129. Ora absolute

add ORA absolute opcode ``0x0D``.

Lesson 129 of 356 · tests/chapter_01_cpu/test_129_ORA_absolute.py

Why this step exists

Absolute ORA extends the operation beyond zero page by decoding a full little-endian address and reading its value before updating A and Z/N.

In this step, `or_a and the immediate/zero-page handlers already exist, so add exactly the following to emulator/cpu/opcodes.py`:

def ora_absolute(cpu: CPU):
    addr = absolute(cpu)
    value = cpu.bus.read(addr)
    or_a(cpu, value)

OPCODE_TABLE = {
    ...
    0x0D: ora_absolute,
}

`emulator/cpu/addressing_modes.py::absolute obtains a little-endian word through CPU.fetch_word. Therefore 0D 00 02 reads $0200 and sends that value to instructions.or_a`. A and Z/N change; Carry/Overflow and memory are invariant; opcode plus word advances PC three bytes.

Misconception: the operand word is an address, not an immediate value, and its low byte comes first. Out of scope: indexed absolute and indirect ORA modes (lessons 130-133).

Run this lesson

uv run pytest tests/chapter_01_cpu/test_129_ORA_absolute.py -v

Example implementation

Why this step exists

def ora_absolute(cpu: CPU):
    addr = absolute(cpu)
    value = cpu.bus.read(addr)
    or_a(cpu, value)

OPCODE_TABLE = {
    ...
    0x0D: ora_absolute,
}

Show test codetest_129_ORA_absolute.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_absolute_handler_exists_and_is_in_opcode_table():
    """Objective: create ora_absolute(cpu) and add 0x0D to OPCODE_TABLE."""
    assert hasattr(opcodes, "ora_absolute")
    assert callable(opcodes.ora_absolute)
    assert list(inspect.signature(opcodes.ora_absolute).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0x0D] is opcodes.ora_absolute


def test_opcode_0D_ora_absolute_reads_memory_value():
    """Objective: 0D 00 02 means ORA value at RAM[$0200]."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x0D)
    rom.write(0x0001, 0x00)
    rom.write(0x0002, 0x02)
    bus.write(0x0200, 0x0F)

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

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

Test functions (2)

test_ora_absolute_handler_exists_and_is_in_opcode_table
test_opcode_0D_ora_absolute_reads_memory_value