Lesson 06 · NOP and the step loop - Build a Game Boy Emulator

NOP and the step loop

Today you wire together fetch, decode, and execute into a single step loop, starting with the simplest possible instruction, NOP. This loop is the CPU's heartbeat, and every other opcode you add for the rest of the project is just another case inside it.

The goal

Decode and execute opcode 0x00 (NOP) so one CPU step advances PC and reports the cycles it took.

Start here - the target

TO DO

Scenario: Executing a single NOP

GivenPC is 0x0100 and memory at 0x0100 holds 0x00

Whenthe CPU executes one step

ThenPC is 0x0101

Andthe step reports 4 cycles taken

Background

Now the pieces connect. A step is one full instruction: fetch the opcode byte at PC, decode it to decide what to do, execute it, and report how many clock cycles it consumed. Timing matters because later the graphics and timer hardware advance in lockstep with the CPU’s cycle count.

Start with the simplest possible instruction: NOP, opcode 0x00, which does nothing and costs 4 cycles. It looks trivial, but it forces the whole machinery

fetch, a decode switch, a cycle return - into place. Every other opcode you add is just another case in this same loop.

Make it work

// fetch the opcode, decode it, run it, return cycles spent
func (c *CPU) Step() int {
    op := c.fetch()
    switch op {
    case 0x00: // NOP
        return 4
    }
    return 0
}