152. Cmp absolute y

add CMP absolute,Y opcode ``0xD9``.

Lesson 152 of 356 · tests/chapter_01_cpu/test_152_CMP_absolute_y.py

Why this step exists

Absolute,Y provides the corresponding indexed comparison through Y while reusing the shared CMP behavior.

In this step, lesson 151 has already added absolute,X. Add exactly the following to `emulator/cpu/opcodes.py`:

def cmp_absolute_y(cpu: CPU):
    addr = absolute_y(cpu)
    value = cpu.bus.read(addr)
    cmp(cpu, value)

OPCODE_TABLE = {
    ...
    0xD9: cmp_absolute_y,
}

`emulator/cpu/addressing_modes.py::absolute_y fetches the little-endian base word and adds Y before the bus read. instructions.cmp` changes only C/Z/N; A, Y, memory, and Overflow remain invariant, while opcode plus word advances PC three bytes.

Misconception: Y modifies the effective address, not the fetched value, and the base bytes are not big-endian. Out of scope: CMP indirect,X and indirect,Y (153-154), then CPX/CPY (155-162).

Run this lesson

uv run pytest tests/chapter_01_cpu/test_152_CMP_absolute_y.py -v

Example implementation

Why this step exists

def cmp_absolute_y(cpu: CPU):
    addr = absolute_y(cpu)
    value = cpu.bus.read(addr)
    cmp(cpu, value)

OPCODE_TABLE = {
    ...
    0xD9: cmp_absolute_y,
}

Show test codetest_152_CMP_absolute_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_absolute_y_handler_exists_and_is_in_opcode_table():
    """Objective: create cmp_absolute_y(cpu) and add 0xD9 to OPCODE_TABLE."""
    assert hasattr(opcodes, "cmp_absolute_y")
    assert callable(opcodes.cmp_absolute_y)
    assert list(inspect.signature(opcodes.cmp_absolute_y).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0xD9] is opcodes.cmp_absolute_y


def test_opcode_D9_cmp_absolute_y_reads_indexed_memory_value():
    """Objective: D9 00 02 with Y=0x04 compares A with RAM[$0204]."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0xD9)
    rom.write(0x0001, 0x00)
    rom.write(0x0002, 0x02)
    bus.write(0x0204, 0x10)

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

    assert (cpu.p & CARRY_FLAG) != 0
    assert cpu.pc == 0x8003

Test functions (2)

test_cmp_absolute_y_handler_exists_and_is_in_opcode_table
test_opcode_D9_cmp_absolute_y_reads_indexed_memory_value