Lesson 40 · Servicing an interrupt - Build a Game Boy Emulator

Servicing an interrupt

Today you make the interrupt gate actually open - when an interrupt is both enabled and pending, the CPU disables further interrupts, pushes the current PC, and jumps to that interrupt's fixed vector, just like a CALL. This is the mechanism that lets a game react to events like VBlank without polling for them.

The goal

Dispatch a pending, enabled interrupt by pushing PC and jumping to its fixed vector.

Start here - the target

TO DO

Scenario: Diverting to the VBlank handler

GivenIME is true, IE bit 0 is set, IF bit 0 is set, PC is 0x0350, and SP is 0xFFFE

Wheninterrupts are serviced

ThenPC is 0x0040 (the VBlank vector), SP is 0xFFFC, and the return address 0x0350 sits on the stack (0x03 at 0xFFFD, 0x50 at 0xFFFC)

AndIME is now false and IF bit 0 has been cleared

Background

Now the gate actually opens. Between instructions, the CPU checks for a pending and enabled interrupt - IE & IF with the corresponding bit set - and if IME is on, it services the highest-priority one. Servicing looks exactly like a CALL: disable further interrupts (IME off), acknowledge by clearing that IF bit, push the current PC, and jump to the interrupt’s fixed vector. VBlank’s vector is 0x0040.

The handler ends with RETI - a RET that also re-enables IME - popping the saved PC so the interrupted code resumes as if nothing happened. This is the mechanism that lets a game react to the screen finishing a frame or a button being pressed without polling. With it working, the CPU can finally respond to the hardware you are about to build.

Make it work

func (c *CPU) serviceInterrupts() {
    pending := c.mem.Read(0xFFFF) & c.mem.Read(0xFF0F)
    if c.IME && pending&0x01 != 0 { // VBlank
        c.IME = false
        c.clearIF(0)      // acknowledge
        c.push16(c.PC)
        c.PC = 0x0040
    }
}