src/player/decode.s parses DLX1 and decodes straight into GVRAM. Verified
pixel-exact over a 120-frame sequential run of the worst sustained window on
the disc -- all four block modes, full temporal recursion, so the last frame
is only right if all 120 were. In check.sh.
It costs a mean of 81.7% of a 12fps frame budget, and 31% of frames exceed
100% (42% at scsi). CPU is now the binding constraint. FINDINGS 28.
Three things that were believed and are not true:
- The dual-display-path plan of FINDINGS 24.5/25.6 is incoherent. The compose
path needs a RAM copy of the previous reconstruction; the direct path's
selling point is that it keeps none. Mixing them shows stale pixels on 70 of
120 frames, worst frame 18.8% of the screen. Every coherent repair is dearer
than not mixing, and 24.5's two figures were both copies with no decode in
either, so there was never a crossover to find. One path ships, and the 96KB
reference frame is gone. tools/analysis/10_pathmix_drift.py keeps the
counterexample runnable; check.sh asserts it still reproduces.
- The four block modes do not cost the same. V1 300, V4 448, RAW 400 cycles
against the old model's flat 207.8. V4 is 25% of blocks and 50% of the
cycles, and the mode decision charges it bytes it does not charge cycles for.
tools/analysis/11_cpu_budget.py reproduces all four frames timed on the
68000 to within 1 point. Hand-derived timings agree to 0.5% on V1.
- The container is big-endian but not aligned. Variable-length records laid end
to end put frame 1's length field at an odd address, and move.l (a0)+ there
is an address error: frame 0 decoded perfectly and then vectored into the
IPL for 59 emulated seconds looking like a hang. Found by dumping PC, not by
reading the source.
Also: an all-V1 frame, the cheapest possible full redraw, is 110.5% of budget.
No mode assignment fits a scene cut at 12fps. That one needs a decision, not a
measurement.
Next: charge cycles in the mode decision and bisect against 833,333 per frame,
the way session 6 bisects lam against bytes -- but with no bucket, because a
late frame cannot be banked.
Claude-Session: https://claude.ai/code/session_01194oWYW8DQXK1SZ2DnChW6
