125. Instruction or a

implement addressing-independent ORA behavior.

Lesson 125 of 356 · tests/chapter_01_cpu/test_125_instruction_or_a.py

Why this step exists

A single value-oriented ORA primitive centralizes inclusive-OR result and flag rules before opcode handlers introduce the individual addressing forms.

In this step, after lessons 116-124 complete AND, add only this symbol to `emulator/cpu/instructions.py`:

def or_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 API keeps ORA semantics independent of addressing. The eight-bit result is stored in A; Zero reflects equality to zero and Negative reflects result bit 7. Carry, Overflow, memory, X, Y, and PC are invariant because `or_a` does not touch them.

Misconception: ORA means inclusive bitwise OR, not XOR, and `value is not a bus address. Out of scope: importing or_a into emulator/cpu/opcodes.py` and all ORA handlers (lessons 126-133), plus the EOR and BIT symbols introduced in lessons 134 and 143.

Run this lesson

uv run pytest tests/chapter_01_cpu/test_125_instruction_or_a.py -v

Example implementation

Why this step exists

def or_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_125_instruction_or_a.py

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


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


def test_or_a_stores_bitwise_or_result_in_accumulator():
    """Objective: A becomes A | value."""
    cpu = make_cpu()
    cpu.a = 0b1100_0000

    or_a(cpu, 0b0000_1010)

    assert cpu.a == 0b1100_1010


def test_or_a_sets_zero_flag_when_result_is_zero():
    """Objective: when A | value is 0, Zero flag is set."""
    cpu = make_cpu()
    cpu.a = 0x00

    or_a(cpu, 0x00)

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


def test_or_a_sets_negative_flag_from_result_bit_7():
    """Objective: if result bit 7 is set, Negative flag is set."""
    cpu = make_cpu()
    cpu.a = 0x01

    or_a(cpu, 0x80)

    assert cpu.a == 0x81
    assert (cpu.p & NEGATIVE_FLAG) != 0
    assert (cpu.p & ZERO_FLAG) == 0


def test_or_a_does_not_modify_carry_or_overflow_flags():
    """Objective: ORA only updates Z/N; Carry and Overflow are preserved."""
    cpu = make_cpu()
    cpu.a = 0x00
    cpu.p |= CARRY_FLAG
    cpu.p |= OVERFLOW_FLAG

    or_a(cpu, 0x0F)

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

Test functions (4)

test_or_a_stores_bitwise_or_result_in_accumulator
test_or_a_sets_zero_flag_when_result_is_zero
test_or_a_sets_negative_flag_from_result_bit_7
test_or_a_does_not_modify_carry_or_overflow_flags