# 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, extracted
from 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 — `src/player/stream.s`, no Lua in the decode path. Source on the left,
the machine's screen on the right.
[`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 (one display path, FINDINGS
28.1 — the dual-path plan is kept runnable as a counterexample precisely because
it corrupts frames), 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 actually doing.** The same window with the block-mode map
beside it: **black = SKIP** (costs nothing, draws nothing — the previous frame
stands), **blue = V1** (one codebook index for a whole 4x4 block), **amber = V4**
(four indices), **red = 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
since session 8 the mode decision is charged both bytes *and* cycles, which is
why a byte-rich profile buys its way out to RAW instead of V4.
[`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.
---
**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).
**The delivery path is built and tested too** (FINDINGS 49). `src/player/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 turned out to be **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*, which
is a condition no byte-counting buffer simulation can see.
**And building it caught a live defect**, then cost the project a constant.
The shipping candidate is 496.7 KB/s; the pipe figure the design had been
simulated against since session 2 was smaller, and nothing in the tree was
comparing the two — the rate controller binds on clocks and has no pipe term at
all, while the buffer sizing kept standing on a constant the design had stopped
enforcing.
**So the pipe constant is retired (session 18, USER DECISION).** It was never a
bus measurement — user-supplied, no provenance, 10% of SCSI-1's asynchronous
rating (FINDINGS 42.1). It is gone as a default from every analysis tool and
from `stream.lua`; `--bus` / `--kbps` / `DLX_STREAM_KBPS` are now **required
arguments**, so no table can be scored against a rate its own output does not
state. **There is no working delivery figure, and that is the honest state.**
What replaces it is a requirement rather than a constant:
`tools/analysis/19_ring_stream.py` reports the **zero-prefill pipe**, the rate a
medium must clear for a container to need no prefill. For the candidate that is
**513.2 KB/s** — a hardware acceptance test to measure a BlueSCSI against.
**Bytes are not free, and the number that said they were was in the wrong
unit.** Session 13 found the pipe figure the design was built against was never
a bus figure (SCSI-1 is 1.5 MB/s asynchronous) and concluded the span pass
saturates at ~837 KB/s, 0/120 frames over budget. Session 14 found the disk
debit behind that was charged **per word of stream to a byte-wide port** — the
MB89352 is an 8-bit SPC, so the DMAC pays per BYTE, and the debit is 2x every
table since FINDINGS 5. No 68000 bus cycle is shorter than four clocks, so the
old figure was below a physical floor.
**What survives, re-encoded honestly: 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. The remaining lever is not ours: **whether the CZ-6BS1 wires the SPC's
DACK to the bus's `#EXACK`**, which decides 5 clocks/byte against 9, and with
it 242 KB/s and 0.69 dB. Read FINDINGS 43 before quoting any rate figure.
**And session 20 read the answer the machine already had.** The X68000's IPL ROM
programs all four HD63450 channels itself, and MAME boots the rig with it, so
`tools/analysis/21_iplrom_dmac.py` decodes the configuration straight out of the
image and gates on the bytes still being there. The audio channel is
dual-address, 8-bit port, cycle steal **without hold**, one external request per
byte: **16..19 clocks a byte, not 5** — which prices the ADPCM stream at
1.25%..1.48% of a frame and closes ROADMAP's "do this first" item. The disk
channel is programmed **identically**. That is 16..19 clocks per delivered byte,
above the whole 5..12 bracket the project costs the transport in, and at that
price nothing fits. It is SASI and not the MB89352, so it does not settle the
question — but **a cheap configuration is now the thing that has to be shown,
not the thing assumed.** FINDINGS 52.
**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/ROADMAP.md`** — the remaining work to a completion target, and which
milestone that target is. Read it with STATUS, not instead of it: STATUS holds
the measurements, ROADMAP holds the shape and goes stale first.
- **`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.
18 measures what a 16-colour text-plane literal would cost in
dB, and closes that direction (FINDINGS 46.3).
19 is the RING-BUFFER simulation, and it supersedes 09_buffer_
sim.py's question rather than repeating it: it models the ring's
ADDRESSES, because src/player/ needs each record contiguous and
not merely resident. It reports the ZERO-PREFILL PIPE -- the
rate a medium must clear for a container to need no prefill --
and warns explicitly when demand exceeds supply on the MEAN,
where a "required prefill" figure would flatter a sustained
overrun. Its wrap count and hole size match tools/bench/
stream.lua's, measured on a real 68000, to the digit.
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.
`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
no longer 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.
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, PRELOADED-stream front-end.
stream.s: the same decoder behind a bounded RING (FINDINGS 49).
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 in FINDINGS 24/30/40/41 is fitted to. check.sh
asserts decode.s still assembles to the same 1,296 bytes.
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).
**But at `--spans all` none of that binds.** 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: the rate is set by the span pass and by `mu`,
not by `--kbps` or the bucket (FINDINGS 44.3). Two known unit inconsistencies on
that side are implemented and default OFF because they measure as a wash --
`--joint-decide` (the per-block lagrangian prices a byte at `lam + mu*c` rather
than `lam`) and `--joint-bucket` (the bucket may not lend clocks it cannot
repay). FINDINGS 44.
An encode is ~95% k-means; a 120-frame window is ~29 s, of which ~22 s is
training the two codebooks (FINDINGS 44.5).
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.