207. Opcodes implícitos de control de flags

conectar los opcodes implícitos de control de flags.

Lección 207 de 356 · tests/chapter_01_cpu/test_207_flag_control_implied_opcodes.py

Por qué existe este paso

En este paso, actualiza la importación de instrucciones y `OPCODE_TABLE de emulator/cpu/opcodes.py. Las funciones del paso 206 se vuelven ejecutables a través de emulator/cpu/cpu.py::CPU.step`.

Implementación sugerida

# Add clc, sec, cli, sei, cld, sed, clv to the instruction import.
OPCODE_TABLE = {
    # existing entries
    0x18: clc,
    0x38: sec,
    0x58: cli,
    0x78: sei,
    0xD8: cld,
    0xF8: sed,
    0xB8: clv,
}

Invariante: cada mapeo despacha directamente a una operación de un argumento; cada instrucción ocupa un byte, así que solo la obtención del opcode avanza PC al byte siguiente y únicamente cambia el flag correspondiente. El error conceptual habitual es añadir un modo de direccionamiento que consuma un operando, o intercambiar los pares de opcodes de activación/desactivación.

Fuera de alcance: NOP y el opcode $EA corresponden a los pasos 208-209, y la semántica de entrega de interrupciones es comportamiento posterior.

Ejecutar esta lección

uv run pytest tests/chapter_01_cpu/test_207_flag_control_implied_opcodes.py -v

Implementación de ejemplo

Implementación sugerida

# Add clc, sec, cli, sei, cld, sed, clv to the instruction import.
OPCODE_TABLE = {
    # existing entries
    0x18: clc,
    0x38: sec,
    0x58: cli,
    0x78: sei,
    0xD8: cld,
    0xF8: sed,
    0xB8: clv,
}

Ver código del testtest_207_flag_control_implied_opcodes.py

import inspect

import pytest

from emulator.bus.cpu_bus import CpuBus
from emulator.cpu import opcodes
from emulator.cpu.cpu import CPU
from emulator.cpu.instructions import clc, cld, cli, clv, sec, sed, sei
from emulator.memory.fake_rom import FakeROM


CARRY_FLAG = 1 << 0
INTERRUPT_DISABLE_FLAG = 1 << 2
DECIMAL_FLAG = 1 << 3
OVERFLOW_FLAG = 1 << 6


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


@pytest.mark.parametrize(
    ("opcode", "instruction"),
    [
        (0x18, clc),
        (0x38, sec),
        (0x58, cli),
        (0x78, sei),
        (0xD8, cld),
        (0xF8, sed),
        (0xB8, clv),
    ],
)
def test_flag_control_opcodes_are_in_opcode_table(opcode, instruction):
    """Objective: each official flag-control opcode maps to the right function."""
    assert opcodes.OPCODE_TABLE[opcode] is instruction


@pytest.mark.parametrize("instruction", [clc, sec, cli, sei, cld, sed, clv])
def test_flag_control_instruction_signatures_take_only_cpu(instruction):
    """Objective: implied instructions do not need operand arguments."""
    assert list(inspect.signature(instruction).parameters) == ["cpu"]


@pytest.mark.parametrize(
    ("opcode", "initial_p", "expected_flag", "flag_mask"),
    [
        (0x18, CARRY_FLAG, False, CARRY_FLAG),
        (0x38, 0x00, True, CARRY_FLAG),
        (0x58, INTERRUPT_DISABLE_FLAG, False, INTERRUPT_DISABLE_FLAG),
        (0x78, 0x00, True, INTERRUPT_DISABLE_FLAG),
        (0xD8, DECIMAL_FLAG, False, DECIMAL_FLAG),
        (0xF8, 0x00, True, DECIMAL_FLAG),
        (0xB8, OVERFLOW_FLAG, False, OVERFLOW_FLAG),
    ],
)
def test_flag_control_opcodes_change_their_target_flag(
    opcode,
    initial_p,
    expected_flag,
    flag_mask,
):
    """Objective: executing each opcode changes exactly its intended flag."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, opcode)

    cpu.reset()
    cpu.p = initial_p
    cpu.step()

    assert bool(cpu.p & flag_mask) is expected_flag


def test_flag_control_opcodes_do_not_fetch_operand_bytes():
    """
    Objective:
    Flag-control opcodes are one byte long.

    The byte after the opcode is the next instruction, not an operand.
    """
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x38)  # SEC
    rom.write(0x0001, 0x99)

    cpu.reset()
    cpu.p = 0x00
    cpu.step()

    assert cpu.flags.get_carry_flag() is True
    assert cpu.pc == 0x8001

Funciones de test (4)

test_flag_control_opcodes_are_in_opcode_table
test_flag_control_instruction_signatures_take_only_cpu
test_flag_control_opcodes_change_their_target_flag
test_flag_control_opcodes_do_not_fetch_operand_bytes