Lesson 03 · Main memory - Build a Game Boy Emulator

Main memory

Today you build the 64 KB address space the CPU sees as its entire world, from cartridge and RAM to hardware registers. Every future instruction that reads or writes memory ultimately calls the two methods you build here.

The goal

Build a 64 KB address space that can be read from and written to at any address.

Start here - the target

TO DO

Scenario: Reading back a written byte

Givena fresh 64 KB memory, all zero

When0x42 is written to address 0xC000

Thenreading address 0xC000 returns 0x42

Andreading the untouched address 0xC001 returns 0x00

Andthe boundary addresses round-trip too - 0x01 written to 0x0000 and 0xFF written to 0xFFFF read back unchanged

Background

The CPU sees the whole machine - cartridge, work RAM, video RAM, and hardware registers - as one flat address space of 65,536 bytes, from 0x0000 to 0xFFFF. Today it is just a plain array; later lessons will give special regions special behavior, but every one of them still answers a read or a write at some address.

Address 0xC000 is the start of work RAM, the CPU’s general scratch memory, which is why it is a safe place to prove the round trip. Keep the interface tiny

one read, one write - because every instruction that touches memory will go through exactly these two doors.

Make it work

// 0x0000..0xFFFF - one byte per address
type Memory struct{ data [0x10000]uint8 }
func (m *Memory) Read(addr uint16) uint8  { return m.data[addr] }
func (m *Memory) Write(addr uint16, v uint8) { m.data[addr] = v }