reala-misaki
65112b9305
Verified GVRAM is one word-access per pixel in ALL color modes; chose 256-color 256x192 with movem.l bursts (page 1 sacrificed as double-buffer). Measured 8 scenes from the Blu-ray source: blit costs under 8% of the 12fps cycle budget, so I/O is the bottleneck, not CPU. Naive delta+RLE reaches only 3.2:1 (365 KB/s, 470MB) -> decision to use 4x4 vector quantization (~30 KB/s). "Shot on twos" assumption failed: the transfer has zero duplicate frames, so 12fps requires explicit decimation. Documents three false measurement results and their root causes (per-frame Floyd-Steinberg dithering, temporal denoise, exact-match dedupe on noisy source). MAME Lua injection harness works and is reusable for cycle-cost measurement; the IOCS _B_READ disk benchmark is blocked returning -1. Claude-Session: https://claude.ai/code/session_01194oWYW8DQXK1SZ2DnChW6
140 lines
6.3 KiB
Markdown
140 lines
6.3 KiB
Markdown
# Status & next-session handoff — end of session 1 (2026-08-23)
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## Decisions locked
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| decision | value | why |
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|---|---|---|
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| Target CPU | 68000 @ 10MHz (stock) | hardest honest constraint |
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| Display mode | 256 colors, 256x192 in 256x256 CRTC mode | every mode is 1 word-access/pixel, so 256c is free vs 16c |
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| Double buffer | **none** — page 1 sacrificed | enables `movem.l` 24px bursts; delta coding needs a RAM reference frame anyway |
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| Codec | 4x4 vector quantization, per-scene codebook + block delta | CPU is idle, I/O is the ceiling — spend cycles to buy bandwidth |
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| Framerate | 12 fps, **explicit decimation** | source has zero duplicate frames; no free "twos" win |
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| Medium | SCSI HDD image (.hds) | but see SASI/SCSI split below |
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| Emulator | MAME 0.277 x68000 | accurate enough that measured cycles mean something |
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**OPEN QUESTION for the user:** stock 10MHz machines are **SASI**, not SCSI.
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Three options, not yet chosen:
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1. Stock 10MHz + SASI (purist) — VQ becomes mandatory
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2. Stock 10MHz + CZ-6BS1 SCSI board — relieves I/O, keeps CPU honest
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3. Super/XVI baseline — built-in SCSI, still a 10MHz 68000
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Recommendation: make the codec's bitrate ceiling a **build parameter**, so one
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encoder serves all three and the target is chosen at package time.
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---
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## Working setup
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**MAME ROMs** — `~/mame/roms/x68000.zip` (present, working).
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Must pass **`-bios ipl10`**; the default BIOS is `cz600ce`, whose split
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even/odd IPL halves (`rh-ix0897cezz.ic12` / `rh-ix0898cezz.ic11`) are absent.
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`-verifyroms` will still report those two as missing — this is expected and harmless.
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Boots headless at ~430-480% speed:
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```
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mame x68000 -bios ipl10 -video none -sound none -nothrottle -seconds_to_run 3
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```
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**Assembler** — vasm built from source, binary at `tools/vasm/vasmm68k_mot`
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(source tarball alongside it). Verified correct 68000 output.
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```
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tools/vasm/vasmm68k_mot -Fbin -o out.bin in.s
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```
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**Blu-ray** — mount with:
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```
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udisksctl loop-setup -r -f DRAGONS_LAIR.iso # -> /media/reala-misaki/BDROM
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```
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NOTE: this loop mount is still active from session 1. Re-mount if the machine rebooted.
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---
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## MAME Lua harness — WORKING, reusable
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`tools/bench/*.lua` inject 68000 machine code straight into emulated RAM and time
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it against the emulated clock. No bootable disk or OS required. This is the
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measurement rig for all future cycle-cost work (blit timing, decoder benchmarks).
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Pattern:
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```
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mame x68000 -bios ipl10 -video none -sound none -nothrottle \
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-seconds_to_run 30 -plugins -autoboot_script yourscript.lua
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```
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### Three MAME Lua gotchas — all cost real time, all now solved
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1. **Retain the notifier subscription.** `emu.add_machine_frame_notifier()` returns
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a token; if you drop it into a chunk-local it is garbage-collected and the
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callback **silently stops firing**. Assign it to a **global** (`SUB = ...`).
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2. **The stack pointer is `SP`, not `A7`** in `cpu.state[...]`.
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Full list: A0-A6, D0-D7, PC, SP, SR, USP, CURPC, CURFLAGS, IR.
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3. **`autoboot_script` fires at time=0, before boot** (PC=0). Wait until
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`machine.time` >= ~5s before injecting, or IOCS is not yet initialised.
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Also: piping MAME through `grep` block-buffers output — write raw to a file when
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backgrounding, or you will see an empty log and assume a hang.
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And never `pkill -f 'mame x68000'` — the pattern matches your own shell and kills it
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(exit 144). Use `pkill -x mame`.
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---
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## BLOCKED: disk throughput benchmark
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**Goal:** measure real SASI/SCSI KB/s to replace the folklore figures in FINDINGS.md §5.
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**Status:** harness fully working; the IOCS call itself fails.
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`IOCS _B_READ` ($46 via `TRAP #15`; d1.hb=PDA, d2.l=position, d3.l=bytes, a1=buffer)
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returns **`FFFFFFFF` (-1), zero reads**, uniformly across:
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- all 16 PDA values $80-$8F
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- both d1 encodings (PDA in bits 31-24 and bits 15-8)
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- image sizes 10MB / 20MB / 40MB
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The uniformity is the diagnostic: calls are **dispatched and cleanly rejected**,
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so `TRAP #15` and IOCS are reachable. MAME does mount the image
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(`:x68k_hdc: opened image file bench.hdf`).
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**Untested hypotheses, in rough order of likelihood:**
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1. The raw image has no X68000 SASI format, so the IPL's boot scan never registered
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a usable drive and IOCS refuses. Would need Human68k to format one — **we have
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no Human68k image on this system.**
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2. MAME's `x68k_hdc` SASI implementation may be too partial for IOCS-level reads.
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3. `SP=$8000` may put the injected stack on top of the IOCS work area in low RAM.
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Try a much higher stack.
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4. The **SCSI path was never tried** — this is the obvious next move and is more
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relevant to the target anyway:
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`-exp1 cz6bs1 -hard disk.chd` with `exp1:cz6bs1:scsi:0 harddisk`
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(`-listmedia` gains a `harddisk` slot accepting .chd/.hd/.hdv/.2mg/.hdi).
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**Honest assessment: this benchmark is NOT on the critical path.** The VQ codec
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(~30 KB/s) is correct whether SASI does 300 or 600 KB/s. Do not let it block the
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encoder. Its real value is deciding whether the *simpler* row-span codec could
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have sufficed.
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Caveat if resumed: MAME idealizes drive seek latency. That's acceptable because the
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realistic deployment is BlueSCSI/SCSI2SD (SD-backed, no mechanical seek), so what
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gets measured is the bus/DMAC/controller path — the genuine ceiling. The caveat
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only bites for a real period spinning drive.
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---
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## Next steps, in priority order
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1. **Build the VQ encoder** (`tools/encoder/`) — 4x4 blocks, per-scene codebook,
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block delta. Emit sample PNGs for visual evaluation. **The open question is
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whether VQ softens Bluth's ink linework unacceptably — decide by eye before
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committing to the architecture.**
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2. **Full-disc survey** — all 224 streams, not 5s samples, to firm up bitrate
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(current numbers are +/-30%) and map streams onto the arcade scene graph.
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3. **Check the SNES project's license**, then evaluate reusing `data/events/`
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(516 chapters / 29 scenes) as the scene-graph and input-timing layer.
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4. Resolve the SASI/SCSI target question with the user.
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5. Optionally unblock the disk benchmark via the SCSI path.
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6. 68000 player skeleton: CRTC init for 256x192x256c, `movem.l` blitter,
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ADPCM via HD63450 DMA.
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## Not yet started
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- Any 68000 player code
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- ADPCM audio extraction/encoding (MSM6258, 15.6kHz mono, ~7.8 KB/s, ~10MB for 22min)
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- Disk image packaging / container format
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- Game logic (scene branching, input windows, death clips)
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