68k interrupts

There are 3 interrupt levels on the AES and MVS. Only 2 on the CD hardware.

Interrupts need to be acknowledged by writing to register $3C000C (REG_IRQACK). bit2: Ack VBlank bit1: Ack RLI bit0: Ack level 3

Bits 8~10 of the SR register are used to mask them. $2000 enables them all, $2700 disables them.

Vertical Blank Interrupt

VBlank interrupt is almost always used. It occurs everytime a new frame is traced (~60Hz).

Raster Line Interrupt

The RLI interrupt's behavior can be programmed through memory mapped registers. It's made to occur at (the start/end ? of) specific horizontal lines of the video output. See the Sammy logo at the start of Viewpoint for an example of its use. Sangoku uses it a lot to do stretches too.

AES/MVS interrupts

VBlank: 0 RLI: 1 ???: 2

CD interrupts

VBlank: 1 RLI: 0