Lesson 22 · Absolute jumps - Build a Game Boy Emulator

Absolute jumps

Today you implement JP nn, the unconditional absolute jump that lets a program's execution move somewhere other than straight ahead - the first step toward real branching and loops.

The goal

Implement JP nn so it reads a 16-bit absolute address and loads it into PC.

Start here - the target

TO DO

Scenario: Jumping to an absolute address

Givenmemory at 0x0100 holds 0xC3, 0x00, 0x20

Whenthe CPU executes one step

ThenPC is 0x2000

Andthe step reports 16 cycles

Background

A program is not just a straight line - it needs to move execution around. JP nn (opcode 0xC3) is the simplest way: it reads a 16-bit absolute address (little-endian, as always) and loads it straight into PC, so the next fetch happens there. The two operand bytes 0x00, 0x20 become the target 0x2000.

The subtle part is that PC is not advanced past the instruction the usual way - the whole point is to replace it. Because you fetched the two operand bytes first, then overwrote PC, the arithmetic just works out. This is the unconditional jump; the next two lessons add relative and conditional forms that turn jumping into real branching and looping.

Make it work

case 0xC3: // JP nn
    lo := c.fetch()
    hi := c.fetch()
    c.PC = uint16(hi)<<8 | uint16(lo)
    return 16