028. Sta indirect y

Add indirect-indexed STA ($91, written `(d),Y`).

Lesson 28 of 356 · tests/chapter_01_cpu/test_028_STA_indirect_y.py

File to update

emulator/cpu/opcodes.py

Locations

opcodes.sta_indirect_y
opcodes.OPCODE_TABLE[$91]

Why this step exists

This completes the STA modes in this sequence by reusing Test 021's indirect_y address resolution. The handler obtains the final Y-indexed destination and delegates the write to sta.

Complete example implementation

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


OPCODE_TABLE = {
    # Preserve existing entries.
    0x91: sta_indirect_y,
}

Important invariants

the zero-page pointer is read before Y is added
the pointer high-byte read wraps from $00FF to $0000
Y indexes the assembled 16-bit destination
$91 stores A without changing flags

Common misconception

(d),Y does not index the zero-page pointer location. Read the pointer at d first, then add Y to the resulting address.

Out of scope

changes to indirect_y or sta
page-cross cycle behavior
refactoring the STA handlers into a generic helper

Run this lesson

uv run pytest tests/chapter_01_cpu/test_028_STA_indirect_y.py -v

Example implementation

Complete example implementation

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


OPCODE_TABLE = {
    # Preserve existing entries.
    0x91: sta_indirect_y,
}

Show test codetest_028_STA_indirect_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_indirect_y_handler_exists_and_is_in_opcode_table():
    """
    Objective:
    Create sta_indirect_y(cpu) and add 0x91 to OPCODE_TABLE.
    """
    assert hasattr(opcodes, "sta_indirect_y")
    assert callable(opcodes.sta_indirect_y)
    assert list(inspect.signature(opcodes.sta_indirect_y).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0x91] is opcodes.sta_indirect_y


def test_opcode_91_sta_indirect_y_stores_register_a():
    """
    Objective:
    91 20 means STA ($20),Y.
    If RAM[$0020-$0021] points to $0200 and Y is 0x04,
    store A into RAM $0204.
    """
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x91)
    rom.write(0x0001, 0x20)
    bus.write(0x0020, 0x00)
    bus.write(0x0021, 0x02)

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

    assert bus.read(0x0204) == 0x42
    assert cpu.pc == 0x8002

Test functions (2)

test_sta_indirect_y_handler_exists_and_is_in_opcode_table
test_opcode_91_sta_indirect_y_stores_register_a