089. Instruction asl a

Add accumulator-targeted ASL behavior.

Lesson 89 of 356 · tests/chapter_01_cpu/test_089_instruction_asl_a.py

In this step, add only asl_a, using the memory form from Test 088 as context and leaving ASL opcode mapping to Tests 090-094.

Production location and symbol

emulator/cpu/instructions.py: `asl_a(cpu: CPU)`

Why this step exists

ASL A has the same bit and flag rules as memory ASL but its destination is the accumulator. A separate function avoids pretending A is a bus address.

Suggested implementation

def asl_a(cpu: CPU):
    value = cpu.a
    result = cpu.a << 1
    result_8 = result & 0xFF

    # Set flags
    cpu.flags.set_carry_flag((value & 0b1000_0000) != 0)
    cpu.flags.set_negative_flag((result_8 & 0b1000_0000) != 0)
    cpu.flags.set_zero_flag(result_8 == 0)

    cpu.a = result_8

Important invariants

Carry receives A's original bit 7
A is masked to eight bits; Z and N use that masked result
no memory address is decoded and no bus location is modified
status bits other than C, Z, and N are preserved

Common misconception

Do not call memory asl(cpu, cpu.a): the accumulator's value is data, not an address, and ASL A must not touch memory.

Out of scope

importing asl_a and mapping opcode 0x0A (test 090)
memory opcode wrappers and mappings (tests 091-094)
cycle timing

Run this lesson

uv run pytest tests/chapter_01_cpu/test_089_instruction_asl_a.py -v

Example implementation

Suggested implementation

def asl_a(cpu: CPU):
    value = cpu.a
    result = cpu.a << 1
    result_8 = result & 0xFF

    # Set flags
    cpu.flags.set_carry_flag((value & 0b1000_0000) != 0)
    cpu.flags.set_negative_flag((result_8 & 0b1000_0000) != 0)
    cpu.flags.set_zero_flag(result_8 == 0)

    cpu.a = result_8

Show test codetest_089_instruction_asl_a.py

from emulator.cpu.instructions import asl_a
from tests.helpers import NEGATIVE_FLAG, ZERO_FLAG, make_cpu


CARRY_FLAG = 1 << 0


def test_asl_a_shifts_accumulator_left():
    """Objective: A value 0b0000_0011 becomes 0b0000_0110."""
    cpu = make_cpu()
    cpu.a = 0b0000_0011

    asl_a(cpu)

    assert cpu.a == 0b0000_0110


def test_asl_a_sets_carry_from_old_accumulator_bit_7():
    """Objective: old A bit 7 is moved into Carry before the 8-bit wrap."""
    cpu = make_cpu()
    cpu.a = 0b1000_0001

    asl_a(cpu)

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


def test_asl_a_clears_carry_when_old_accumulator_bit_7_was_clear():
    """Objective: Carry is cleared when original A did not have bit 7 set."""
    cpu = make_cpu()
    cpu.p |= CARRY_FLAG
    cpu.a = 0b0100_0000

    asl_a(cpu)

    assert cpu.a == 0b1000_0000
    assert (cpu.p & CARRY_FLAG) == 0


def test_asl_a_sets_zero_flag_when_result_is_zero():
    """Objective: A=0x80 shifts to 0x00 and sets Zero flag."""
    cpu = make_cpu()
    cpu.a = 0x80

    asl_a(cpu)

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


def test_asl_a_sets_negative_flag_from_result_bit_7():
    """Objective: A=0x40 shifts to 0x80, so Negative flag is set."""
    cpu = make_cpu()
    cpu.a = 0x40

    asl_a(cpu)

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


def test_asl_a_does_not_write_shifted_result_to_memory():
    """Objective: ASL A writes to A only; it must not treat A as a memory address."""
    cpu = make_cpu()
    cpu.a = 0x20
    cpu.bus.write(0x0020, 0x99)

    asl_a(cpu)

    assert cpu.a == 0x40
    assert cpu.bus.read(0x0020) == 0x99

Test functions (6)

test_asl_a_shifts_accumulator_left
test_asl_a_sets_carry_from_old_accumulator_bit_7
test_asl_a_clears_carry_when_old_accumulator_bit_7_was_clear
test_asl_a_sets_zero_flag_when_result_is_zero
test_asl_a_sets_negative_flag_from_result_bit_7
test_asl_a_does_not_write_shifted_result_to_memory