108. Rol absolute x

wire ROL Absolute,X (opcode ``0x3E``).

Lesson 108 of 356 · tests/chapter_01_cpu/test_108_ROL_absolute_x.py

In this step, complete the ROL sequence by adding only the Absolute,X adapter and dispatch entry after lesson 107.

Complete example implementation in the production locations

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

def rol_absolute_x(cpu: CPU):
    addr = absolute_x(cpu)
    rol(cpu, addr)

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

0x3E: rol_absolute_x,

Why this step exists

`absolute_x centralizes little-endian decoding and X indexing; rol` handles only operation semantics.

Invariants: consume two operand bytes, decode the 16-bit base, then add X; `3E 00 02 with X=0x04 targets $0204` and PC ends at start+3. The memory primitive's C/Z/N behavior remains unchanged.

Misconception: do not index a single operand byte or apply zero-page wrapping; this mode indexes the decoded 16-bit absolute base.

Out of scope: ROR primitives begin at 109 and ROR opcodes at 111; page-cross and cycle-accurate read/modify/write behavior are later work.

Run this lesson

uv run pytest tests/chapter_01_cpu/test_108_ROL_absolute_x.py -v

Example implementation

Complete example implementation in the production locations

def rol_absolute_x(cpu: CPU):
    addr = absolute_x(cpu)
    rol(cpu, addr)

Show test codetest_108_ROL_absolute_x.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_x_handler_exists_and_is_in_opcode_table():
    """Objective: create rol_absolute_x(cpu) and add 0x3E to OPCODE_TABLE."""
    assert hasattr(opcodes, "rol_absolute_x")
    assert callable(opcodes.rol_absolute_x)
    assert list(inspect.signature(opcodes.rol_absolute_x).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0x3E] is opcodes.rol_absolute_x


def test_opcode_3E_rol_absolute_x_rotates_indexed_memory_value():
    """Objective: 3E 00 02 with X=0x04 rotates RAM[$0204]."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x3E)
    rom.write(0x0001, 0x00)
    rom.write(0x0002, 0x02)
    bus.write(0x0204, 0b0000_0011)

    cpu.reset()
    cpu.x = 0x04
    cpu.step()

    assert bus.read(0x0204) == 0b0000_0110
    assert cpu.pc == 0x8003

Test functions (2)

test_rol_absolute_x_handler_exists_and_is_in_opcode_table
test_opcode_3E_rol_absolute_x_rotates_indexed_memory_value