025. Sta absolute x

Add absolute,X STA ($9D).

Lesson 25 of 356 · tests/chapter_01_cpu/test_025_STA_absolute_x.py

File to update

emulator/cpu/opcodes.py

Locations

opcodes.sta_absolute_x
opcodes.OPCODE_TABLE[$9D]

Why this step exists

This applies the already-tested absolute_x address calculation to stores. The opcode layer selects X indexing; sta remains independent of addressing mode.

Complete example implementation

# emulator/cpu/opcodes.py
def sta_absolute_x(cpu) -> None:
    address = absolute_x(cpu)
    sta(cpu, address)


OPCODE_TABLE = {
    # Preserve existing entries.
    0x9D: sta_absolute_x,
}

Important invariants

two operand bytes form the base address before X is added
the full indexed address is used; indexing is not zero-page wrapped
the handler stores A and consumes three instruction bytes
STA does not update flags

Common misconception

Absolute,X does not mask the sum to eight bits. That wrapping rule belongs to zero-page,X, not to $hhhh,X.

Out of scope

absolute,Y and indirect STA opcodes
page-cross cycle behavior
changes to absolute_x or sta

Run this lesson

uv run pytest tests/chapter_01_cpu/test_025_STA_absolute_x.py -v

Example implementation

Complete example implementation

# emulator/cpu/opcodes.py
def sta_absolute_x(cpu) -> None:
    address = absolute_x(cpu)
    sta(cpu, address)


OPCODE_TABLE = {
    # Preserve existing entries.
    0x9D: sta_absolute_x,
}

Show test codetest_025_STA_absolute_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_sta_absolute_x_handler_exists_and_is_in_opcode_table():
    """
    Objective:
    Create sta_absolute_x(cpu) and add 0x9D to OPCODE_TABLE.
    """
    assert hasattr(opcodes, "sta_absolute_x")
    assert callable(opcodes.sta_absolute_x)
    assert list(inspect.signature(opcodes.sta_absolute_x).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0x9D] is opcodes.sta_absolute_x


def test_opcode_9D_sta_absolute_x_stores_register_a():
    """
    Objective:
    9D 00 02 means STA $0200,X.
    If X is 0x04, store A into RAM $0204.
    """
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x9D)
    rom.write(0x0001, 0x00)
    rom.write(0x0002, 0x02)

    cpu.reset()
    cpu.a = 0x42
    cpu.x = 0x04
    cpu.step()

    assert bus.read(0x0204) == 0x42
    assert cpu.pc == 0x8003

Test functions (2)

test_sta_absolute_x_handler_exists_and_is_in_opcode_table
test_opcode_9D_sta_absolute_x_stores_register_a