SYSTEM INT1

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SYSTEM_INT1 ($C00438): Multi-purpose VBL call

The SYSTEM_INT1 call is the system ROM's v-blank interrupt routine.

It will call MESS_OUT and SYSTEM_IO.

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