Lesson 39 · Interrupt enables - Build a Game Boy Emulator

Interrupt enables

Today you wire up the interrupt-enable plumbing - the IME master flag toggled by EI/DI, plus the IE and IF registers that track which interrupt types are allowed and pending. This is the gate that, once opened tomorrow, lets hardware events like VBlank pull the CPU away from its current work.

The goal

Model the IME master switch and the IE/IF registers, toggled by the EI and DI instructions.

Start here - the target

TO DO

Scenario: Enabling and disabling interrupts

Giventhe IME master flag is false

WhenEI (opcode 0xFB) runs

ThenIME is true

AndDI (opcode 0xF3) sets IME back to false, and a write of 0x01 to IF (0xFF0F) requests the VBlank interrupt

Background

An interrupt lets hardware pull the CPU away from its current work to handle an event. Three things gate one: the IME master flag (a single on/off switch toggled by EI and DI), the IE register at 0xFFFF (which interrupt types are allowed), and the IF register at 0xFF0F (which are currently pending). An interrupt fires only when IME is on and the same bit is set in both IE and IF.

Today just wire up the plumbing: the IME flag with its two instructions, and IE/IF as readable, writable registers. IE (0xFFFF) and IF (0xFF0F) need no new storage - they are just addresses your memory already backs, so a plain read/write is all they require. Bit 0 of each is VBlank, the interrupt the screen raises 60 times a second - the one your finished emulator will lean on to pace frames. Tomorrow you make a set request actually divert the CPU.

Make it work

case 0xFB: // EI - enable interrupts
    c.IME = true
    return 4
case 0xF3: // DI - disable interrupts
    c.IME = false
    return 4
// IE at 0xFFFF = which interrupts are allowed; IF at 0xFF0F = which are pending