debug_server_record_frame (every vblank) and the sio_state handler read
SIO_STAT/CTRL/RX via the GUEST-BUS sio_read(), which has hardware
side-effects: sio_tick(0) advances the delayed-IRQ state machine, reading
SIO_STAT decrements sio_ack_visible_reads and clears the ACK bit, reading
SIO_RX_DATA pops the RX FIFO. So the observer silently mutated the SIO/pad
handshake every frame in dev builds (the cardinal observability-alters-state
violation the event_ring header itself warns against).
Add side-effect-free sio_peek_stat/ctrl/rx_data() and use them on the debug
path; the guest bus keeps sio_read() with its real side effects. Also add an
fmv_state TCP command reporting the resolved config path + auto_skip_fmv +
frame-count-table params + live MDEC/XA/pad state, so the FMV-skip + config
state can be observed rather than inferred.
Note: this removes a real per-vblank SIO perturbation but is NOT the full fix
for the dev-build FMV-skip divergence (residual is the cycle-timing-core
class; see precise_irq_slice_state). It is correct observability hygiene.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Matthew Stanley
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