116. Instruction and a

implement addressing-independent AND behavior.

Lesson 116 of 356 · tests/chapter_01_cpu/test_116_instruction_and_a.py

In this step, add only `and_a to emulator/cpu/instructions.py`. Opcode imports and AND addressing modes follow in lessons 117-124.

Why this step exists

Defining AND as a value-oriented primitive keeps accumulator and flag semantics in one place so every addressing-mode handler can reuse them consistently.

Suggested implementation

def and_a(cpu: CPU, value: int):
    result_8 = (cpu.a & value) & 0xFF

    # Flags:
    cpu.flags.set_zero_flag(result_8 == 0)
    cpu.flags.set_negative_flag((result_8 & 0b1000_0000) != 0)

    cpu.a = result_8

The value-oriented signature keeps instruction semantics separate from address decoding. The result is constrained to eight bits, stored in A, Zero reflects equality to zero, and Negative reflects result bit 7. Carry, Overflow, memory, X, Y, and PC are invariant because this function never touches them.

Misconception: `value is not an address to read through cpu.bus; opcode handlers perform such reads. Out of scope: importing and_a into emulator/cpu/opcodes.py` and every AND opcode (lessons 117-124), plus the ORA, EOR, and BIT symbols in lessons 125-145.

Run this lesson

uv run pytest tests/chapter_01_cpu/test_116_instruction_and_a.py -v

Example implementation

Suggested implementation

def and_a(cpu: CPU, value: int):
    result_8 = (cpu.a & value) & 0xFF

    # Flags:
    cpu.flags.set_zero_flag(result_8 == 0)
    cpu.flags.set_negative_flag((result_8 & 0b1000_0000) != 0)

    cpu.a = result_8

Show test codetest_116_instruction_and_a.py

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


CARRY_FLAG = 1 << 0
OVERFLOW_FLAG = 1 << 6


def test_and_a_stores_bitwise_and_result_in_accumulator():
    """Objective: A becomes A & value."""
    cpu = make_cpu()
    cpu.a = 0b1100_1010

    and_a(cpu, 0b1010_1100)

    assert cpu.a == 0b1000_1000


def test_and_a_sets_zero_flag_when_result_is_zero():
    """Objective: when A & value is 0, Zero flag is set."""
    cpu = make_cpu()
    cpu.a = 0b0000_1111


    and_a(cpu, 0b1111_0000)


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


def test_and_a_sets_negative_flag_from_result_bit_7():
    """Objective: if result bit 7 is set, Negative flag is set."""
    cpu = make_cpu()
    cpu.a = 0b1000_1111

    and_a(cpu, 0b1111_0000)

    assert cpu.a == 0b1000_0000
    assert (cpu.p & NEGATIVE_FLAG) != 0
    assert (cpu.p & ZERO_FLAG) == 0


def test_and_a_does_not_modify_carry_or_overflow_flags():
    """Objective: AND only updates Z/N; Carry and Overflow are preserved."""
    cpu = make_cpu()
    cpu.a = 0xFF
    cpu.p |= CARRY_FLAG
    cpu.p |= OVERFLOW_FLAG

    and_a(cpu, 0x0F)

    assert cpu.a == 0x0F
    assert (cpu.p & CARRY_FLAG) != 0
    assert (cpu.p & OVERFLOW_FLAG) != 0

Test functions (4)

test_and_a_stores_bitwise_and_result_in_accumulator
test_and_a_sets_zero_flag_when_result_is_zero
test_and_a_sets_negative_flag_from_result_bit_7
test_and_a_does_not_modify_carry_or_overflow_flags