Dragon-s-Lair-X68k/README.md

# Dragon's Lair — Sharp X68000 port

Porting Dragon's Lair to a stock X68000 (68000 @ 10MHz, 2MB, SCSI).

This is fundamentally a **video codec problem**, not a game-logic problem: the
game logic is a scene table with branching input windows; the difficulty is
pushing ~22 minutes of Don Bluth animation through a 10MHz 68000.

**And the binding resource is the 68000's local BUS, not its clock.** The
decoder occupies 86.7% of it once instruction prefetch is counted, and 52 of the
53 frames that miss the 12fps budget miss it on the bus, not the CPU
(FINDINGS 38). Read that before optimising anything for cycles.

The **literal span with a fine tail** (v7) is now IN the player: `decode.s`
paints it, pixel-exact under both CPU cores, and it costs inside the decoder
what `blit.s` said it would to 0.2% (FINDINGS 41).

**It only pays if the stream is allowed to run near the pipe.** Spans buy the
68000's deadline with bytes, and at the 280 KB/s profile the mode decision has
already spent them: 77/120 frames over budget against 86 without spans. Given
the full 488 KB/s pipe it is **34/120, and 0.36 dB better** — so the next
decision is a rate point, not an optimisation (FINDINGS 41.2, docs/STATUS.md).

**Green-light check:** `./tools/bench/check.sh` (~3 min, needs the Blu-ray
mounted) re-runs both display regression tests, the rate-control drift test, the
display-path coherency counterexample and a 120-frame 68000 decode, then prints
`ALL GREEN`.

## Read first
- **`docs/FINDINGS.md`** — measured hardware facts, content statistics, codec
  decision, and a section on measurement traps that produced three separate
  false results. Read §4 before trusting any pipeline number.
- **`docs/STATUS.md`** — current state, working setup, blockers, next steps.
  **Start here.** It also lists what has been explicitly abandoned, so old ideas
  do not get re-proposed.
- **`docs/BENCHMARK.md`** — how to measure the storage subsystem, and why a
  bandwidth figure out of MAME would be meaningless.
- **`docs/HARDWARE.md`** — X68000 GVRAM/CRTC reference.

## Layout
```
docs/            findings, status, hardware reference
tools/analysis/  measurement scripts, numbered in the order they were written
                 (01/02 marked BROKEN deliberately, kept as regression refs).
                 Run from the repo root — they import from tools/encoder/.
                 07 finds the hottest sustained window in a stream; 08 renders
                 source | decoded | block-mode map as .webm; 09 is the
                 rate-control drift gate (FINDINGS 26/27) and is part of
                 check.sh -- it exits non-zero if the encoder ever again
                 reports a reconstruction no decoder would produce.
                 10 is a COUNTEREXAMPLE, and exits non-zero by design: it
                 demonstrates that the two-display-path plan of FINDINGS
                 24.5/25.6 corrupts 70 of 120 frames (FINDINGS 28.1).
                 11 scores a container against the MEASURED per-mode block
                 costs without needing MAME; 12 prices the literal-span mode of
                 FINDINGS 30 against those same mode maps, and prints whether a
                 scene cut still fits at 12fps; 13 measures what fitting the
                 CPU budget costs in dB (FINDINGS 31) and caches H.build so the
                 search loop is seconds, not minutes.
                 14 prices the HD63450 array-chain against the v6 and v7
                 spans (FINDINGS 39/40) and prints the sensitivity that decides
                 it -- v7 is measured, and takes 37 of the 43 frames the DMAC
                 would, so the DMAC stays dropped;
                 15 measures how much of the 68000's LOCAL bus the decoder
                 occupies (FINDINGS 38) and exits non-zero if its derived
                 model stops matching the harness's measurement.
                 16 is the DLX3 span container ROUND-TRIP gate (part of
                 check.sh): it encodes, writes the container, reads it back with
                 the reference decoder and fails if a pixel differs -- or if it
                 emitted too few spans to have tested anything. 17 prices the
                 spans the encoder ACTUALLY emitted, with no selection model,
                 which is what 12 and 14 could only simulate.
                 buscost.py is the shared bus-cycle table both import; the
                 per-BLOCK constants live in tools/encoder/vq_hybrid.py and are
                 imported, never copied (session 12 corrected one of them).
tools/bench/     MAME Lua injection harness + 68000 benchmark sources.
                 `check.sh` re-runs both display regression tests (~40 s).
                 `blit.s`/`blit.lua` time the full-frame GVRAM blit on the
                 68000 itself (FINDINGS 24) — not part of check.sh, because
                 wall timings would make the green-light check host-sensitive.
                 `span.sh` (prep_spans.py + span.lua + blit.s v5/v6/v7)
                 measures the literal-span mode the same way (FINDINGS 30 and
                 40, ~30 s); it also asserts that every one of its 36 timing
                 configs drew a pixel-exact frame, the count taken from the
                 generated metadata so a new config cannot weaken the gate.
                 v7 is v6 with a second, 2-pixel chain for the span tail:
                 66.0 cycles/span + 9.143 per coarse pixel + 9.978 per fine
                 pixel, MEASURED, which is the win FINDINGS 39.4 predicted.
                 `crtc_mode.lua` is the single source of truth for CRTC R00-R08
                 and R20 — do not write CRTC values anywhere else.
                 `prep_dlx.py`/`decode.lua`/`verify_decode.py` load, time and
                 verify `src/player/decode.s`; the verify pass is in check.sh.
tools/bench/c68k/ headless px68k C68K harness -- a SECOND emulator for every
                 68000 cycle figure (FINDINGS 37). Links only px68k's CPU core:
                 no SDL, no ROMs, no emulated machine. `make PX68K=~/src/px68k`
                 then `run.sh`; `verify_c68k.py` checks the decode is
                 pixel-exact, which is what licenses the cycle numbers. It also
                 counts BUS cycles, which MAME cannot report.
                 The Makefile's -no-pie and the harness's MAP_32BIT arena are
                 load-bearing: C68K truncates host pointers to 32 bits.
tools/vasm/      vasm m68k assembler (built from source)
tools/encoder/   hybrid VQ encoder + DLX3 container writer (working).
                 spans.py is the v7 span geometry, selection and serialiser, and
                 the single place the chain layout is stated on the encoder side
                 -- it must match blit.s/decode.s (11 coarse units of 24 px, 11
                 fine of 2).
                 DLX2 4-byte-aligns every frame record: an odd `move.l` is an
                 ADDRESS ERROR on a 68000, not a slow read (FINDINGS 28.3).
                 dlx.py is the reference DECODER -- ground truth for the 68000.
src/player/      decode.s: the 68000 DLX3 decoder. Pixel-exact under MAME and
                 px68k's C68K core, blocks and v7 literal spans both. The span
                 pass is blit.s v7 verbatim -- the same instruction sequence the
                 66.0/9.143/9.978 fit was measured on, so do not tidy it.
                 See FINDINGS 28, 31, 40 and 41.
assets/          extracted frames/audio (gitignored)
```

## Encoder

```
python3 tools/encoder/extract.py 00020 /tmp/fr 12 crop
python3 tools/encoder/encode.py  /tmp/fr out.dlx --profile scsi --preview p.png
```

**Two budgets, not one.** `--kbps` is the quality rate point and `--span-kbps`
is the ceiling the span pass may draw on. They are different things: the profile
is chosen, the pipe is hardware, and bytes between them buy a better picture if
spent on `lam`, the 68000's deadline if spent on spans, and nothing if left
unspent. Spans run before `mu` because a span pays in bytes and `mu` pays in
picture (FINDINGS 41.2).

**One profile: `scsi`, 280 KB/s.** The 110 KB/s `sasi` profile was dropped in
session 9 on capacity, not bandwidth — a SASI volume is limited to 40 MB, and
the game's 22.8 minutes of footage is 146 MiB even at that rate (FINDINGS 32).
The rate point may return under another name once the delivery medium is
settled, because a 1x CD-ROM sustains ~150 KB/s and CD-ROM is the only period
medium with the capacity.

The profile bitrate is a **ceiling**: lam is bisected per frame under a leaky
bucket, so the profile's `lam` is a quality floor rather than a setting
(`--fixed-lam` opts out).

There are **two** ceilings, on two different axes. The second is the 68000's
decode budget: `mu` is bisected per frame against 833,333 cycles so the frame
also *decodes* in time, which takes the worst sustained window from 37 frames
over budget to 1 for 0.62 dB at `scsi` (FINDINGS 31). It is on by default; `--no-cpu-fit`
restores session 7 behaviour. Unlike bytes, cycles have no bucket — there is no
double buffer to decode ahead into, so it is a hard per-frame ceiling. The codec is
a Cinepak-style hybrid: each 4x4 block is coded as SKIP, one 4x4 codeword, four
2x2 codewords, or RAW literal pixels, chosen per block by rate-distortion.

The RAW escape means `lam=0` is pixel-exact against the palettised frame, so the
quality knob spans lossless to heavily-compressed without changing the bitstream.

Profiles are derived from a bandwidth figure, not chosen by eye:

```
python3 tools/encoder/profile_gen.py --bw-mbps 4 --name scsi
```

> **On reading `docs/FINDINGS.md`:** it is append-only and several later sections
> overturn earlier ones. Superseded sections carry a blockquote at the top
> pointing to the correction — heed those, especially 18 (reversed by 21).

Source media (`DRAGONS_LAIR.iso`) and ROMs are gitignored — supply your own.

**Not every large stream is game footage.** `00216` is the feature with a
burned-in commentary picture-in-picture and `00215` is the commentary itself —
the two largest files on the disc. The clean 9.4-minute animation is **`00223`**.
See FINDINGS 25.1 before running any size-ranked survey.