087. Dey 隐含寻址

接入 DEY 隐含寻址操作码。

第 87 / 356 · tests/chapter_01_cpu/test_087_DEY_implied.py

在这一步中，使用测试 086 中的 instructions.dey，仅添加其操作码接入。

生产代码位置与符号

emulator/cpu/opcodes.py: imported `dey` and `OPCODE_TABLE[0x88]`

本步骤存在的原因

DEY 的 Y 寄存器操作数是隐含的，可以直接进行表分发，无需寻址模式包装器。

本步骤的实现建议

# emulator/cpu/opcodes.py
from emulator.cpu.instructions import dey  # alongside existing imports

OPCODE_TABLE = {
    # ... existing entries ...
    0x88: dey,
}

重要不变量

操作码 0x88 分发到精确的 dey(cpu) 函数
不消耗操作数，PC 推进一个字节
Y 的回绕及 Z/N 更新仍由测试 086 的 dey 负责

常见误解

不要围绕 dec 构建一个内存递减包装器；DEY 修改的是寄存器，不进行任何总线读或写。

不在本步骤范围内

测试 084-085 中此前的 INY 行为/映射
从测试 088 开始的 ASL 行为及操作码
周期计时

运行本课

uv run pytest tests/chapter_01_cpu/test_087_DEY_implied.py -v

示例实现

本步骤的实现建议

# emulator/cpu/opcodes.py
from emulator.cpu.instructions import dey  # alongside existing imports

OPCODE_TABLE = {
    # ... existing entries ...
    0x88: dey,
}

查看测试代码test_087_DEY_implied.py

import inspect

from emulator.bus.cpu_bus import CpuBus
from emulator.cpu import opcodes
from emulator.cpu.cpu import CPU
from emulator.memory.fake_rom import FakeROM
from tests.helpers import NEGATIVE_FLAG, ZERO_FLAG


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


def test_dey_opcode_exists_and_is_in_opcode_table():
    """Objective: add 0x88 to OPCODE_TABLE and point it to dey."""
    assert hasattr(opcodes, "dey")
    assert callable(opcodes.dey)
    assert list(inspect.signature(opcodes.dey).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0x88] is opcodes.dey


def test_opcode_88_dey_implied_decrements_y_register():
    """Objective: 88 means DEY, so Y is decremented and PC advances by 1."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x88)

    cpu.reset()
    cpu.y = 0x10
    cpu.step()

    assert cpu.y == 0x0F
    assert cpu.pc == 0x8001


def test_opcode_88_dey_implied_updates_zero_flag():
    """Objective: Y=0x01 becomes 0x00 and sets Zero flag."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x88)

    cpu.reset()
    cpu.y = 0x01
    cpu.step()

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


def test_opcode_88_dey_implied_updates_negative_flag():
    """Objective: Y=0x00 wraps to 0xFF and sets Negative flag."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x88)

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

    assert cpu.y == 0xFF
    assert (cpu.p & NEGATIVE_FLAG) != 0
    assert (cpu.p & ZERO_FLAG) == 0

测试函数 (4)

test_dey_opcode_exists_and_is_in_opcode_table
test_opcode_88_dey_implied_decrements_y_register
test_opcode_88_dey_implied_updates_zero_flag
test_opcode_88_dey_implied_updates_negative_flag