141. Eor indirect x

add EOR (indirect,X) opcode ``0x41``.

Lesson 141 of 356 · tests/chapter_01_cpu/test_141_EOR_indirect_x.py

Why this step exists

Pre-indexed indirect EOR enables pointer-table access through zero page while preserving the split between effective-address resolution and exclusive OR.

In this step, lessons 134-140 already provide `or_e, its opcode import, and the earlier EOR modes. Add exactly the following to emulator/cpu/opcodes.py`:

def eor_indirect_x(cpu: CPU):
    addr = indirect_x(cpu)
    value = cpu.bus.read(addr)
    or_e(cpu, value)

OPCODE_TABLE = {
    ...
    0x41: eor_indirect_x,
}

`emulator/cpu/addressing_modes.py::indirect_x adds X to the operand in zero page, reads the wrapped little-endian pointer there, and returns its target. The handler reads that target and delegates XOR semantics to emulator/cpu/instructions.py::or_e`. A and Z/N may change; Carry, Overflow, X, Y, and memory remain invariant; opcode plus operand advances PC two bytes.

Misconception: X indexes the zero-page pointer location, not the final target. Out of scope: EOR (indirect),Y is lesson 142; BIT begins in lesson 143.

Run this lesson

uv run pytest tests/chapter_01_cpu/test_141_EOR_indirect_x.py -v

Example implementation

Why this step exists

def eor_indirect_x(cpu: CPU):
    addr = indirect_x(cpu)
    value = cpu.bus.read(addr)
    or_e(cpu, value)

OPCODE_TABLE = {
    ...
    0x41: eor_indirect_x,
}

Show test codetest_141_EOR_indirect_x.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_indirect_x_handler_exists_and_is_in_opcode_table():
    """Objective: create eor_indirect_x(cpu) and add 0x41 to OPCODE_TABLE."""
    assert hasattr(opcodes, "eor_indirect_x")
    assert callable(opcodes.eor_indirect_x)
    assert list(inspect.signature(opcodes.eor_indirect_x).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0x41] is opcodes.eor_indirect_x


def test_opcode_41_eor_indirect_x_reads_pointed_memory_value():
    """Objective: 41 20 with X=0x04 reads pointer at zero-page $24/$25."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x41)
    rom.write(0x0001, 0x20)
    bus.write(0x0024, 0x00)
    bus.write(0x0025, 0x02)
    bus.write(0x0200, 0x0F)

    cpu.reset()
    cpu.x = 0x04
    cpu.a = 0xF0
    cpu.step()

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

Test functions (2)

test_eor_indirect_x_handler_exists_and_is_in_opcode_table
test_opcode_41_eor_indirect_x_reads_pointed_memory_value