshot_directory ./snap_verify -snapview native -seconds_to_run 6
cd .. && python3 tools/bench/verify_frame.py

Verified cold from the Blu-ray at end of session 3: exact match, 38.88 dB. (That 38.88 is correct for this test: show_frame.lua still packs I = 1. The 40.81 dB ceiling comes from show_frame256.lua, which picks I per entry.)

tmp/ is gitignored scratch. The frames are NOT in the repo — regenerate them with extract.py; the earlier ones lived in /tmp and do not survive a reboot.

Reference material on this box (not in the repo)

• MAME 0.277 source: ~/src/mame-mame0277/ (tarball ~/src/mame0277.tar.gz). Downloaded this session to settle the graphics-layer question. The files that matter are src/mame/sharp/x68k_v.cpp, x68k_crtc.cpp, x68k_crtc.h, x68k.cpp. Read these before theorising about X68000 video behaviour — six register-poking attempts failed against a gate that one grep found.
• Blu-ray mounted at /media/reala-misaki/BDROM via udisksctl loop-setup -r -f DRAGONS_LAIR.iso.

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Parked ideas (not scheduled, not abandoned)

• Cliff Hanger, retitled as Lupin III (user, session 4). Stern's 1983 laserdisc game was cut from Castle of Cagliostro and Mystery of Mamo with the Lupin branding stripped; a port would restore it. Technically cheaper than this project: same content class (cel animation, flat colour, hard cuts), ~13 min of footage vs Dragon's Lair's ~22, and flatter linework than Bluth's, so fewer blocks should escape to V4/RAW. The codec, the display path, and crtc_mode.lua would all drop straight in. The real cost is media prep, not code: there is no clean master cut to Stern's scene boundaries the way DRAGONS_LAIR.iso is, so the footage would have to be sourced and cut to match. Not to be started until the CPU path is proven — it changes nothing about whether this design works.

Reproducing the blit measurement (session 5)

python3 tools/encoder/extract.py 00020 tmp/fr_00020 12 crop
python3 tools/bench/prep_frame.py tmp/fr_00020 tmp/frame256.bin 0 --reserve-black
tools/vasm/vasmm68k_mot -Fbin -o tmp/blit.bin tools/bench/blit.s
mkdir -p tmp/snap_blit && cd tmp && SDL_VIDEODRIVER=dummy timeout -k 5 900 mame x68000 \
  -bios ipl10 -video soft -window -sound none -nothrottle -plugins \
  -autoboot_script ../tools/bench/blit.lua \
  -snapshot_directory ./snap_blit -snapview native -seconds_to_run 120

~25 s wall. Prints cycles/frame and % of a 12fps budget for V1-V4, and snapshots V1's output. To check that snapshot is still pixel-exact: sed 's|snap256|snap_blit|' tools/bench/verify_frame256.py | python3 -

Not added to check.sh: check.sh asserts pixel-exactness, and asserting wall timings there would make the green-light check sensitive to host load.