107. Rol absolute

wire ROL Absolute (opcode ``0x2E``).

Lesson 107 of 356 · tests/chapter_01_cpu/test_107_ROL_absolute.py

In this step, after the zero-page modes, add only the absolute adapter and table entry.

Complete example implementation in the production locations

`emulator/cpu/opcodes.py::rol_absolute`::

def rol_absolute(cpu: CPU):
    addr = absolute(cpu)
    rol(cpu, addr)

`emulator/cpu/opcodes.py::OPCODE_TABLE`::

0x2E: rol_absolute,

Why this step exists

Operand decoding stays in `absolute while rol` remains a destination-address operation shared by all memory modes.

Invariants: two operand bytes are consumed little-endian; `2E 00 02 targets $0200`; PC ends at start+3; only target memory and C/Z/N change.

Misconception: bytes `00 02 encode $0200, not $0002` and not the literal value to rotate.

Out of scope: Absolute,X is lesson 108, ROR begins at 109, and page/cycle timing was not part of this transition.

Run this lesson

uv run pytest tests/chapter_01_cpu/test_107_ROL_absolute.py -v

Example implementation

Complete example implementation in the production locations

def rol_absolute(cpu: CPU):
    addr = absolute(cpu)
    rol(cpu, addr)

Show test codetest_107_ROL_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_rol_absolute_handler_exists_and_is_in_opcode_table():
    """Objective: create rol_absolute(cpu) and add 0x2E to OPCODE_TABLE."""
    assert hasattr(opcodes, "rol_absolute")
    assert callable(opcodes.rol_absolute)
    assert list(inspect.signature(opcodes.rol_absolute).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0x2E] is opcodes.rol_absolute


def test_opcode_2E_rol_absolute_rotates_memory_value():
    """Objective: 2E 00 02 means ROL $0200, so RAM[$0200] is rotated."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x2E)
    rom.write(0x0001, 0x00)
    rom.write(0x0002, 0x02)
    bus.write(0x0200, 0b0000_0011)

    cpu.reset()
    cpu.step()

    assert bus.read(0x0200) == 0b0000_0110
    assert cpu.pc == 0x8003

Test functions (2)

test_rol_absolute_handler_exists_and_is_in_opcode_table
test_opcode_2E_rol_absolute_rotates_memory_value