Interrupts

[jeremyBanks]

What are interrupts?

The Game Boy defines five types of "interrupts". Each is a type of event that can occur during execution. Each interrupt type has a constant handler address. When an interrupt is triggered, the CPU jumps to the handler instead of executing the next instruction. It will also temporarily disable interrupts, so that your interrupt doesn't itself get interrupted, although this can be toggled back if desired. Each interrupt handler routine typically ends with a RETI instruction to jump back to the original instruction address and re-enable interrupts.

An interrupt is "triggered", either manually or automatically, by setting the appropriate bit of the interrupt trigger flag register at 0xFF0F. The corresponding bit in the interrupt enable flag register at 0xFFFF must be set, or else the request is ignored. If the interrupt is triggered, the corresponding request flag bit is cleared, but if it's ignored it remains set, and may be triggered if the corresponding enable bit becomes set later.

• 0b00000001 0x0040 V-Blank
  Triggered after each frame, when VRAM becomes available for general writing again.
• 0b00000010 0x0048 LCD Status
  Triggered at different points during a frame based on the value of the "STAT" register at 0xFF41.
• 0b00000100 0x0050 Timer Overflow
  Triggered when the timer overflows and resets, based on the the frequency and limits set in the timer registers at 0xFF04, 0xFF05, and 0xFF06.
• 0b00001000 0x0058 Serial Transfer
  Triggered after a byte of data has been received over the serial port. The byte will be available at 0xFF01.
• 0b00100000 0x0060 Button Released
  Triggered after a button has been released. Often requires debouncing due to duplicate triggers on physical hardware.

What do I need to implement?

• [x] Jump to interrupts when they're triggered. This should be a pretty simple new branch at the beginning of each CPU tick. If interrupt trigger register bitwise-and interrupt enable register is nonzero, then clear the first set bit and jump to the corresponding handler instead of running the next instruction.

• Trigger interrupts

  • [ ] V-Blank
    The VideoController already keeps track of our draw line, so we just needs it to signal the CPU every time that value reaches the first "off-screen" v-blank line.
  • LCD Status
    I'd like to punt on this, at least for a little while. My currently very-primitive drawing routines do not work like they really do on the game boy, and probably don't have any notion of the timings that should trigger this.
  • [ ] Timer Overflow
    This shouldn't be very complicated, and I'm sure it's very widely used.
  • Serial Transfer
    When hell freezes over.
  • [ ] Button Released
    It'll probably be a while before we get any game to the point where it's actually waiting on this, but connecting our UI input events into the Game Boy state should be a useful, feasible, and neat.

[jeremyBanks]

• [ ] DI instruction, F3 opcode: disable interrupts after next instruction

• [ ] EI instruction, FB opcode: enable interrupts after next instruction

• [ ] TAC register, FF07 address: timer enable and interval selection