Lesson 21 · The high page - Build a Game Boy Emulator

The high page

Today you implement LDH (n), A, the compact one-byte-addressed load that reaches the 0xFF00 I/O page where every hardware register - video, sound, joypad - lives, bridging the CPU you have built to the hardware you are about to build.

The goal

Implement LDH (n), A and the read-direction LDH A, (n) so the CPU can address the 0xFF00 I/O register page with a single immediate byte.

Start here - the target

TO DO

Scenario: Writing to a high-page register

GivenA is 0x91 and memory at 0x0100 holds 0xE0, 0x40

Whenthe CPU executes one step

Thenmemory at 0xFF40 holds 0x91

AndPC is 0x0102 and the step reports 12 cycles

Andthe read direction LDH A, (n) (opcode 0xF0) with n = 0x40 loads that 0x91 from 0xFF40 back into A

Background

The most important 256 bytes on the machine - the hardware I/O registers - all live in the top page, 0xFF00 to 0xFFFF. Because a full 16-bit address for every I/O access would be wasteful, the CPU has a shortcut: LDH (n), A (opcode 0xE0) takes a single immediate byte n and writes to 0xFF00 + n.

So the byte 0x40 reaches 0xFF40, which is LCDC - the LCD control register you will program the display through. LDH A, (n) (0xF0) reads back the same way. This one-byte addressing is how games poke the screen, sound, and joypad, so it is the bridge between the CPU you have built and the hardware you are about to build.

Make it work

case 0xE0: // LDH (n), A  ->  write A to 0xFF00 + n
    n := c.fetch()
    c.mem.Write(0xFF00+uint16(n), c.A)
    return 12
case 0xF0: // LDH A, (n) - the read direction