039. Stx absolute

Add STX absolute ($8E).

Lesson 39 of 356 · tests/chapter_01_cpu/test_039_STX_absolute.py

File to update

emulator/cpu/opcodes.py

Locations

opcodes.stx_absolute
opcodes.OPCODE_TABLE[$8E]

Why this step exists

This lesson adds STX's full 16-bit destination form. The existing absolute helper decodes the little-endian address, while stx remains responsible only for writing X.

Complete example implementation

# emulator/cpu/opcodes.py
def stx_absolute(cpu: CPU):
    addr = absolute(cpu)
    stx(cpu, addr)


OPCODE_TABLE = {
    # Preserve existing entries.
    0x8E: stx_absolute,
}

Important invariants

$8E maps to stx_absolute
absolute consumes low byte then high byte and returns the destination address
the address itself is passed to stx, which writes X through the bus
the full instruction advances PC by three and does not modify flags

Common misconception

8E 00 02 stores X at $0200; the operand is a little-endian destination, not a value to load into X or a big-endian address.

Out of scope

indexed absolute STX encodings, which are not part of the supported STX set
changes to absolute or stx
cycle timing

Run this lesson

uv run pytest tests/chapter_01_cpu/test_039_STX_absolute.py -v

Example implementation

Complete example implementation

# emulator/cpu/opcodes.py
def stx_absolute(cpu: CPU):
    addr = absolute(cpu)
    stx(cpu, addr)


OPCODE_TABLE = {
    # Preserve existing entries.
    0x8E: stx_absolute,
}

Show test codetest_039_STX_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
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_stx_absolute_handler_exists_and_is_in_opcode_table():
    """
    Objective:
    Create in opcodes.py:
        def stx_absolute(cpu):
            addr = absolute(cpu)
            stx(cpu, addr)

    Then add:
        0x8E: stx_absolute
    """
    assert hasattr(opcodes, "stx_absolute")
    assert callable(opcodes.stx_absolute)
    assert list(inspect.signature(opcodes.stx_absolute).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0x8E] is opcodes.stx_absolute


def test_opcode_8E_stx_absolute_stores_register_x():
    """
    Objective:
    8E 00 02 means STX $0200.
    Store register X into RAM $0200.
    """
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x8E)
    rom.write(0x0001, 0x00)
    rom.write(0x0002, 0x02)

    cpu.reset()
    cpu.x = 0x42
    cpu.step()

    assert bus.read(0x0200) == 0x42
    assert cpu.pc == 0x8003


def test_opcode_8E_stx_absolute_does_not_change_flags():
    """
    Objective:
    STX Absolute stores X but does not update flags.
    """
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x8E)
    rom.write(0x0001, 0x00)
    rom.write(0x0002, 0x02)

    cpu.reset()
    cpu.x = 0x00
    cpu.p = NEGATIVE_FLAG
    cpu.step()

    assert bus.read(0x0200) == 0x00
    assert (cpu.p & ZERO_FLAG) == 0
    assert (cpu.p & NEGATIVE_FLAG) != 0

Test functions (3)

test_stx_absolute_handler_exists_and_is_in_opcode_table
test_opcode_8E_stx_absolute_stores_register_x
test_opcode_8E_stx_absolute_does_not_change_flags