154. Cmp indirect y

add CMP (indirect),Y opcode ``0xD1``.

Lesson 154 of 356 · tests/chapter_01_cpu/test_154_CMP_indirect_y.py

Why this step exists

The (indirect),Y form completes CMP addressing by indexing a resolved zero-page pointer rather than the pointer location itself.

In this step, close the CMP opcode sequence by adding to `emulator/cpu/opcodes.py`:

def cmp_indirect_y(cpu: CPU):
    addr = indirect_y(cpu)
    value = cpu.bus.read(addr)
    cmp(cpu, value)

OPCODE_TABLE = {
    ...
    0xD1: cmp_indirect_y,
}

`emulator/cpu/addressing_modes.py::indirect_y` reads a little-endian pointer from zero page (wrapping the pointer high-byte lookup), then adds Y to that 16-bit address. Only C/Z/N change; A, Y, pointer bytes, target memory, and Overflow are invariant; opcode plus operand advances PC two bytes.

Misconception: unlike (indirect,X), Y is added after pointer dereference, not to the zero-page operand. Out of scope: CPX and CPY in lessons 155-162.

Run this lesson

uv run pytest tests/chapter_01_cpu/test_154_CMP_indirect_y.py -v

Example implementation

Why this step exists

def cmp_indirect_y(cpu: CPU):
    addr = indirect_y(cpu)
    value = cpu.bus.read(addr)
    cmp(cpu, value)

OPCODE_TABLE = {
    ...
    0xD1: cmp_indirect_y,
}

Show test codetest_154_CMP_indirect_y.py

import inspect

from emulator.bus.cpu_bus import CpuBus
from emulator.cpu import opcodes
from emulator.cpu.cpu import CPU
from emulator.memory.fake_rom import FakeROM


CARRY_FLAG = 1 << 0


def make_cpu_with_rom():
    rom = FakeROM()
    rom.write(0x7FFC, 0x00)
    rom.write(0x7FFD, 0x80)
    bus = CpuBus(program_rom=rom)
    return CPU(bus), bus, rom


def test_cmp_indirect_y_handler_exists_and_is_in_opcode_table():
    """Objective: create cmp_indirect_y(cpu) and add 0xD1 to OPCODE_TABLE."""
    assert hasattr(opcodes, "cmp_indirect_y")
    assert callable(opcodes.cmp_indirect_y)
    assert list(inspect.signature(opcodes.cmp_indirect_y).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0xD1] is opcodes.cmp_indirect_y


def test_opcode_D1_cmp_indirect_y_reads_pointed_indexed_memory_value():
    """Objective: D1 20 reads pointer at $20/$21, then adds Y."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0xD1)
    rom.write(0x0001, 0x20)
    bus.write(0x0020, 0x00)
    bus.write(0x0021, 0x02)
    bus.write(0x0204, 0x10)

    cpu.reset()
    cpu.y = 0x04
    cpu.a = 0x20
    cpu.step()

    assert (cpu.p & CARRY_FLAG) != 0
    assert cpu.pc == 0x8002

Test functions (2)

test_cmp_indirect_y_handler_exists_and_is_in_opcode_table
test_opcode_D1_cmp_indirect_y_reads_pointed_indexed_memory_value