SYSTEM INT1

SYSTEM_INT1 ($C00438): Multi-purpose VBL call
Needs to be called at the beginning of the vblank period. Sets values in RAM according to buttons states, checks CDDA commands, calls MESS_OUT, and probably other things...

If called outside of vblank, interrupts need to be disabled before.

Memory locations set (all 8bit):

Bit definitions: see Joypad