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Pace the ring, then read the DMAC config out of the IPL ROM: audio is cheap and the disk is not

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Two sessions that were never separated in the working tree, so they land as one
commit. check.sh ALL GREEN before and after both.

SESSION 19 -- the ring rig gets a frame clock (FINDINGS 51).

src/player/stream.s had no frame clock: it asked for record i the instant it
finished i-1, outran any finite pipe, and never let the ring back up. The 49.1
sweep passing at 48 KB was therefore a wrap-correctness result and nothing else.
PACE/PACEON ($18034/$18038) hold the decoder to 12 fps, so FR_HEAD-FR_TAIL
finally means what it reads as: whole frames the decoder could still draw with
delivery stopped dead. PACEON=0 free-runs and is what the wrap gate still uses,
so every figure in 49 is unmoved.

Paced, on the gate container: 64 KB holds 2 frames, 256 KB holds 7-8, 512 KB
holds 14-15, all pixel-exact. Tolerance is ceiling-1, measured by cutting the
pipe: 256 KB buys 500 ms of dead pipe, not 583.

SLACK IS ACCUMULATED, NOT OWNED. It is built out of pipe-wire and a seek spends
all of it. At 488 KB/s a 256 KB ring needs 4.83 s of play to reach its ceiling
from empty; 512 KB needs 8.42 s to reach 14. A bigger ring raises the ceiling
AND lengthens the climb, so a branch point does not ask "is the buffer big
enough" but "has there been enough play since the last one" -- and Dragon's
Lair's decision points are seconds apart. The rig now also says WHICH resource
is binding: at 460 KB/s every ring from 192 KB to 512 KB is rate-bound at
ceiling 4 and never fills, so larger rings are dead RAM in that scene.
20_seek_slack.py is the same model rewritten in Python from record sizes,
sharing no code with the Lua producer: 35/35 ceilings inside its bracket.

SESSION 20 -- the DMAC configuration was in the IPL ROM the whole time
(FINDINGS 52).

ROADMAP's "do this first" was to put the ADPCM stream on the bus. That needs a
clocks-per-byte figure for the audio channel, and 11_cpu_budget.py was charging
audio the DISK's rate -- 5 clk/B, its own help text calling it "single-address,
bus held". Audio was being charged the favourable end of B3, a 242 KB/s open
question.

It never had to be a guess. The IPL ROM programs all four HD63450 channels
itself and MAME boots the rig with it, so 21_iplrom_dmac.py reads the
configuration out of the image and decodes the MC68450 fields. Eight
(address, expected bytes, meaning) sites; a mismatch or an unknown revision
exits non-zero. In check.sh, no emulator, milliseconds.

ch3 DCR=$80, OCR=$32: dual address, 8-bit port, cycle steal WITHOUT hold,
REQG=10 external request. The DMAC arbitrates once per byte with no burst to
amortise the 5..8 + 2 over, so an audio byte is 16..19 clocks, not 5 -- the old
debit was 3.2x..3.8x small. And on the bus it is still nothing: 651 B/frame is
1.25%..1.48% of a frame, about 4% of what the decoder leaves. P6's bus risk
does not materialise. The unit worry was worth checking and nearly right: 15.6
kHz is 8 MHz/512 = 15,625 samples/s, two 4-bit samples to a byte = 7,812.5 B/s
exactly, and AUDIO_KBPS=7.8 is that in decimal kB while the tool multiplied by
1024.

THE DISK CHANNEL IS PROGRAMMED IDENTICALLY. ch1 (SASI) is DCR=$80 too, and so
is ch0. That is 16..19 clocks per delivered byte, where 42.4 brackets W at 5..12
and 42.5 has W=8 already missing 47/120 frames. The only worked example of a
disk DMA configuration on this machine sits above the entire bracket, and at
that price nothing fits at any container size. It is not scsiexrom.bin so B3
stays open -- what changed is that a cheap configuration is now the thing that
has to be SHOWN. W <= 12 is a requirement on the player's DMAC programming, not
a range the hardware hands us, and it is now the largest open number in the
project, ahead of the rate.

An unforced cross-check fell out: 15_bus_occupancy.py's new W sweep puts W=8 at
105.7% of the frame, agreeing with 42.5's 47/120, from mode histograms and bus
clocks respectively, two models sharing no code.

Also: ADPCM outranks the disk at the arbiter (CPR 1 against 2), so an audio byte
never waits and a video byte does -- relevant to 51's smooth-rate delivery model.

README MEDIA.

stream.lua gains DLX_SNAP_EVERY=1 (needs DLX_PACE, off by default, on no path
check.sh takes) and tools/media/make_readme_media.py turns the PNGs into
docs/img/. The stills and both clips are MAME's own screen pixels.

Building it turned up something worth recording. 116 of 119 captured frames are
pixel-exact against dlx.py; three are TORN -- frame n on top, frame n-1 below
the tear line -- because MAME captured the screen while the block loop was
partway down it. decode.s writes straight to the displayed page (one display
path, 28.1), so a real player tears the same way, and this is the first time
that consequence has been visible rather than argued. The script ASSERTS the
tear and refuses to build otherwise, rather than trimming three frames and
reporting "every frame I kept is exact". Second correction the capture forced:
the snapshot fires before frame n is decoded, so the obvious reading is that it
holds frame n-1 -- it does not, because MAME renders the screen at the end of
the machine frame, by which time the 68000 has finished frame n.

11_cpu_budget.py's "validated to within 1 pt" line is also corrected: the model
reads 2..10 pt HIGH and by more as the frame gets harder, which was already true
before either session.

src/player/decode.s is unchanged; decode.bin is still 1,296 B at the same MD5.

Claude-Session: https://claude.ai/code/session_01194oWYW8DQXK1SZ2DnChW6
This commit is contained in:
prosolis
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# Roadmap — remaining work to a completion target
Written end of session 19 (2026-08-24), against a tree that is ALL GREEN.
**THE COMPLETION TARGET IS M3, THE VERTICAL SLICE** (USER DECISION): one scene
tree — a decision point, two outcomes, a death clip — with audio, streaming from
a real SCSI volume on a stock 2 MB machine, playable. That is the point at which
every layer of this design has been shown to work at once. M4 is listed because
it is real work, but past M3 it is content grinding rather than open questions.
`docs/STATUS.md` remains the session-by-session record and the handoff. This file
is the shape of what is left; where the two disagree about what is done, STATUS
is the one with the measurements and this one is the one that goes stale. Both
were wrong about two encoder gaps until this file was written — see "What was
already done" below.
---
## Status of the four resources
The project's own framing, restated because every item below is priced in one of
these units:
| resource | state |
|---|---|
| **68000 local bus** | the binding one. Decoder occupies 86.7%; 52 of 53 missed frames miss on the bus, not the clock (FINDINGS 38). |
| **68000 clocks** | measured, and the rate controller binds on them. |
| **Delivery rate** | **no working figure, deliberately** (FINDINGS 50, USER DECISION). Every tool REQUIRES an explicit rate. |
| **Seek time** | **no figure at all, and never had one.** 51.3/51.4 made it matter. |
| **W, clocks stolen per delivered byte** | bracketed 5..12 (42.4); the IPL ROM's own disk channel is **16..19** (52.5). **The largest open number in the project.** |
---
## What was already done, and was still on the list
Found while inventorying for this file. Both had been closed in code for several
sessions and were still listed as open gaps in `docs/STATUS.md`:
- **4-byte record padding.** `DLX2`, `encode.py:139-156`, inside rate-control
accounting, reported per frame and per second.
- **CPU cost in the mode decision.** `vq_hybrid.py:218`, priced against measured
per-mode cycles with the exact clustered SKIP rule.
Both entries are now struck in STATUS. **The lesson is procedural: a gap list
that is only ever appended to manufactures phantom work.** Anything crossed off
below should be crossed off in STATUS in the same sitting.
---
## Blocked on hardware this tree does not have
None of these block M2 or M3 software work, because session 18 forced every rate
to be an explicit argument. They set constants, and two of them decide how much
headroom the finished player has.
**B1. Measure the BlueSCSI — throughput AND seek time.**
Throughput has an acceptance test already derived from real record sizes:
**513.2 KB/s** for the session-14 candidate, **451.4 KB/s** for the gate
container (`19_ring_stream.py`, FINDINGS 49.5). Seek time has nothing.
51.3/51.4 is why the second half matters: slack is *accumulated* out of
`pipe - wire`, so what a branch point costs is set by the rate and the time since
the last branch, not by the ring size. At 460 KB/s every ring from 192 KB to
512 KB is rate-bound and never fills. **Do not substitute a guess** — run at
several explicit rates and report the sensitivity. That is exactly how the
retired pipe constant survived five sessions after 42.1 called it folklore.
**B2. Does buffer mode blank the display?** `probe_bit11_blank.lua` is written
and settles it in minutes on a real board. FINDINGS 48 shifted the prior toward
MAME and toward "unusable" — **do not pre-build on 1.0 B/pixel**. Same sitting:
the priority register `0xE82500` at `0x0000` (47.3).
**B3. Single-address vs dual-address DMA.** 242 KB/s and 0.69 dB. Needs
`scsiexrom.bin` (8 KB, CRC `7be488de`) sourced, then its DMAC init disassembled
for DCR's DTYP: `10`/`11` = single (5.0 clk/B), `00`/`01` = dual (9.0).
FINDINGS 48.4. Not on this machine (checked, session 18).
**This is also P4's input** — the handshake the player drives is the same
question from the software side.
> **Session 20 moved the prior hard, and it moved the wrong way (FINDINGS 52.5).**
> The IPL ROM *is* on this machine, and `tools/analysis/21_iplrom_dmac.py` reads
> its HD63450 setup: the on-board disk channel (ch1, SASI) is `DCR = $80` —
> **dual address, 8-bit port, cycle steal WITHOUT hold**, with `REQG = 10`
> external request, i.e. a full bus arbitration per byte. That is **16..19
> clocks per delivered byte**, above the whole 5..12 bracket 42.4 costs P4 in.
> Same vendor, same DMAC, same class of 8-bit port — but it is *not*
> `scsiexrom.bin`, so B3 stays open. What it changes is that a cheap
> configuration is now the thing that has to be **shown**, not assumed.
---
## M2 — a player, as opposed to a decoder
`decode.s` draws pixel-exact frames from RAM Lua pre-loaded; `stream.s` decodes
out of a bounded ring fed by a host file on a paced clock. Neither is a player.
**Exit criterion: boots from a real SCSI volume on a stock 2 MB X68000, plays
one scene at 12 fps from disc, no host-file pipe, no Lua in the loop. Silent.**
**P1. Codebook expansion on the 68000.** `dlxload.py:19` expands CB1 to 32 B per
entry and CB4 to 8 B, host-side, because at the time it was a load-time cost that
would have flattered or damned the inner loop. The player must do it: **8 KB +
2 KB per scene**. Note where that lands — *at a scene change, when the ring is
empty because of the seek*. It compounds with 51.3 and should be priced against
the refill climb, not treated as free setup.
**P2. Palette packing on the 68000.** The encoder still emits RGB888; the X68000
word packing is Lua-side. Whatever writes real palette words must pick `I` per
entry by minimum squared error (**1.96 dB**, FINDINGS 23.3) and reserve index 0
as black with `I = 0` (23.4).
**P3. A real frame clock.** `stream.s` has `PACE`/`PACEON` (`$18034`/`$18038`)
but the 12 fps tick comes from the Lua producer. Needs MFP timer or VBL. Keep
`PACEON=0` free-run working — the wrap gate uses it and every FINDINGS 49 figure
depends on it.
**P4. Real transport.** Drive the MB89352 instead of a host file. The `W`
handshake — clocks stolen per delivered byte, bracketed 5..12 by MC68450 Fig
4-25 — is listed in "Decisions locked" as UNDECIDED and as the thing that
decides the project: `W<=6` fits 0/120 frames, `W=8` misses 47/120. It is a
property of how the player drives the SPC, **so it is ours to choose, not to
receive** (FINDINGS 42.4-42.6). B3 informs it.
**Session 20 promoted this to the project's biggest open number.** FINDINGS 52.5
found the only worked example of a disk DMA configuration on this machine — the
IPL ROM's own — sitting at 16..19 clk/B, outside the bracket entirely, where the
whole design fails at any container size (`15_bus_occupancy.py` sweeps it).
`W <= 12` is now a **requirement on the player's DMAC programming**, not a range
the hardware hands us, and demonstrating a configuration that meets it is P4's
first job rather than its last.
**P5. Seek and branch.** Per-record index (the `aligned` producer needs one
anyway, 49.3), prefill policy, and the accumulated-slack rule from 51.3 made
explicit in the player rather than implied by the rig.
**P7. Boot.** The player as an executable loading from the SCSI volume.
---
## M3 — the vertical slice, and the completion target
**Exit criterion: one decision point, two outcomes, a death clip, with audio,
playing from disc on stock hardware.**
**P6. Audio — and it is the largest unpriced risk left in the project.**
MSM6258 ADPCM, 15.6 kHz mono, **7.8 KB/s**. That figure is in `ratectl.py`'s
budget and nowhere else: not extracted, not encoded, not interleaved into the
container, and **never priced on the bus**. Two reasons to treat it as a risk
rather than a task:
1. A second DMA consumer attacks **the bus** — the resource this project already
established is the binding one, at 86.7% occupied. Clock headroom says
nothing about whether it fits.
2. 7.8 KB/s is a *byte* figure. The last time a byte/word unit error went
unexamined in a delivery budget it cost the project a 2x error in every table
since FINDINGS 5 (session 14, the MB89352 being an 8-bit SPC).
~~**Price it before writing it**: add the ADPCM DMA stream to `15_bus.py` and see
what it does to the 86.7%.~~ **DONE, session 20 — FINDINGS 52.** It is in
`15_bus_occupancy.py` and the answer is **1.25%..1.48% of the frame**, about 4%
of what the decoder leaves. The per-byte cost is no longer a guess borrowed from
the disk: `tools/analysis/21_iplrom_dmac.py` reads the IPL ROM's own HD63450
configuration and finds ch3 dual-address, 8-bit port, cycle steal without hold,
external request — **16..19 clocks per byte**, where `11_cpu_budget.py` had been
charging audio the disk's 5. Both worries above resolve:
1. **The bus concern does not materialise.** A second DMA consumer at 7.8 kB/s
is not what a bus at 88% occupancy is short of.
2. **The unit was checked and is nearly right.** 15.6 kHz = 8 MHz ÷ 512 =
15,625 samples/s, 4 bits each, two to a byte = **7,812.5 B/s exactly**. The
7.8 was decimal kB being multiplied by 1024; 2.4% high, now derived from the
sample rate in `buscost.ADPCM_BYTES_PER_S`.
**What is still open in P6 is everything except the bus:** extraction, encode,
container interleave, and what a second stream does to `wire` — and therefore to
`pipe - wire`, and therefore to 51.3's refill climb. That last one is the
interaction to price next, and it is E2's question with a second consumer in it.
**E6. Container v2** — audio interleave, per-record index, scene table. Depends
on P6's answer and on P5's index.
**G1. Import the scene graph — early, because it is a measurement input.**
SNES project `data/events/` (MIT, cleared) diffed against DirkSimple (zlib),
which transcribed the same data independently, to catch transcription errors
before anything reaches 68000 tables. **Neither is on this box** — both need
fetching.
The reason to pull this ahead of the game logic that consumes it: 51.3 says
4.83 s of play to refill a 256 KB ring at 488 KB/s, and Dragon's Lair's decision
points are seconds apart. **Nothing in this tree can currently say what the worst
gap between consecutive decision points is** — only the scene table knows, and
until it is imported, whether this design survives a back-to-back branch is an
open question nobody is able to ask.
---
## M4 — the whole game
Listed for completeness; past M3 these are scope, not risk.
- **C1. Full-disc survey**, 22.8 minutes. Classify **content / menu / bonus** —
not menu vs content: the two largest streams are bonus material and look like
content by size, duration and bitrate alike (25.1). Run
`07_motion_survey.py` per stream first for a hot-window shortlist.
**Gated by E4.**
- **E4. `H.build` k-means**, 51 s of a 55 s run, once per scene. The thing to
attack before C1, and not anything in the per-frame path (27.6).
- **E2. `--spans all` as default.** Still a recommendation, not a measurement
(43.6.1), and the only loaded lever on the encoder's byte side (44.3). **It
spends every profitable byte, which raises `wire`, which shrinks `pipe - wire`,
which lengthens the refill climb after every branch.** That interaction is not
priced, and M3 is where it becomes measurable.
- **E3. Re-derive span selection jointly with `lam`** (39.3).
- **C2. Framing** — crop vs squash vs wide (FINDINGS 12). Needs an eyeball
against arcade reference, not a measurement. Cheap; blocks only final encodes.
- **C3. Disk image packaging**, ~1.09 GiB at the candidate rate.
- **G2/G3.** Branching, input windows, death clips, attract mode; playtest.
---
## Dependency summary
```
B1 seek+rate ─┐
B3 DTYP ──────┴─> P4 transport ─┐
├─> M2 ─> M3 (COMPLETION TARGET) ─> M4
P1 P2 P3 P5 P7 ─────────────────┘ ^
│
P6 (bus cost DONE, 52) ──────────────────┤
G1 scene graph (fetch, do early) ─────────┘
B2 blanking ─> (page 1; do not pre-build on it)
```
## Standing rules that apply to all of it
- **Green light first and last.** `./tools/bench/check.sh`, ALL GREEN, before and
after. **Never two MAME jobs at once** — session 18 did it, two `decode.lua`
runs shared a log file, and it produced a 0-byte log and 15 wasted minutes.
- **Name the layer.** Emulated, or real hardware. Every progress claim.
- **Label measured / estimated / folklore.** A rate with no provenance is
folklore even when it is plausible, and this project has already paid for that
twice.
- **No new default constants.** Rates stay explicit arguments. If a measurement
is not available, report the sensitivity across several rates rather than
picking one.