158. Cpx absolute

add CPX absolute opcode ``0xEC``.

Lesson 158 of 356 · tests/chapter_01_cpu/test_158_CPX_absolute.py

Why this step exists

Absolute CPX extends the comparison to a byte anywhere in CPU memory while keeping little-endian address decoding outside the CPX primitive.

In this step, complete CPX by adding exactly the following to `emulator/cpu/opcodes.py`:

def cpx_absolute(cpu: CPU):
    addr = absolute(cpu)
    value = cpu.bus.read(addr)
    cpx(cpu, value)

OPCODE_TABLE = {
    ...
    0xEC: cpx_absolute,
}

`emulator/cpu/addressing_modes.py::absolute obtains a little-endian word through CPU.fetch_word; EC 00 02 therefore compares X with the value at $0200`. Only C/Z/N change; X, memory, and Overflow are invariant, and opcode plus word advances PC three bytes.

Misconception: the operand word is an address, not a 16-bit compared value. Out of scope: the CPY instruction and its three opcodes (lessons 159-162).

Run this lesson

uv run pytest tests/chapter_01_cpu/test_158_CPX_absolute.py -v

Example implementation

Why this step exists

def cpx_absolute(cpu: CPU):
    addr = absolute(cpu)
    value = cpu.bus.read(addr)
    cpx(cpu, value)

OPCODE_TABLE = {
    ...
    0xEC: cpx_absolute,
}

Show test codetest_158_CPX_absolute.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


CARRY_FLAG = 1 << 0


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


def test_opcode_EC_cpx_absolute_reads_memory_value():
    """Objective: EC 00 02 means compare X with RAM[$0200]."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0xEC)
    rom.write(0x0001, 0x00)
    rom.write(0x0002, 0x02)
    bus.write(0x0200, 0x10)

    cpu.reset()
    cpu.x = 0x20
    cpu.step()

    assert (cpu.p & CARRY_FLAG) != 0
    assert cpu.pc == 0x8003

Test functions (2)

test_cpx_absolute_handler_exists_and_is_in_opcode_table
test_opcode_EC_cpx_absolute_reads_memory_value