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 interrupt
- bit1: Ack Timer interrupt
- bit0: Ack level 3
move #$0007,REG_IRQACK ; Acknowledge all interrupts
Bits 8~10 of the SR register are used to mask them.
move #$2000,SR ; Enable all interrupts (+Supervisor mode) move #$2700,SR ; Disable all interrupts (+Supervisor mode)
Vertical Blank interrupt
The VBlank interrupt is almost always used. It occurs everytime a new frame is traced (~60Hz). See display timing.
Timer interrupt
The Timer interrupt's behavior can be programmed through the GPU's memory mapped registers. It is made of an up-counting 32bit counter clocked by the 6MHz pixel clock, and a match register. It can be used to generate interrupts at intervals ranging from 166.7µs to 11.9 minutes (?). The Sammy logo at the start of Viewpoint is 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 levels
- VBlank: Level 0
- Timer: Level 1
- Pending at startup ?: Level 2
CD interrupts levels
- VBlank: Level 1
- Timer: Level 0