085. Iny 隐含寻址

接入 INY 隐含寻址操作码。

第 85 / 356 · tests/chapter_01_cpu/test_085_INY_implied.py

在这一步中，使用测试 084 中的 instructions.iny，仅添加分发逻辑。

生产代码位置与符号

emulator/cpu/opcodes.py: imported `iny` and `OPCODE_TABLE[0xC8]`

本步骤存在的原因

由于 Y 是 INY 的隐含操作数，操作码表可以直接分发到指令函数，而无需解码操作数。

本步骤的实现建议

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

OPCODE_TABLE = {
    # ... existing entries ...
    0xC8: iny,
}

重要不变量

操作码 0xC8 映射到精确的 iny(cpu) 函数
这条单字节指令将 PC 推进一位
回绕及 Z/N 行为仍保留在测试 084 的指令函数中

常见误解

不要仅仅因为表需要一个可调用对象就使用 immediate(cpu)；INY 没有操作数字节，立即数解码会破坏 PC。

不在本步骤范围内

DEY 行为及操作码映射（测试 086-087）
后续的 ASL 指令形式
周期计时

运行本课

uv run pytest tests/chapter_01_cpu/test_085_INY_implied.py -v

示例实现

本步骤的实现建议

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

OPCODE_TABLE = {
    # ... existing entries ...
    0xC8: iny,
}

查看测试代码test_085_INY_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_iny_opcode_exists_and_is_in_opcode_table():
    """Objective: add 0xC8 to OPCODE_TABLE and point it to iny."""
    assert hasattr(opcodes, "iny")
    assert callable(opcodes.iny)
    assert list(inspect.signature(opcodes.iny).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0xC8] is opcodes.iny


def test_opcode_C8_iny_implied_increments_y_register():
    """Objective: C8 means INY, so Y is incremented and PC advances by 1."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0xC8)

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

    assert cpu.y == 0x11
    assert cpu.pc == 0x8001


def test_opcode_C8_iny_implied_updates_zero_flag():
    """Objective: Y=0xFF wraps to 0x00 and sets Zero flag."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0xC8)

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

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


def test_opcode_C8_iny_implied_updates_negative_flag():
    """Objective: Y=0x7F becomes 0x80 and sets Negative flag."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0xC8)

    cpu.reset()
    cpu.y = 0x7F
    cpu.step()

    assert cpu.y == 0x80
    assert (cpu.p & NEGATIVE_FLAG) != 0
    assert (cpu.p & ZERO_FLAG) == 0

测试函数 (4)

test_iny_opcode_exists_and_is_in_opcode_table
test_opcode_C8_iny_implied_increments_y_register
test_opcode_C8_iny_implied_updates_zero_flag
test_opcode_C8_iny_implied_updates_negative_flag