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.

## What it looks like

![Blu-ray source next to the 68000's output](docs/img/source-vs-decoded.png)

Left, the Blu-ray frame cropped to 256x192. Right, the same frame **as the
emulated 68000 actually drew it**: 256 colours out of the X68000's 65536, one
16-colour-per-4x4-block codebook, decoded by `src/player/decode.s` from the
container. Not a re-render. These are the pixels MAME had on screen, pulled out
of its own snapshot, 2x nearest-neighbour, no filtering.

**The player, running.** 119 frames out of a **256 KB ring buffer on an emulated
stock 2 MB X68000**, paced to a 12 fps frame clock, streamed from a host file at
488 KB/s by `src/player/stream.s` with no Lua in the decode path. Source on the
left, the machine's screen on the right. (This recording was paced by the host;
the 68000 now keeps that clock itself, off the CRTC's V-DISP, and the same 120
frames decode pixel-exact under it — `src/player/clock.i`, FINDINGS 54.)

<video src="docs/img/player.webm" controls muted loop width="100%"></video>

[`docs/img/player.webm`](docs/img/player.webm) (119 frames, 12 fps, VP9)

116 of those 119 frames are **pixel-exact** against `tools/encoder/dlx.py`'s
reference reconstruction. The other three are **torn**: the top of the picture
is frame *n* and the bottom still holds frame *n-1*, because MAME captured the
screen while the block loop was partway down it. That is not a rig artefact.
`decode.s` writes straight to the displayed page, so a real player tears the
same way. `tools/media/make_readme_media.py` asserts the tear rather than
trimming it: every differing pixel has to come from the previous frame, or it
refuses to build.

**What the decoder is doing.** The same window with the block-mode map beside
it. **Black is SKIP** (costs nothing, draws nothing, the previous frame stands),
**blue is V1** (one codebook index for a whole 4x4 block), **amber is V4** (four
indices), **red is RAW** (sixteen bytes verbatim). The mode mix is what every
cost table in `docs/FINDINGS.md` is really about: V4 costs 1.5x V1, and the mode
decision is charged both bytes *and* cycles, which is why a byte-rich profile
buys its way out to RAW rather than V4.

<video src="docs/img/modes.webm" controls muted loop width="100%"></video>

[`docs/img/modes.webm`](docs/img/modes.webm) (the same 119 frames, with the mode map)

**Name the layer.** Everything above is **emulated**: MAME 0.277 `x68000`,
`-bios ipl10`, stock 10 MHz / 2 MB, cross-checked frame for frame on a second
CPU core (px68k's C68K). Nothing in this project has run on real hardware yet.

## Where it stands

**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 on the bus (FINDINGS 38). Read that
before optimising anything for cycles.

**The decoder works and is measured.** `decode.s` draws blocks and v7 literal
spans pixel-exact under both CPU cores, and costs inside the player what the
standalone blit benchmark said it would, to 0.2% (FINDINGS 41).

**The delivery path works too.** `stream.s` decodes the whole 120-frame window
out of a 256 KB ring on a stock 2 MB machine, final frame pixel-exact, with the
container in a host file rather than preloaded into RAM. The constraint is
**contiguity, not byte count**: the block loop reads with a monotonically
increasing `a0` and no bounds check, so the ring needs the whole next record
resident *and contiguous*, a condition no byte-counting buffer simulation can
see (FINDINGS 49).

**Seek slack is accumulated, not owned.** A ring's lookahead is built out of
`pipe - wire` and a seek spends all of it. At 488 KB/s a 256 KB ring needs 4.83
seconds of play to reach its 7-frame ceiling from empty, and 512 KB needs 8.42
seconds to reach 14, so a bigger ring raises the ceiling *and* lengthens the
climb. A branch point therefore asks "has there been enough play since the last
one", not "is the buffer big enough" (FINDINGS 51).

**There is no working delivery rate figure, deliberately.** `--bus`, `--kbps`
and `DLX_STREAM_KBPS` are required arguments with no defaults, so no table can
be scored against a rate its own output does not state. What replaces a constant
is a requirement: `tools/analysis/19_ring_stream.py` reports the **zero-prefill
pipe**, the rate a medium must clear for a container to need no prefill, which
is **513.2 KB/s** for the current candidate. That is a hardware acceptance test
to measure a BlueSCSI against (FINDINGS 50).

**The largest open number is W, the clocks stolen per delivered byte.** The
MB89352 is an 8-bit SPC, so the DMAC pays per byte rather than per word, which
is a 2x correction the project has already paid for once (FINDINGS 43). What W
costs is set by how the player programs the DMAC: 5 clocks a byte single
address with the bus held, 9 dual address held, 12 single address arbitrating
per byte, 16..19 dual address arbitrating per byte. The design's fate changes
completely across that ladder, and it is ours to choose.

**The one worked example on the machine is expensive.** The X68000 IPL ROM
programs all four HD63450 channels itself, and
`tools/analysis/21_iplrom_dmac.py` decodes that configuration out of the ROM
image and gates on the bytes still being there. Both the audio channel and the
on-board disk channel are dual address, 8-bit port, cycle steal *without* hold,
one external request per byte: **16..19 clocks a byte**, the top of the ladder.
For audio that is a settled figure and a small one, 1.25%..1.48% of a frame. For
the disk it is where nothing fits at any container size. The ROM drives SASI
rather than the MB89352, so it does not settle W, but a cheap configuration is
now the thing that has to be shown rather than assumed (FINDINGS 52).

**The player builds its own codebooks and palette now.** The two load-time
transforms — codebooks to word-per-pixel form, palette to `GGGGGRRRRRBBBBBI`
with the shared LSB picked per entry — ran host-side until session 21 and now
run on the 68000, out of the raw container header, byte-exact against the host
implementation on both CPU cores and with the palette read back out of the
hardware registers. A scene change costs **18.96 ms**, a third of one 12fps
frame slot. The finding underneath it is a cost nothing had counted: a scene
header is **5,920 bytes** that must arrive before frame 0, and in the currency
of seek slack those bytes lengthen the refill climb by 138 ms at 488 KB/s and by
**1.099 s at 451.4 KB/s**, because the surplus they are divided by goes to zero
(FINDINGS 53).

**The 68000 fills its own ring now, and the player's request loop costs more
than the medium does.** `src/player/ring.i` places records, prefills, keeps the
slack rule and seeks, out of a per-record index the container carries (DLX4).
The channel only moves bytes while it has a request and only the CPU can issue
one, so the disc **stands still between records** by an amount the player sets:
at 488 KB/s a one-deep request queue gives away **6.8% of the pipe and underruns
59 of 120 frames**, a two-deep one gives away 3.4% and underruns none — on a
container whose whole surplus over the wire is 8.7% (FINDINGS 55).

**Current encode:** 496.7 KB/s at 29.19 dB, 1 frame of 120 over the 12fps
budget, and that one is frame 0, the intra frame, late on purpose.

**Green-light check:** `./tools/bench/check.sh` (~4 min, needs the Blu-ray
mounted) re-runs both display regression tests, the rate-control drift gate, the
display-path coherency counterexample, a 120-frame 68000 decode on two CPU
cores, the ring and paced-ring passes, the DMAC configuration gate and the
load-time transforms on both cores, then prints `ALL GREEN`.

## Reproducing this

**No media ships in this repo and none of it is redistributable.** Bring your
own Dragon's Lair Blu-ray. Everything else needed to rebuild every number and
every picture above is either here or is packaged.

You need:

| | |
|---|---|
| the disc | loop-mounted read-only: `udisksctl loop-setup -r -f DRAGONS_LAIR.iso`. The tree was built against a decrypted UDF 2.x image. 7-Zip cannot read UDF 2.x, so use the loop mount |
| `python3` | plus **numpy** and **Pillow**, and nothing else. The k-means is hand-rolled rather than pulling in sklearn |
| `ffmpeg` / `ffprobe` | frame extraction, and the clips above |
| **MAME** | tested on 0.277, with the `x68000` ROM set. The rigs drive it headless via `-autoboot_script` |
| vasm (m68k, Motorola syntax) | **vendored**: `tools/vasm/vasmm68k_mot` is a Linux x86-64 binary, with the source tarball beside it to rebuild elsewhere |

Then:

```sh
export DLX_BDROM=/path/to/your/mounted/bluray    # if not /media/$USER/BDROM
./tools/bench/check.sh                           # ~3 min, prints ALL GREEN
```

`DLX_BDROM` is honoured by every tool that reads the disc. Two stages are
optional and **skip rather than fail** when their input is absent, because both
live outside this repo:

- `PX68K=/path/to/px68k` for the second-CPU-core gate. This is the cheapest
  strong test in the tree (seconds, no MAME, no ROMs) and it is what licenses
  the bus and cycle figures.
- `IPLROM=/path/to/iplrom.dat` for the DMAC configuration gate. Defaults to
  `~/mame/roms/iplrom.dat`.

To rebuild the stills and clips in `docs/img/` you also need a paced recording
run; see the header of `tools/media/make_readme_media.py`.

**Scene selection is a hard-coded stream number, not a search.** The gates use
streams `00020` and `00223` of the disc's 224 `.m2ts` files. A different
pressing may number them differently, and if so the green light will extract the
wrong footage rather than fail, so check that `tmp/fr_singe/` looks like the
Singe encounter before trusting any figure.

**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`**
(FINDINGS 25.1).

## 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
```

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.

**Two byte 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).

**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 and
`--no-cpu-fit` turns it off. Unlike bytes, cycles have no bucket: there is no
double buffer to decode ahead into, so it is a hard per-frame ceiling.

**One profile, `scsi`, at 280 KB/s.** The 110 KB/s `sasi` profile was dropped on
capacity rather than bandwidth, since a SASI volume is limited to 40 MB and the
game's 22.8 minutes 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). At `--spans all` none of that binds, though. A
32-frame bucket emits the same container byte for byte as an 8-frame one and
`lam` never leaves its floor on any frame of the reference window, because the
rate is set by the span pass and by `mu` (FINDINGS 44.3). Two known unit
inconsistencies on that side are implemented and default off because they
measure as a wash: `--joint-decide` prices a byte at `lam + mu*c` rather than
`lam`, and `--joint-bucket` stops the bucket lending clocks it cannot repay.

An encode is ~95% k-means. A 120-frame window is ~29 s, of which ~22 s is
training the two codebooks.

Profiles are derived from a bandwidth figure rather than chosen by eye:

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

## Documentation

- **`docs/STATUS.md`** is the current state, working setup, blockers and next
  steps. **Start here.** It also lists what has been explicitly abandoned, so
  old ideas do not get re-proposed.
- **`docs/ROADMAP.md`** is the remaining work to a completion target, and which
  milestone that target is. Read it with STATUS rather than instead of it:
  STATUS holds the measurements, ROADMAP holds the shape and goes stale first.
- **`docs/FINDINGS.md`** is measured hardware facts, content statistics, the
  codec decision, and a section on measurement traps that produced three
  separate false results. Read §4 before trusting any pipeline number. It is
  append-only and later sections overturn earlier ones; superseded sections
  carry a blockquote pointing at the correction.
- **`docs/BENCHMARK.md`** is how to measure the storage subsystem, and why a
  bandwidth figure out of MAME would be meaningless.
- **`docs/HARDWARE.md`** is the X68000 GVRAM/CRTC reference.

## Layout

```
docs/            findings, status, roadmap, hardware reference
docs/img/        the stills and clips above, built from a real emulated run
tools/analysis/  measurement scripts, numbered in the order they were written.
                 Run from the repo root; they import from tools/encoder/.
                 01 and 02 are marked BROKEN deliberately and kept as
                 regression references.
                 10 is a COUNTEREXAMPLE and exits non-zero by design: it
                 demonstrates that the two-display-path plan corrupts 70 of 120
                 frames, which is why decode.s has one display path.
                 15 measures how much of the 68000's local bus the decoder
                 occupies and exits non-zero if its derived model stops
                 matching the harness's measurement.
                 16 is the DLX3 span container round-trip gate: 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.
                 19 models the ring's ADDRESSES rather than its occupancy,
                 because each record must be contiguous and not merely resident,
                 and reports the zero-prefill pipe.
                 20 is an independent Python re-derivation of the seek-slack
                 model, sharing no code with the Lua producer it checks.
                 21 decodes the IPL ROM's HD63450 configuration and gates on the
                 bytes being where it says they are.
                 22 prices a scene change: header bytes, load-time clocks and
                 what both cost in accumulated seek slack, across explicit
                 rates. Its cycle counts are PARSED out of the rig's log, not
                 pasted in, so they cannot go stale silently.
                 buscost.py is the shared bus-cycle table. The per-block
                 constants live in tools/encoder/vq_hybrid.py and are imported,
                 never copied.
tools/bench/     MAME Lua injection harness and 68000 benchmark sources.
                 check.sh is the green light.
                 blit.s/blit.lua time the full-frame GVRAM blit on the 68000
                 itself. Not part of check.sh, because wall timings would make
                 the green light host-sensitive.
                 span.sh measures the literal-span mode the same way and
                 asserts that every one of its 36 timing configs drew a
                 pixel-exact frame, the count taken from generated metadata so
                 a new config cannot weaken the gate.
                 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
                 decode.s. prep_stream.py/stream.lua do the same for stream.s,
                 but lay the container out as a DISK in a host file and feed it
                 through a bounded ring at a modelled pipe rate, so the rig is
                 not bounded by the emulated machine's RAM and a stock 2 MB
                 machine runs the whole window. dlxload.py holds the
                 codebook/palette load-time maths both preps share -- and
                 the reference src/player/load.i is gated against.
                 prep_load.py/load.lua/verify_load.py/load_run.sh run those
                 transforms ON the 68000 and compare all 10,752 output bytes
                 with dlxload.py's, palette words read back out of the palette
                 registers rather than a RAM shadow.
tools/bench/c68k/ headless px68k C68K harness, a SECOND emulator for every
                 68000 cycle figure. 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/media/     builds docs/img/ from a paced recording run
tools/vasm/      vasm m68k assembler, binary plus source tarball
tools/encoder/   hybrid VQ encoder and DLX3 container writer.
                 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 and decode.s: 11 coarse units of
                 24 px, 11 fine of 2.
                 DLX2 4-byte-aligns every frame record, because an odd move.l
                 is an ADDRESS ERROR on a 68000, not a slow read.
                 dlx.py is the reference DECODER, ground truth for the 68000.
                 24 models the ring with the 68000 owning it: the request
                 queue, the poll-only-when-not-decoding rule and 54.4's frame
                 cadence, and reports the pipe the player's own loop gives away.
src/player/      decode.s is the 68000 DLX3 decoder with a preloaded-stream
                 front-end. stream.s is the same decoder behind a bounded ring.
                 load.i is the LOAD-time half: codebook expansion and palette
                 packing, out of the raw container header, with loadgate.s as
                 its rig front-end. Its three scratch tables describe the
                 machine rather than the scene, so they are a separate entry
                 point a player calls once at boot.
                 ring.i is the RING PRODUCER: `aligned` placement, the
                 descriptor ring, the prefill policy, 51.2's slack rule as
                 arithmetic (ring_may_seek) and a seek. It reads the DLX4 record
                 index because a player cannot learn a record's length by
                 walking a stream it has not fetched.
                 Both include frame.i (the block loop and span chain) and
                 geom.i (the constants), so there is exactly ONE copy of the
                 bytes every cycle constant is fitted to. The span pass is
                 blit.s v7 verbatim, the same instruction sequence the
                 66.0/9.143/9.978 clock fit was measured on, so do not tidy it.
                 check.sh asserts decode.s still assembles to the same 1,296
                 bytes.
assets/          extracted frames and audio (gitignored)
```

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