138. Eor absolute

add EOR absolute opcode ``0x4D``.

Lesson 138 of 356 · tests/chapter_01_cpu/test_138_EOR_absolute.py

Why this step exists

Absolute EOR allows exclusive OR against any CPU memory location while keeping little-endian address decoding outside the instruction primitive.

In this step, after the two zero-page forms, add exactly the following to `emulator/cpu/opcodes.py`:

def eor_absolute(cpu: CPU):
    addr = absolute(cpu)
    value = cpu.bus.read(addr)
    or_e(cpu, value)

OPCODE_TABLE = {
    ...
    0x4D: eor_absolute,
}

`emulator/cpu/addressing_modes.py::absolute uses CPU.fetch_word to decode low byte then high byte. The handler reads the resulting address and delegates the XOR and Z/N flags to instructions.or_e`. Carry/Overflow, memory, X, and Y remain invariant; opcode plus word advances PC three bytes.

Misconception: the little-endian word is the location of the value, not the value itself. Out of scope: indexed/indirect EOR modes (139-142) and BIT (143-145).

Run this lesson

uv run pytest tests/chapter_01_cpu/test_138_EOR_absolute.py -v

Example implementation

Why this step exists

def eor_absolute(cpu: CPU):
    addr = absolute(cpu)
    value = cpu.bus.read(addr)
    or_e(cpu, value)

OPCODE_TABLE = {
    ...
    0x4D: eor_absolute,
}

Show test codetest_138_EOR_absolute.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


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_eor_absolute_handler_exists_and_is_in_opcode_table():
    """Objective: create eor_absolute(cpu) and add 0x4D to OPCODE_TABLE."""
    assert hasattr(opcodes, "eor_absolute")
    assert callable(opcodes.eor_absolute)
    assert list(inspect.signature(opcodes.eor_absolute).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0x4D] is opcodes.eor_absolute


def test_opcode_4D_eor_absolute_reads_memory_value():
    """Objective: 4D 00 02 means EOR value at RAM[$0200]."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x4D)
    rom.write(0x0001, 0x00)
    rom.write(0x0002, 0x02)
    bus.write(0x0200, 0x0F)

    cpu.reset()
    cpu.a = 0xF0
    cpu.step()

    assert cpu.a == 0xFF
    assert cpu.pc == 0x8003

Test functions (2)

test_eor_absolute_handler_exists_and_is_in_opcode_table
test_opcode_4D_eor_absolute_reads_memory_value