132. Ora indirect x

add ORA (indirect,X) opcode ``0x01``.

Lesson 132 of 356 · tests/chapter_01_cpu/test_132_ORA_indirect_x.py

Why this step exists

Pre-indexed indirect ORA supports zero-page pointer tables and confirms that pointer lookup is completed before the resulting memory value is ORed with A.

In this step, lesson 131 has completed the direct ORA modes. Add exactly the following to `emulator/cpu/opcodes.py`:

def ora_indirect_x(cpu: CPU):
    addr = indirect_x(cpu)
    value = cpu.bus.read(addr)
    or_a(cpu, value)

OPCODE_TABLE = {
    ...
    0x01: ora_indirect_x,
}

`emulator/cpu/addressing_modes.py::indirect_x fetches the operand, adds X with zero-page wrapping, and reads a little-endian pointer whose high-byte lookup also wraps in zero page. The handler reads the pointed-to value and delegates ORA flags/result to instructions.or_a`. A and Z/N may change; Carry/Overflow, memory, X, and Y remain invariant; PC advances two bytes.

Misconception: X is added before pointer dereferencing, not to the final 16-bit address. Out of scope: ORA (indirect),Y (lesson 133) and EOR/BIT (134-145).

Run this lesson

uv run pytest tests/chapter_01_cpu/test_132_ORA_indirect_x.py -v

Example implementation

Why this step exists

def ora_indirect_x(cpu: CPU):
    addr = indirect_x(cpu)
    value = cpu.bus.read(addr)
    or_a(cpu, value)

OPCODE_TABLE = {
    ...
    0x01: ora_indirect_x,
}

Show test codetest_132_ORA_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_ora_indirect_x_handler_exists_and_is_in_opcode_table():
    """Objective: create ora_indirect_x(cpu) and add 0x01 to OPCODE_TABLE."""
    assert hasattr(opcodes, "ora_indirect_x")
    assert callable(opcodes.ora_indirect_x)
    assert list(inspect.signature(opcodes.ora_indirect_x).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0x01] is opcodes.ora_indirect_x


def test_opcode_01_ora_indirect_x_reads_pointed_memory_value():
    """Objective: 01 20 with X=0x04 reads pointer at zero-page $24/$25."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x01)
    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_ora_indirect_x_handler_exists_and_is_in_opcode_table
test_opcode_01_ora_indirect_x_reads_pointed_memory_value