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

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.)

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.

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).

The scene graph is in, and the worst gap between two decision points is zero. tools/import/scenegraph.py imports the arcade scene graph — 40 scenes, 516 sequences, 906 input windows — and 5.4% of the game's 612 branch transitions open an input window on the first frame of a clip the disc seeked to, so two seeks can fall back to back with no play between them. A rule of the form "has there been enough play since the last branch" can therefore be answered no by the content, not by the buffer. It does not break the design: a branch on an empty ring costs the 2-record prefill, 149.7 ms at 488 KB/s, not the climb. What it removes is margin — at that rate in a 256 KB ring, 76% of this game's branch points arrive before the ring has refilled, and a 512 KB ring makes it 90%, because doubling the ceiling does not touch pipe - wire (FINDINGS 56).

Nothing outside-derived is committed here. The scene graph is not redistributable from this tree; it is regenerated from a reader's own clones into gitignored tmp/, and tools/import/scenegraph.py is the single file in the repo coupled to those projects — everything downstream reads DLXSCENE1, this project's own schema, with the sources' attribution carried in it. DirkSimple is zlib (Ryan C. Gordon); the SNES chapter set is MIT (Chad Doebelin) and, by its own README, derived from DirkSimple rather than an independent transcription, which struck a cross-check this project had planned on for eight sessions.

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 imports and gates the scene graph when a DirkSimple checkout is present, 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:

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.
                 25 imports nothing itself: it reads the DLXSCENE1 scene
                 table and reports the worst gap between two decision points,
                 what the input layer has to survive, and what both cost in
                 51.3's accumulated slack across explicit rates.
tools/import/    the ONLY code in this tree coupled to somebody else's source.
                 scenegraph.py reads a DirkSimple checkout (and optionally the
                 SNES chapter XMLs) and writes tmp/scenegraph.json in this
                 project's own DLXSCENE1 schema, with the sources' licences and
                 attribution inside it. Nothing is vendored and the output is
                 gitignored derived data.
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.