026. Sta absolute y

Add absolute,Y STA ($99).

Lesson 26 of 356 · tests/chapter_01_cpu/test_026_STA_absolute_y.py

File to update

emulator/cpu/opcodes.py

Locations

opcodes.sta_absolute_y
opcodes.OPCODE_TABLE[$99]

Why this step exists

This is the Y-indexed counterpart to Test 025. It reuses absolute_y so the handler only coordinates destination resolution and the existing sta write.

Complete example implementation

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


OPCODE_TABLE = {
    # Preserve existing entries.
    0x99: sta_absolute_y,
}

Important invariants

two operand bytes form the base before Y is added
Y, not X, selects the final destination
the full indexed address is passed to sta
STA does not update flags

Common misconception

Copying the absolute,X handler without changing the addressing helper silently uses the wrong index register when X and Y differ.

Out of scope

indirect STA opcodes
page-cross cycle behavior
changes to absolute_y or sta

Run this lesson

uv run pytest tests/chapter_01_cpu/test_026_STA_absolute_y.py -v

Example implementation

Complete example implementation

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


OPCODE_TABLE = {
    # Preserve existing entries.
    0x99: sta_absolute_y,
}

Show test codetest_026_STA_absolute_y.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_y_handler_exists_and_is_in_opcode_table():
    """
    Objective:
    Create sta_absolute_y(cpu) and add 0x99 to OPCODE_TABLE.
    """
    assert hasattr(opcodes, "sta_absolute_y")
    assert callable(opcodes.sta_absolute_y)
    assert list(inspect.signature(opcodes.sta_absolute_y).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0x99] is opcodes.sta_absolute_y


def test_opcode_99_sta_absolute_y_stores_register_a():
    """
    Objective:
    99 00 02 means STA $0200,Y.
    If Y is 0x04, store A into RAM $0204.
    """
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x99)
    rom.write(0x0001, 0x00)
    rom.write(0x0002, 0x02)

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

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

Test functions (2)

test_sta_absolute_y_handler_exists_and_is_in_opcode_table
test_opcode_99_sta_absolute_y_stores_register_a