HC ($F00004) free-runs 0..HP once per half-line on hardware (JTRM), so
its low bits at a CPU read are effectively random. Alien vs Predator
uses them as its marines' fire roll -- `move.w $F00004,d0 / and #7 /
cmp #2` at 68K $015ADA -- while its Predators roll the game's software
RNG. The core returned a per-read counter reset to 0 / (HP+1)/2 every
half-line, so a once-per-AI-tick read always saw $0000 or $05A6: low
bits 0 or 6, never 2. Marines walked up to an Alien player and never
fired (30 of 30 rolls measured from the corridor savestate).
The readback is now the elapsed fraction of the current half-line:
time to the next half-line event at slice start, less how far the 68K
has run into the slice (JaguarSliceElapsedUs, stall and clock scale
included), scaled to HP+1 ticks. Byte reads of $F00004/5 use it too.
tomRam8[HC] is unchanged (the OP's second-half-line test reads bit 10),
and the read-phase counter is gone; nothing new is serialized.
Measured after: the same marine rolls all eight HC&7 values (2 on 12
of 111), crouches, fires, and the Alien's health drops.
Co-authored-by: Claude Opus 5.5 <[email protected]>
