090. Asl accumulator

Wire the ASL accumulator opcode.

Lesson 90 of 356 · tests/chapter_01_cpu/test_090_ASL_accumulator.py

In this step, use asl_a from Test 089 and add only accumulator dispatch. Tests 091-094 add the memory forms.

Production location and symbols

emulator/cpu/opcodes.py: imported `asl_a` and `OPCODE_TABLE[0x0A]`

Why this step exists

Opcode 0x0A selects the accumulator itself, so dispatch can call asl_a(cpu) directly rather than resolving a memory address.

Suggested implementation for this step

# emulator/cpu/opcodes.py
from emulator.cpu.instructions import asl_a  # alongside existing imports

OPCODE_TABLE = {
    # ... existing entries ...
    0x0A: asl_a,
}

Important invariants

opcode 0x0A maps to the exact asl_a(cpu) function
no addressing helper or memory read/write occurs
this one-byte instruction advances PC by exactly one
test 089's function owns A masking and C/Z/N updates

Common misconception

ASL A is not zero-page ASL with an omitted operand; it is a distinct accumulator form and therefore needs no asl_* addressing wrapper.

Out of scope

asl_zero_page, asl_zero_page_x, asl_absolute, and asl_absolute_x
mappings 0x06, 0x16, 0x0E, and 0x1E from tests 091-094
cycle timing and bus-accurate read-modify-write behavior

Run this lesson

uv run pytest tests/chapter_01_cpu/test_090_ASL_accumulator.py -v

Example implementation

Suggested implementation for this step

# emulator/cpu/opcodes.py
from emulator.cpu.instructions import asl_a  # alongside existing imports

OPCODE_TABLE = {
    # ... existing entries ...
    0x0A: asl_a,
}

Show test codetest_090_ASL_accumulator.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
from tests.helpers import NEGATIVE_FLAG, ZERO_FLAG


CARRY_FLAG = 1 << 0


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_asl_accumulator_opcode_exists_and_is_in_opcode_table():
    """Objective: import asl_a and add 0x0A to OPCODE_TABLE."""
    assert hasattr(opcodes, "asl_a")
    assert callable(opcodes.asl_a)
    assert list(inspect.signature(opcodes.asl_a).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0x0A] is opcodes.asl_a


def test_opcode_0A_asl_accumulator_shifts_a_and_advances_pc_by_one():
    """Objective: 0A means ASL A, so only A is shifted and PC advances by 1."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x0A)

    cpu.reset()
    cpu.a = 0b0000_0011
    cpu.step()

    assert cpu.a == 0b0000_0110
    assert cpu.pc == 0x8001


def test_opcode_0A_asl_accumulator_sets_carry_from_old_a_bit_7():
    """Objective: old A bit 7 is copied into Carry before the result wraps."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x0A)

    cpu.reset()
    cpu.a = 0b1000_0001
    cpu.step()

    assert cpu.a == 0b0000_0010
    assert (cpu.p & CARRY_FLAG) != 0


def test_opcode_0A_asl_accumulator_updates_zero_flag():
    """Objective: A=0x80 shifts to 0x00 and sets Zero flag."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x0A)

    cpu.reset()
    cpu.a = 0x80
    cpu.step()

    assert cpu.a == 0x00
    assert (cpu.p & ZERO_FLAG) != 0
    assert (cpu.p & NEGATIVE_FLAG) == 0


def test_opcode_0A_asl_accumulator_updates_negative_flag():
    """Objective: A=0x40 shifts to 0x80 and sets Negative flag."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x0A)

    cpu.reset()
    cpu.a = 0x40
    cpu.step()

    assert cpu.a == 0x80
    assert (cpu.p & NEGATIVE_FLAG) != 0
    assert (cpu.p & ZERO_FLAG) == 0

Test functions (5)

test_asl_accumulator_opcode_exists_and_is_in_opcode_table
test_opcode_0A_asl_accumulator_shifts_a_and_advances_pc_by_one
test_opcode_0A_asl_accumulator_sets_carry_from_old_a_bit_7
test_opcode_0A_asl_accumulator_updates_zero_flag
test_opcode_0A_asl_accumulator_updates_negative_flag