136. Eor zero page

add EOR zero-page opcode ``0x45``.

Lesson 136 of 356 · tests/chapter_01_cpu/test_136_EOR_zero_page.py

Why this step exists

EOR needs a compact memory form that interprets its operand as a zero-page address, reads that location, and applies the common exclusive-OR behavior.

In this step, `or_e and EOR immediate already exist. Add exactly the following to emulator/cpu/opcodes.py`:

def eor_zero_page(cpu: CPU):
    addr = zero_page(cpu)
    value = cpu.bus.read(addr)
    or_e(cpu, value)

OPCODE_TABLE = {
    ...
    0x45: eor_zero_page,
}

`emulator/cpu/addressing_modes.py::zero_page fetches the one-byte address; the handler performs the separate data read and passes that value to instructions.or_e`. A and Z/N may change; Carry/Overflow, memory, X, and Y remain invariant; opcode plus operand advances PC two bytes.

Misconception: `$nn` is an address in page zero, not the literal XOR value. Out of scope: indexed and wider EOR modes (137-142) and BIT (143-145).

Run this lesson

uv run pytest tests/chapter_01_cpu/test_136_EOR_zero_page.py -v

Example implementation

Why this step exists

def eor_zero_page(cpu: CPU):
    addr = zero_page(cpu)
    value = cpu.bus.read(addr)
    or_e(cpu, value)

OPCODE_TABLE = {
    ...
    0x45: eor_zero_page,
}

Show test codetest_136_EOR_zero_page.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_zero_page_handler_exists_and_is_in_opcode_table():
    """Objective: create eor_zero_page(cpu) and add 0x45 to OPCODE_TABLE."""
    assert hasattr(opcodes, "eor_zero_page")
    assert callable(opcodes.eor_zero_page)
    assert list(inspect.signature(opcodes.eor_zero_page).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0x45] is opcodes.eor_zero_page


def test_opcode_45_eor_zero_page_reads_memory_value_and_updates_a():
    """Objective: 45 10 means EOR value at RAM[$0010]."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x45)
    rom.write(0x0001, 0x10)
    bus.write(0x0010, 0x0F)

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

    assert cpu.a == 0xFF
    assert cpu.pc == 0x8002

Test functions (2)

test_eor_zero_page_handler_exists_and_is_in_opcode_table
test_opcode_45_eor_zero_page_reads_memory_value_and_updates_a