207. 标志控制隐含寻址操作码

接通隐含寻址的标志控制操作码。

第 207 / 356 · tests/chapter_01_cpu/test_207_flag_control_implied_opcodes.py

本步骤存在的原因

在本步骤中，更新 `emulator/cpu/opcodes.py 的指令导入以及 OPCODE_TABLE。步骤 206 中的函数由此可以通过 emulator/cpu/cpu.py::CPU.step` 执行。

建议实现

# 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,
}

不变式：每个映射都直接分派到一个单参数操作；每条指令都是单字节，因此仅操作码取指就使 PC 前进到下一字节，并且只有目标标志发生变化。常见的误解是添加一个会消耗操作数的寻址模式，或者把设置/清除操作码对弄反。

范围之外：NOP 及操作码 $EA 属于步骤 208-209，中断传递语义是后续的行为。

运行本课

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

示例实现

建议实现

# 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,
}

查看测试代码test_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

测试函数 (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