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Handoff: green-light script, reconciled figures, a smaller first step for the decoder

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Prepares session 4 for handoff. No new measurements; this reconciles the docs
with what session 4 changed and makes the next session's entry point cheaper.

- tools/bench/check.sh re-runs both display regression tests from the Blu-ray in
  ~40 s and prints ALL GREEN. Verified green cold, after wiping tmp/ and
  re-extracting. STATUS and README both open with it, because everything
  downstream assumes the display path is pixel-exact and nothing previously
  checked that in one command.

- Reconciled the figures session 4 invalidated. Session 3's 38.88 dB ceiling is
  struck through in STATUS with a pointer to 40.81; the "three facts the player
  must honour" table no longer quotes R20 = 0x0116, which was the 768-wide IPL
  timing and would have been copied into the player as if it were the shipping
  value. crtc_mode.lua is now named as the single source of truth for CRTC
  registers, in both STATUS and README.

  The 38.88 dB in the session-3 reproduce section is left alone and annotated
  instead: it is correct for that test, which still packs I = 1. The two numbers
  disagree for a reason and a reader should be able to see which is which.

- Next-step 2 now leads with something smaller than "write the decoder": a dumb
  full-frame RAM->GVRAM blit in 68000 code, timed. That number alone confirms or
  kills the 38% estimate, and needs no bitstream, codebooks, or DLX1 parsing.
  The reference image and its checker already exist.

- Parked the user's Cliff Hanger / Lupin III follow-on in STATUS so it is not
  lost and not mistaken for scheduled work. Cheaper than this project on every
  axis except media prep, which is where it would actually stall.

Claude-Session: https://claude.ai/code/session_01194oWYW8DQXK1SZ2DnChW6
This commit is contained in:
prosolis
2026-08-23 13:31:11 -07:00
parent 34f9ee341d
commit 7ba979a236
3 changed files with 95 additions and 11 deletions
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docs/STATUS.md
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@@ -1,4 +1,14 @@
# Status & next-session handoff — session 4 in progress (2026-08-23)
# Status & next-session handoff — end of session 4 (2026-08-23)
## Start here: is the tree still green?
```
./tools/bench/check.sh
```
~40 s, needs the Blu-ray mounted. Re-runs both display regression tests from
source media and prints `ALL GREEN`. Verified green at end of session 4.
If it fails, fix that before doing anything else — everything downstream assumes
the display path is pixel-exact.
## Decisions locked
@@ -65,14 +75,14 @@ rate-distortion curve, not two codecs.
MAME 0.277 source**, not folklore: the palette word format, the 1024-byte
GVRAM line stride, and the 256-colour page aliasing in `HARDWARE.md`. All
three were already written down correctly; they are now cited.
4. **A new quality ceiling was measured** — the 15-bit+I palette alone costs
38.88 dB, the same order as the `scsi` profile's own codec error. FINDINGS
22.4. This bounds how much further `scsi` is worth raising.
4. ~~**A new quality ceiling was measured** — the 15-bit+I palette alone costs
38.88 dB.~~ **Superseded by session 4:** that figure assumed the shared LSB
`I` is always 1. Chosen per entry, the ceiling is **40.81 dB**. FINDINGS 23.3.
5. **Two shell traps that wedged session 2's background jobs** are documented in
the working-setup section below. They cost ~1.5 h of wall clock and a wedged
CPU core, and one of them was hit again this session.
## What session 4 settled (in progress)
## What session 4 settled
1. **A real 256x256 CRTC mode exists and is verified.** `crtc_mode.lua`, derived
from `x68k_crtc.cpp`'s divisor ladder rather than recalled — the derivation is
@@ -220,9 +230,14 @@ Three facts the player MUST honour, none of which were guessable:
| what | where | value |
|---|---|---|
| **Un-hide the graphics layer** | CRTC R20 `$E80028` | clear bit 11 ("G-VRAM set to buffer"); IPL leaves `0x0B16` |
| Colour setup (256c) | CRTC R20 bits 9-8 | `0x0100` -> `R20 = 0x0116` |
| Colour setup (256c) | CRTC R20 bits 9-8 | `0x0100` |
| **Monitor contrast** | `$E8E001` bits 3-0 | IPL leaves **14**; write **15** or everything renders 7% dark |
The `R20 = 0x0116` value quoted here in session 3 is the **768-wide IPL timing**
with the gate cleared. The shipping value is **`R20 = 0x0110`** — see the mode
table in `tools/bench/crtc_mode.lua`, which is now the single source of truth
for all of R00-R08 and R20.
Bit 11 is the one that cost the most time: GVRAM writes land and read back
correctly while the layer is invisible, so the video controller looks guilty and
is not. Contrast `0` blanks the screen — free fade-to-black for transitions.
@@ -231,10 +246,11 @@ Palette format is now **confirmed from MAME source**, not assumed:
`GGGGGRRRRRBBBBBI` (G 15:11, R 10:6, B 5:1, shared LSB I), expanded as
`pal6bit((field<<1)|I)`. With contrast at 15 the render is **pixel-exact**.
New ceiling: the 15-bit+I palette alone costs **38.88 dB** against the 24-bit
palettised source — the same order as the `scsi` profile's own codec error
(39.4 dB). `scsi` is close to display-transparent on real hardware. See
FINDINGS 22.4 before considering raising quality further.
Ceiling: the 15-bit+I palette costs **40.81 dB** against the 24-bit palettised
source, once `I` is chosen per entry (FINDINGS 23.3 — session 3's 38.88 dB
assumed `I = 1`). Still the same order as the `scsi` profile's own codec error
(39.4 dB), so `scsi` remains near display-transparent, with ~2 dB more headroom
than session 3 thought.
Snapshot recipe that works (`-video none` CANNOT snapshot):
```
@@ -261,6 +277,20 @@ SDL_VIDEODRIVER=dummy mame x68000 -bios ipl10 -video soft -window \
validates the 38% full-frame blit estimate the whole CPU budget rests on.
**This is now the top priority** — it is the only remaining unknown that can
still invalidate the design.
**Concrete first step, deliberately smaller than "write the decoder":** do
not start by parsing `DLX1`. Start by making 68000 code do the dumbest
possible full-frame blit — copy 256x192 bytes from RAM to GVRAM through the
mode set by `crtc_mode.lua` — and time it with the existing Lua harness.
That single number either confirms or kills the 38% estimate, and it needs
no bitstream, no codebooks, and no container parsing. `show_frame256.lua`
already produces the exact reference image to diff the result against, and
`verify_frame256.py` already knows how to check it. Only once that number is
in hand is it worth writing the mode dispatch.
Assembler: `tools/vasm/vasmm68k_mot -Fbin -o out.bin in.s`. The harness
pattern for loading and running 68000 code is in `tools/bench/one.lua` and
`tools/bench/bench.lua` (working, from session 1).
2a. ~~CRTC mode table for 256x192-in-256x256.~~ **DONE, session 4.** Derived from
the CRTC divisor ladder (not recalled), verified by snapshot, pixel-exact.
`tools/bench/crtc_mode.lua`; write-up in FINDINGS 23; regression test
@@ -323,6 +353,8 @@ mkdir -p tmp/snap_verify && cd tmp && SDL_VIDEODRIVER=dummy mame x68000 -bios ip
cd .. && python3 tools/bench/verify_frame.py
```
Verified cold from the Blu-ray at end of session 3: exact match, 38.88 dB.
(That 38.88 is correct *for this test*: `show_frame.lua` still packs `I = 1`.
The 40.81 dB ceiling comes from `show_frame256.lua`, which picks `I` per entry.)
`tmp/` is gitignored scratch. The frames are NOT in the repo — regenerate them
with `extract.py`; the earlier ones lived in `/tmp` and do not survive a reboot.
@@ -336,3 +368,19 @@ with `extract.py`; the earlier ones lived in `/tmp` and do not survive a reboot.
six register-poking attempts failed against a gate that one grep found.
- Blu-ray mounted at `/media/reala-misaki/BDROM` via
`udisksctl loop-setup -r -f DRAGONS_LAIR.iso`.
---
## Parked ideas (not scheduled, not abandoned)
- **Cliff Hanger, retitled as Lupin III** (user, session 4). Stern's 1983
laserdisc game was cut from *Castle of Cagliostro* and *Mystery of Mamo* with
the Lupin branding stripped; a port would restore it. Technically **cheaper
than this project**: same content class (cel animation, flat colour, hard
cuts), ~13 min of footage vs Dragon's Lair's ~22, and flatter linework than
Bluth's, so fewer blocks should escape to V4/RAW. The codec, the display path,
and `crtc_mode.lua` would all drop straight in.
**The real cost is media prep, not code:** there is no clean master cut to
Stern's scene boundaries the way `DRAGONS_LAIR.iso` is, so the footage would
have to be sourced and cut to match. Not to be started until the CPU path is
proven — it changes nothing about whether this design works.