Lesson 38 · The divider and timer - Build a Game Boy Emulator

The divider and timer

Today you build the DIV register, the simplest hardware clock, which ticks once every 256 CPU cycles by accumulating the cycle counts your CPU already returns. This cycle-budget pattern is the foundation for the full timer and, later, the display.

The goal

Make the DIV register count up once every 256 CPU cycles.

Start here - the target

TO DO

Scenario: DIV ticks on a cycle budget

Giventhe DIV register (0xFF04) reads 0x00

Whenthe timer is advanced by 256 cycles

Thenreading DIV returns 0x01

Andadvancing 255 more cycles leaves DIV at 0x01 (it only ticks each full 256)

AndDIV is a single byte, so after 256 full ticks (65536 cycles from 0) it wraps from 0xFF back to 0x00

Background

Hardware keeps time by counting CPU cycles, and the simplest clock is the divider register DIV at 0xFF04, which increments once every 256 cycles and wraps around at 0xFF. To drive it, feed the timer the cycle count each instruction returned - the value your Step has been handing back since lesson 6 - and let it accumulate until it crosses a threshold.

The full timer adds TIMA (a counter that ticks at a rate chosen by TAC and raises an interrupt when it overflows to TMA), but they all share this cycle-budget pattern. Getting the accumulator right - ticking only on each complete 256-cycle span, not smearing partial progress - is the idea that makes every time-based feature, from the timer to the display, stay in sync with the CPU.

Make it work

func (t *Timer) Step(cycles int) {
    t.divCounter += cycles
    for t.divCounter >= 256 {
        t.divCounter -= 256
        t.div++ // wraps at 0xFF naturally
    }
}