SYSTEM INT1

SYSTEM_INT1 ($C00438): Multi-purpose VBL call
The SYSTEM_INT1 is the BIOS vblank interrupt routine.

It will call MESS_OUT and SYSTEM_IO

It has it's own frame counter (BIOS_INT1_FRAME_COUNTER, $10FDD4) and a flag to skip MESS_OUT and SYSTEM_IO (BIOS_INT1_SKIP, $10FEE3).