031. Ldx immediate

Add LDX immediate ($A2).

Lesson 31 of 356 · tests/chapter_01_cpu/test_031_LDX_immediate.py

File to update

emulator/cpu/opcodes.py

Locations

opcodes import of ldx
opcodes.ldx_immediate
opcodes.OPCODE_TABLE[$A2]

Why this step exists

Test 030 introduced the value-level ldx instruction. This lesson connects its first machine-code encoding by fetching the literal operand and delegating register and flag behavior to that existing instruction.

Complete example implementation

# emulator/cpu/opcodes.py
from emulator.cpu.instructions import lda, sta, ldx


def ldx_immediate(cpu: CPU):
    ldx(cpu, immediate(cpu))


OPCODE_TABLE = {
    # Preserve existing entries.
    0xA2: ldx_immediate,
}

Important invariants

$A2 maps to ldx_immediate
immediate fetches exactly one byte, so the full instruction advances PC by two
the operand byte itself is loaded into X; it is not used as an address
ldx remains responsible for updating Zero and Negative

Common misconception

Do not read memory at the operand's numeric value. A2 42 loads $42 itself into X.

Out of scope

zero-page, zero-page,Y, and absolute LDX encodings
changes to immediate or ldx
cycle timing

Run this lesson

uv run pytest tests/chapter_01_cpu/test_031_LDX_immediate.py -v

Example implementation

Complete example implementation

# emulator/cpu/opcodes.py
from emulator.cpu.instructions import lda, sta, ldx


def ldx_immediate(cpu: CPU):
    ldx(cpu, immediate(cpu))


OPCODE_TABLE = {
    # Preserve existing entries.
    0xA2: ldx_immediate,
}

Show test codetest_031_LDX_immediate.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
from tests.helpers import NEGATIVE_FLAG, ZERO_FLAG


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_ldx_immediate_handler_exists_and_is_in_opcode_table():
    """Objective: create ldx_immediate(cpu) and add 0xA2 to OPCODE_TABLE."""
    assert hasattr(opcodes, "ldx_immediate")
    assert callable(opcodes.ldx_immediate)
    assert list(inspect.signature(opcodes.ldx_immediate).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0xA2] is opcodes.ldx_immediate


def test_opcode_A2_ldx_immediate_loads_register_x():
    """Objective: A2 42 means LDX #$42, so X becomes 0x42."""
    cpu, _, rom = make_cpu_with_rom()
    rom.write(0x0000, 0xA2)
    rom.write(0x0001, 0x42)

    cpu.reset()
    cpu.step()

    assert cpu.x == 0x42
    assert cpu.pc == 0x8002


def test_opcode_A2_ldx_immediate_updates_flags():
    """Objective: LDX Immediate updates Zero and Negative flags."""
    cpu, _, rom = make_cpu_with_rom()
    rom.write(0x0000, 0xA2)
    rom.write(0x0001, 0x80)

    cpu.reset()
    cpu.step()

    assert cpu.x == 0x80
    assert (cpu.p & ZERO_FLAG) == 0
    assert (cpu.p & NEGATIVE_FLAG) != 0

Test functions (3)

test_ldx_immediate_handler_exists_and_is_in_opcode_table
test_opcode_A2_ldx_immediate_loads_register_x
test_opcode_A2_ldx_immediate_updates_flags