051. 转移操作码

将四条转移指令注册为隐含寻址操作码。

第 51 / 356 · tests/chapter_01_cpu/test_051_transfer_opcodes.py

要更新的文件

emulator/cpu/opcodes.py

要更新的符号

the instruction import and OPCODE_TABLE

为什么需要这一步

测试 050 中的转移操作已经具备 CPU.step 期望从操作码表条目得到的同一种单参数形态。隐含寻址指令没有需要解码的操作数，因此增设中间的操作码处理函数不会带来任何行为。

完整示例实现

# emulator/cpu/opcodes.py
from emulator.cpu.instructions import (
    lda, sta, ldx, stx, ldy, sty, tax, txa, tay, tya,
)

OPCODE_TABLE = {
    # ...the existing load/store entries...
    0xAA: tax,
    0x8A: txa,
    0xA8: tay,
    0x98: tya,
}

重要不变量

每个操作码直接映射到其指令函数
CPU.step 只消耗操作码，使 PC 前进一个字节
转移与 Zero/Negative 行为仍保留在指令层

常见误区

不要创建 tax_implied 式的包装函数，也不要取操作数；这四条指令使用隐含寻址，已经符合分发契约。

范围之外

测试 052 引入的 FlagsHandler 重构
ADC 及其操作码处理函数
周期计数

运行本课

uv run pytest tests/chapter_01_cpu/test_051_transfer_opcodes.py -v

示例实现

完整示例实现

# emulator/cpu/opcodes.py
from emulator.cpu.instructions import (
    lda, sta, ldx, stx, ldy, sty, tax, txa, tay, tya,
)

OPCODE_TABLE = {
    # ...the existing load/store entries...
    0xAA: tax,
    0x8A: txa,
    0xA8: tay,
    0x98: tya,
}

查看测试代码test_051_transfer_opcodes.py

from emulator.bus.cpu_bus import CpuBus
from emulator.cpu import instructions, opcodes
from emulator.cpu.cpu import CPU
from emulator.memory.fake_rom import FakeROM


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_transfer_opcodes_are_in_opcode_table():
    """
    Objective:
    Add these entries to OPCODE_TABLE:

        0xAA: tax,
        0x8A: txa,
        0xA8: tay,
        0x98: tya,

    Why:
    These instructions use implied addressing.
    CPU.step() fetches only the opcode, then calls the instruction.
    """
    assert opcodes.OPCODE_TABLE[0xAA] is instructions.tax
    assert opcodes.OPCODE_TABLE[0x8A] is instructions.txa
    assert opcodes.OPCODE_TABLE[0xA8] is instructions.tay
    assert opcodes.OPCODE_TABLE[0x98] is instructions.tya


def test_opcode_AA_tax_transfers_a_to_x():
    """Objective: AA means TAX. It copies A into X."""
    cpu, _, rom = make_cpu_with_rom()
    rom.write(0x0000, 0xAA)

    cpu.reset()
    cpu.a = 0x42
    cpu.step()

    assert cpu.x == 0x42
    assert cpu.pc == 0x8001


def test_opcode_8A_txa_transfers_x_to_a():
    """Objective: 8A means TXA. It copies X into A."""
    cpu, _, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x8A)

    cpu.reset()
    cpu.x = 0x42
    cpu.step()

    assert cpu.a == 0x42
    assert cpu.pc == 0x8001


def test_opcode_A8_tay_transfers_a_to_y():
    """Objective: A8 means TAY. It copies A into Y."""
    cpu, _, rom = make_cpu_with_rom()
    rom.write(0x0000, 0xA8)

    cpu.reset()
    cpu.a = 0x42
    cpu.step()

    assert cpu.y == 0x42
    assert cpu.pc == 0x8001


def test_opcode_98_tya_transfers_y_to_a():
    """Objective: 98 means TYA. It copies Y into A."""
    cpu, _, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x98)

    cpu.reset()
    cpu.y = 0x42
    cpu.step()

    assert cpu.a == 0x42
    assert cpu.pc == 0x8001

测试函数 (5)

test_transfer_opcodes_are_in_opcode_table
test_opcode_AA_tax_transfers_a_to_x
test_opcode_8A_txa_transfers_x_to_a
test_opcode_A8_tay_transfers_a_to_y
test_opcode_98_tya_transfers_y_to_a