Function wait_vblank 

pub fn wait_vblank()

Block until the next VBlank IRQ.

This is the display-sync primitive: a frame that finishes early sleeps until the blank, and a slow frame snaps to the next one, so presentation quantizes to whole display periods. (psx_gpu::vsync() cannot do this: it reconfigures Timer 1 on every call, and a mode write resets the counter, so it busy-waits a fixed 242 HBlanks from the call site instead of syncing to the display.)

Installs the VBlank counter on first use if the game has not already called install_vblank_counter, but only while the exception vector is still the one the program booted with. A game that has put its own handler there keeps it: this function never overwrites it. Such a game calls install_vblank_counter first and then wraps psx-rt’s handler, or its handler chains to psx-rt’s, which is what advances the count this waits on. The wait is bounded: after about four million reads without the count moving it returns (and asserts in a debug build) instead of hanging, as it would inside a critical section or without the handler. try_wait_vblank takes the bound.