143. Instruction bit

implement addressing-independent BIT behavior.

Lesson 143 of 356 · tests/chapter_01_cpu/test_143_instruction_bit.py

Why this step exists

BIT has unusual flag semantics: Z comes from A AND value while N and V copy operand bits, so one shared primitive prevents addressing handlers from drifting.

In this step, after the EOR lessons, add exactly this symbol to `emulator/cpu/instructions.py`:

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

    # Flags:
    cpu.flags.set_zero_flag(result_8 == 0)
    cpu.flags.set_negative_flag((value & 0b1000_0000) != 0)
    cpu.flags.set_overflow_flag((value & 0b0100_0000) != 0)

BIT separates the zero test from the two copied operand bits: Z reflects `A & value`, while N and V reflect value bits 7 and 6. A, Carry, memory, X, Y, and PC are invariant; only Z/N/V are rewritten.

Misconception: BIT neither stores `A & value in A nor derives N/V from that AND result. Out of scope: importing bit into emulator/cpu/opcodes.py` and wiring zero-page and absolute opcodes are lessons 144-145.

Run this lesson

uv run pytest tests/chapter_01_cpu/test_143_instruction_bit.py -v

Example implementation

Why this step exists

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

    # Flags:
    cpu.flags.set_zero_flag(result_8 == 0)
    cpu.flags.set_negative_flag((value & 0b1000_0000) != 0)
    cpu.flags.set_overflow_flag((value & 0b0100_0000) != 0)

Show test codetest_143_instruction_bit.py

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


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


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

    bit(cpu, 0b1111_0000)

    assert (cpu.p & ZERO_FLAG) != 0


def test_bit_clears_zero_flag_when_a_and_value_is_not_zero():
    """Objective: if A & value is nonzero, Zero flag is cleared."""
    cpu = make_cpu()
    cpu.p |= ZERO_FLAG
    cpu.a = 0b0000_1111

    bit(cpu, 0b0000_0001)

    assert (cpu.p & ZERO_FLAG) == 0


def test_bit_sets_negative_flag_from_value_bit_7():
    """Objective: BIT copies value bit 7 into Negative flag."""
    cpu = make_cpu()
    cpu.a = 0x00

    bit(cpu, 0b1000_0000)

    assert (cpu.p & NEGATIVE_FLAG) != 0


def test_bit_sets_overflow_flag_from_value_bit_6():
    """Objective: BIT copies value bit 6 into Overflow flag."""
    cpu = make_cpu()
    cpu.a = 0x00

    bit(cpu, 0b0100_0000)

    assert (cpu.p & OVERFLOW_FLAG) != 0


def test_bit_clears_negative_and_overflow_when_value_bits_are_clear():
    """Objective: N and V follow value bits 7 and 6, including clearing them."""
    cpu = make_cpu()
    cpu.p |= NEGATIVE_FLAG
    cpu.p |= OVERFLOW_FLAG
    cpu.a = 0xFF

    bit(cpu, 0b0011_1111)

    assert (cpu.p & NEGATIVE_FLAG) == 0
    assert (cpu.p & OVERFLOW_FLAG) == 0


def test_bit_does_not_modify_accumulator_or_carry_flag():
    """Objective: BIT only updates Z/N/V; A and Carry are preserved."""
    cpu = make_cpu()
    cpu.a = 0b1010_1010
    cpu.p |= CARRY_FLAG

    bit(cpu, 0b1111_0000)

    assert cpu.a == 0b1010_1010
    assert (cpu.p & CARRY_FLAG) != 0

Test functions (6)

test_bit_sets_zero_flag_when_a_and_value_is_zero
test_bit_clears_zero_flag_when_a_and_value_is_not_zero
test_bit_sets_negative_flag_from_value_bit_7
test_bit_sets_overflow_flag_from_value_bit_6
test_bit_clears_negative_and_overflow_when_value_bits_are_clear
test_bit_does_not_modify_accumulator_or_carry_flag