122. And absolute y

add AND absolute,Y opcode ``0x39``.

Lesson 122 of 356 · tests/chapter_01_cpu/test_122_AND_absolute_y.py

Why this step exists

This supplies the Y-indexed counterpart to absolute,X so AND can use either index register without duplicating its accumulator and flag semantics.

In this step, following absolute,X in lesson 121, add only this handler and registration to `emulator/cpu/opcodes.py`:

def and_absolute_y(cpu: CPU):
    addr = absolute_y(cpu)
    value = cpu.bus.read(addr)
    and_a(cpu, value)

OPCODE_TABLE = {
    ...
    0x39: and_absolute_y,
}

`emulator/cpu/addressing_modes.py::absolute_y fetches the little-endian base word before adding cpu.y. The resolved memory value feeds the already-existing instructions.and_a`; A and Z/N change, while Y, Carry/Overflow, and memory remain invariant. The opcode and word consume three bytes.

Misconception: absolute,Y is not zero-page,Y and does not constrain indexing to page zero. Out of scope: AND (indirect,X)/(indirect),Y (lessons 123-124) and the ORA/EOR/BIT additions in lessons 125-145.

Run this lesson

uv run pytest tests/chapter_01_cpu/test_122_AND_absolute_y.py -v

Example implementation

Why this step exists

def and_absolute_y(cpu: CPU):
    addr = absolute_y(cpu)
    value = cpu.bus.read(addr)
    and_a(cpu, value)

OPCODE_TABLE = {
    ...
    0x39: and_absolute_y,
}

Show test codetest_122_AND_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_and_absolute_y_handler_exists_and_is_in_opcode_table():
    """Objective: create and_absolute_y(cpu) and add 0x39 to OPCODE_TABLE."""
    assert hasattr(opcodes, "and_absolute_y")
    assert callable(opcodes.and_absolute_y)
    assert list(inspect.signature(opcodes.and_absolute_y).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0x39] is opcodes.and_absolute_y


def test_opcode_39_and_absolute_y_reads_indexed_memory_value():
    """Objective: 39 00 02 with Y=0x04 reads RAM[$0204]."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x39)
    rom.write(0x0001, 0x00)
    rom.write(0x0002, 0x02)
    bus.write(0x0204, 0x0F)

    cpu.reset()
    cpu.y = 0x04
    cpu.a = 0xF3
    cpu.step()

    assert cpu.a == 0x03
    assert cpu.pc == 0x8003

Test functions (2)

test_and_absolute_y_handler_exists_and_is_in_opcode_table
test_opcode_39_and_absolute_y_reads_indexed_memory_value