When the ray moves from the end of the screen to the beginning there's a time frame which can safely be used to update the screen without any flickering, thats the vertical blank. Its actually not an interrupt because in above example the bios polls for that event rather than getting an interrupt when its there.
Is that bit latched or is it a direct indication of the state of the CRTC. If its not latched then due to the reduced speed of the emulator we could miss the bit. But that dosen't really explain why the in-tree has the 90 second and the usermode dosen't.
Could the size compression on the in tree emu affect the speed that much?
-- Richard A. Smith