Lesson 25 · The stack - Build a Game Boy Emulator

The stack

Today you add the stack pointer SP and implement PUSH BC and POP BC, the push-and-pop mechanism the CPU uses to save values temporarily - and the foundation tomorrow's subroutine calls are built from.

The goal

Add the stack pointer SP and implement PUSH BC and POP BC so a register pair survives a round trip through the stack.

Start here - the target

TO DO

Scenario: Pushing then popping a pair

GivenSP is 0xFFFE and BC is 0x1234

WhenPUSH BC (opcode 0xC5) runs

ThenSP is 0xFFFC, memory at 0xFFFD holds 0x12 and memory at 0xFFFC holds 0x34

Anda following POP DE (opcode 0xD1) sets DE to 0x1234 and SP back to 0xFFFE

AndPUSH reports 16 cycles and POP reports 12

Background

The stack is a region of memory the CPU uses to save values temporarily, managed by the stack pointer SP - a 16-bit register you add to the CPU today, alongside the PC from lesson 4. It grows downward: PUSH decrements SP and writes, POP reads and increments. A push stores two bytes - high byte first at the higher address - so a later POP reassembles them in the right order. On boot SP is 0xFFFE, near the top of RAM.

Trace the addresses carefully: from SP = 0xFFFE, pushing BC = 0x1234 writes 0x12 to 0xFFFD and 0x34 to 0xFFFC, leaving SP at 0xFFFC. Popping reverses it exactly. This little dance is the mechanism behind subroutine calls, which is precisely what you build tomorrow - CALL is just a PUSH of the return address followed by a jump.

Make it work

case 0xC5: // PUSH BC
    c.SP--
    c.mem.Write(c.SP, c.B) // high byte first
    c.SP--
    c.mem.Write(c.SP, c.C) // then low byte
    return 16