68k interrupts

The cartridge systems use 3 auto-vectored interrupt levels. The CD systems use 6 vectored interrupts, 3 of which are equivalent to the cartridge system's.

See 68k vector table.

Video-related interrupts must to be acknowledged to re-trigger in the future by writing to the register REG_IRQACK.

• bit 2: Acknowledge v-blank interrupt
• bit 1: Acknowledge timer interrupt
• bit 0: Acknowledge reset interrupt (only occurs on cold boot ?)

Multiple bits can be set:

move	#$0007,REG_IRQACK     ; Acknowledge all interrupts

Bits 8~10 of the 68k's SR register is used to set the minimum priority:

move	#$2000,SR     ; Accept all interrupts (+Supervisor mode)
move	#$2700,SR     ; Ignore all interrupts (+Supervisor mode)

Note that the 68k's interrupt level setting is different from the interrupt configuration bits in REG_LSPCMODE.

Vertical blank interrupt

The v-blank interrupt is almost always used. It occurs when the rendering of a frame is about to start (~60 times per second). See display timing.

Timer interrupt

The timer interrupt's behavior can be programmed through the LSPC's memory mapped registers. It is triggered by a 32-bit down counter clocked by the 6MHz pixel clock, and a corresponding reset register. When the counter reaches 0, an interrupt is generated. Intervals can range from 166.7ns (would cause an interrupt flood !) to a little over 11.9 minutes.

It can be used for special video effects such as scanline effects, for example:

• The interlaced Sammy logo in Viewpoint
• Sengoku 2's intro
• The road in Riding hero
• Neo Turf Masters's ground perspective
• ...

AES/MVS interrupt levels

• Level 1: V-blank
• Level 2: Timer
• Level 3: Pending after reset

CD interrupt levels

The V-blank and Timer levels are swapped compared to the cartridge systems.

• Level 1: Timer
• Level 2: V-blank
• Level 3: Not used ?
• Vector 21: CD host decoder interrupt (data ready)
• Vector 22: CD drive communication start
• Vector 23: Used but not understood