133. Ora indirect y

add ORA (indirect),Y opcode ``0x11``.

Lesson 133 of 356 · tests/chapter_01_cpu/test_133_ORA_indirect_y.py

Why this step exists

This completes ORA's addressing forms by resolving a zero-page pointer and then adding Y, without duplicating the operation's result or flag logic.

In this step, complete ORA after lessons 125-132 by adding the following to `emulator/cpu/opcodes.py`:

def ora_indirect_y(cpu: CPU):
    addr = indirect_y(cpu)
    value = cpu.bus.read(addr)
    or_a(cpu, value)

OPCODE_TABLE = {
    ...
    0x11: ora_indirect_y,
}

`emulator/cpu/addressing_modes.py::indirect_y fetches a zero-page pointer, reads its little-endian target with a wrapping high-byte lookup, and only then adds Y. The handler reads that indexed target and calls instructions.or_a`. A and Z/N may change; Carry/Overflow, memory, X, and Y are invariant; opcode plus operand advances PC two bytes.

Misconception: unlike (indirect,X), Y does not select the pointer location; it indexes the resolved target. Out of scope: EOR and BIT (lessons 134-145).

Run this lesson

uv run pytest tests/chapter_01_cpu/test_133_ORA_indirect_y.py -v

Example implementation

Why this step exists

def ora_indirect_y(cpu: CPU):
    addr = indirect_y(cpu)
    value = cpu.bus.read(addr)
    or_a(cpu, value)

OPCODE_TABLE = {
    ...
    0x11: ora_indirect_y,
}

Show test codetest_133_ORA_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_ora_indirect_y_handler_exists_and_is_in_opcode_table():
    """Objective: create ora_indirect_y(cpu) and add 0x11 to OPCODE_TABLE."""
    assert hasattr(opcodes, "ora_indirect_y")
    assert callable(opcodes.ora_indirect_y)
    assert list(inspect.signature(opcodes.ora_indirect_y).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0x11] is opcodes.ora_indirect_y


def test_opcode_11_ora_indirect_y_reads_pointed_indexed_memory_value():
    """Objective: 11 20 reads pointer at $20/$21, then adds Y."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x11)
    rom.write(0x0001, 0x20)
    bus.write(0x0020, 0x00)
    bus.write(0x0021, 0x02)
    bus.write(0x0204, 0x0F)

    cpu.reset()
    cpu.y = 0x04
    cpu.a = 0xF0
    cpu.step()

    assert cpu.a == 0xFF
    assert cpu.pc == 0x8002

Test functions (2)

test_ora_indirect_y_handler_exists_and_is_in_opcode_table
test_opcode_11_ora_indirect_y_reads_pointed_indexed_memory_value