127. Ora zero page

add ORA zero-page opcode ``0x05``.

Lesson 127 of 356 · tests/chapter_01_cpu/test_127_ORA_zero_page.py

Why this step exists

ORA must distinguish a zero-page address operand from literal data, read the addressed byte, and then apply the already-defined inclusive-OR behavior.

In this step, with `or_a imported by lesson 126, the complete incremental implementation in emulator/cpu/opcodes.py` is:

def ora_zero_page(cpu: CPU):
    addr = zero_page(cpu)
    value = cpu.bus.read(addr)
    or_a(cpu, value)

OPCODE_TABLE = {
    ...
    0x05: ora_zero_page,
}

`emulator/cpu/addressing_modes.py::zero_page fetches the one-byte address, then the handler reads its value before calling instructions.or_a. A and Z/N change; Carry/Overflow and memory remain invariant; 05 10 reads $0010` and advances PC two bytes.

Misconception: pass the byte stored at `$0010 to or_a, not address 0x10`, and do not write the result back to memory. Out of scope: zero-page,X through indirect,Y ORA modes (lessons 128-133).

Run this lesson

uv run pytest tests/chapter_01_cpu/test_127_ORA_zero_page.py -v

Example implementation

Why this step exists

def ora_zero_page(cpu: CPU):
    addr = zero_page(cpu)
    value = cpu.bus.read(addr)
    or_a(cpu, value)

OPCODE_TABLE = {
    ...
    0x05: ora_zero_page,
}

Show test codetest_127_ORA_zero_page.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_zero_page_handler_exists_and_is_in_opcode_table():
    """Objective: create ora_zero_page(cpu) and add 0x05 to OPCODE_TABLE."""
    assert hasattr(opcodes, "ora_zero_page")
    assert callable(opcodes.ora_zero_page)
    assert list(inspect.signature(opcodes.ora_zero_page).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0x05] is opcodes.ora_zero_page


def test_opcode_05_ora_zero_page_reads_memory_value_and_updates_a():
    """Objective: 05 10 means ORA value at RAM[$0010]."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x05)
    rom.write(0x0001, 0x10)
    bus.write(0x0010, 0x0F)

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

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

Test functions (2)

test_ora_zero_page_handler_exists_and_is_in_opcode_table
test_opcode_05_ora_zero_page_reads_memory_value_and_updates_a