TX_DATA writes now route through the SIO0 shifter under
SIO_MODEL_CYCLE_PACED. Pad and card share the single-byte shifter and
TX buffer; bus owner is tagged on the first byte of each SELECT cycle.
sio_advance() is driven from psx_advance_cycles() so the shifter and
ACK pipeline progress on the guest CPU clock without touching the
legacy IRQ countdown.
The SELECT-deassert and TX_EN 1->0 paths now restore TX_RDY|TX_EMPTY
when they kill an in-flight shifter. Without this, sio_stat retained
the TX-clear bits set by SHIFT_START and the next handler busy-waited
forever on TX_RDY=0 inside the exception, deadlocking the BIOS.
Watchdog dump captured the deadlock at chain handler 0x445C with
stat=0x0000 across 4 seconds of PC samples.
Diagnostics added (passive/gated):
- starvation_ring records SIO MMIO events + periodic PC samples,
watchdog dumps to JSONL on TCP-poll starvation
- starvation_walk.py queries the dump (last/last_change/why_idle)
Validation: shell-only sanity stable; one CROSS press no longer
hangs; mc_read_done climbs from 0 -> 30+ with mc_aborts=0.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Matthew Stanley
479c51934a