142. Eor indirect y

add EOR (indirect),Y opcode ``0x51``.

Lesson 142 of 356 · tests/chapter_01_cpu/test_142_EOR_indirect_y.py

Why this step exists

This completes EOR addressing coverage with post-indexed pointer access, using the indirect,Y helper before passing the fetched value to the EOR primitive.

In this step, lesson 141 completes the other indirect EOR form. Add exactly the following to `emulator/cpu/opcodes.py`:

def eor_indirect_y(cpu: CPU):
    addr = indirect_y(cpu)
    value = cpu.bus.read(addr)
    or_e(cpu, value)

OPCODE_TABLE = {
    ...
    0x51: eor_indirect_y,
}

`emulator/cpu/addressing_modes.py::indirect_y first reads the wrapped little-endian pointer from zero page and then adds Y to that target. The handler reads the resulting address and passes its value to or_e`. A and Z/N may change; Carry, Overflow, X, Y, and memory remain invariant; PC advances two bytes.

Misconception: Y does not select the zero-page pointer bytes; it indexes the decoded target. Out of scope: BIT behavior and opcodes are lessons 143-145.

Run this lesson

uv run pytest tests/chapter_01_cpu/test_142_EOR_indirect_y.py -v

Example implementation

Why this step exists

def eor_indirect_y(cpu: CPU):
    addr = indirect_y(cpu)
    value = cpu.bus.read(addr)
    or_e(cpu, value)

OPCODE_TABLE = {
    ...
    0x51: eor_indirect_y,
}

Show test codetest_142_EOR_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


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_y_handler_exists_and_is_in_opcode_table():
    """Objective: create eor_indirect_y(cpu) and add 0x51 to OPCODE_TABLE."""
    assert hasattr(opcodes, "eor_indirect_y")
    assert callable(opcodes.eor_indirect_y)
    assert list(inspect.signature(opcodes.eor_indirect_y).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0x51] is opcodes.eor_indirect_y


def test_opcode_51_eor_indirect_y_reads_pointed_indexed_memory_value():
    """Objective: 51 20 reads pointer at $20/$21, then adds Y."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x51)
    rom.write(0x0001, 0x20)
    bus.write(0x0020, 0x00)
    bus.write(0x0021, 0x02)
    bus.write(0x0204, 0x0F)

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

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

Test functions (2)

test_eor_indirect_y_handler_exists_and_is_in_opcode_table
test_opcode_51_eor_indirect_y_reads_pointed_indexed_memory_value