123. And indirect x

add AND (indirect,X) opcode ``0x21``.

Lesson 123 of 356 · tests/chapter_01_cpu/test_123_AND_indirect_x.py

Why this step exists

AND needs pre-indexed indirect access for pointer tables in zero page, while pointer resolution remains an addressing concern rather than logical behavior.

In this step, after the earlier AND modes, add exactly the following to `emulator/cpu/opcodes.py`:

def and_indirect_x(cpu: CPU):
    addr = indirect_x(cpu)
    value = cpu.bus.read(addr)
    and_a(cpu, value)

OPCODE_TABLE = {
    ...
    0x21: and_indirect_x,
}

`emulator/cpu/addressing_modes.py::indirect_x fetches the operand, computes ptr = (base + cpu.x) & 0xFF, and reads the little-endian pointer from ptr and (ptr + 1) & 0xFF. The handler reads that final address and calls instructions.and_a`. A and Z/N change; X, Carry/Overflow, pointer bytes, and target memory are invariant; PC advances two bytes.

Misconception: (indirect,X) applies X before dereferencing, not to the final 16-bit address. Out of scope: AND (indirect),Y (lesson 124) and all later ORA/EOR/BIT work.

Run this lesson

uv run pytest tests/chapter_01_cpu/test_123_AND_indirect_x.py -v

Example implementation

Why this step exists

def and_indirect_x(cpu: CPU):
    addr = indirect_x(cpu)
    value = cpu.bus.read(addr)
    and_a(cpu, value)

OPCODE_TABLE = {
    ...
    0x21: and_indirect_x,
}

Show test codetest_123_AND_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_and_indirect_x_handler_exists_and_is_in_opcode_table():
    """Objective: create and_indirect_x(cpu) and add 0x21 to OPCODE_TABLE."""
    assert hasattr(opcodes, "and_indirect_x")
    assert callable(opcodes.and_indirect_x)
    assert list(inspect.signature(opcodes.and_indirect_x).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0x21] is opcodes.and_indirect_x


def test_opcode_21_and_indirect_x_reads_pointed_memory_value():
    """Objective: 21 20 with X=0x04 reads pointer at zero-page $24/$25."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x21)
    rom.write(0x0001, 0x20)
    bus.write(0x0024, 0x00)
    bus.write(0x0025, 0x02)
    bus.write(0x0200, 0x0F)

    cpu.reset()
    cpu.x = 0x04
    cpu.a = 0xF3
    cpu.step()

    assert cpu.a == 0x03
    assert cpu.pc == 0x8002

Test functions (2)

test_and_indirect_x_handler_exists_and_is_in_opcode_table
test_opcode_21_and_indirect_x_reads_pointed_memory_value