Lesson 36 · Loading a cartridge - Build a Game Boy Emulator

Loading a cartridge

Today you load a real cartridge ROM into memory and parse its header, pulling out the game title and cartridge-type byte. This is how an emulator learns which game it's running and what hardware - like a memory-bank controller - it needs to emulate.

The goal

Load a ROM image into the low address space and read its header fields.

Start here - the target

TO DO

Scenario: Reading the cartridge header

Givena 32 KB ROM whose bytes at 0x0134 spell "TETRIS" and whose byte at 0x0147 is 0x00

Whenthe ROM is loaded into memory

Thenreading address 0x0134 returns the ASCII 'T' (0x54)

Andthe cartridge-type byte read from 0x0147 is 0x00 (ROM only)

Background

A game lives in a cartridge ROM, and loading it is simply copying its bytes into the 0x0000–0x7FFF region so the CPU can fetch from them. Every cartridge begins with a fixed header in that space: the game title as ASCII at 0x0134, a cartridge-type byte at 0x0147 that says which memory-bank controller (if any) it uses, and the ROM size at 0x0148.

Type 0x00 means “ROM only” - a plain 32 KB game like Tetris with no banking, which fits entirely in the address space at once. Reading the header is how a real emulator decides what hardware to emulate for this specific game. Prove you can load an image and pull 'T' out of the title, and you are ready for the one feature bigger games need: bank switching.

Make it work

// Keep the WHOLE cartridge in a rom field (banks past the first live there,
// ready for tomorrow's bank switching), and copy the low, directly-runnable
// region into the flat backing array from prior lessons.
func (m *Memory) LoadROM(rom []byte) {
    m.rom = rom                 // add a `rom []byte` field to Memory
    copy(m.data[0x0000:], rom)  // 0x0000..0x7FFF is executable straight away
}
title := m.rom[0x0134:0x0143]   // up to 16 ASCII bytes
cartType := m.rom[0x0147]