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

move	#$0007,REG_IRQACK     ; Acknowledges all interrupts

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. Sengoku 3 and Neo Turf Masters are also known to rely on them to do scanline effects.

AES/MVS interrupts

• VBlank: 0
• RLI: 1
• Pending at startup ?: 2

CD interrupts

• VBlank: 1
• RLI: 0