Lesson 41 · The scanline counter - Build a Game Boy Emulator

The scanline counter

Today you build the display's line counter, advancing LY one scanline every 456 cycles and firing the VBlank interrupt the moment the visible lines finish. This clock is what paces every frame the emulator will draw.

The goal

Make the PPU advance the LY register through 154 lines and request the VBlank interrupt at line 144.

Start here - the target

TO DO

Scenario: LY advances and enters VBlank

Giventhe PPU LY register (0xFF44) reads 0 and each scanline takes 456 cycles

Whenthe PPU is advanced by 456 cycles

ThenLY reads 1

Andwhen LY advances from 143 to 144, IF bit 0 (VBlank) is requested

Background

The display is drawn one horizontal scanline at a time, and the LY register at 0xFF44 reports which line the PPU is on. A line takes 456 cycles; after the 144 visible lines (0–143) come 10 lines of VBlank (144–153), the brief pause when nothing is drawn - then LY wraps back to 0. The whole cycle is 70,224 cycles, about 60 frames a second.

The moment LY reaches 144, the PPU raises the VBlank interrupt by setting IF bit 0 - the request your lesson-40 dispatcher will turn into a jump to 0x0040. VBlank is sacred: it is the only safe window to update video memory, so games do almost all their drawing work in that handler. You now have the clock that paces the screen; next you decode what it actually shows.

Make it work

// Like the Timer, the PPU is a cycle-budget counter - but it needs a Memory
// reference so it can raise the VBlank request in IF. Read LY via an accessor
// for now (as with the timer's DIV); it becomes memory-mapped at 0xFF44 when
// the PPU is wired into Memory in a later lesson.
type PPU struct {
    mem *Memory
    ly  uint8
    dot int
}
func NewPPU(mem *Memory) *PPU { return &PPU{mem: mem} }
func (p *PPU) LY() uint8      { return p.ly }
 
func (p *PPU) Step(cycles int) {
    p.dot += cycles
    for p.dot >= 456 {           // one scanline elapsed
        p.dot -= 456
        p.ly = (p.ly + 1) % 154  // 154 lines total, then wrap to 0
        if p.ly == 144 {         // just entered VBlank
            p.requestVBlank()    // set IF (0xFF0F) bit 0
        }
    }
}