069. SBC 间接,X

添加 SBC (间接,X)。

第 69 / 356 · tests/chapter_01_cpu/test_069_SBC_indirect_x.py

要更新的文件

emulator/cpu/opcodes.py

要添加/更新的符号

opcodes.sbc_indirect_x and OPCODE_TABLE[0xE1]

为什么需要这一步

SBC 通过将现有的 indirect_x 解析器与标准的内存读取和指令委托包装器组合，获得了先变址间接访问能力。

完整示例实现

# emulator/cpu/opcodes.py
def sbc_indirect_x(cpu: CPU):
    addr = indirect_x(cpu)
    value = cpu.bus.read(addr)
    sbc(cpu, value)

OPCODE_TABLE = {
    # ... existing entries ...
    0xE1: sbc_indirect_x,
}

重要不变量

indirect_x 在读取指针之前先把 X 加到零页操作数上
指针字节及其零页回绕由解析器处理
包装器读取最终地址并把读到的字节传给 sbc
执行这条两字节指令会使 PC 前进两个字节

常见误解

不要把零页指针位置当作 SBC 的操作数来读取；indirect_x 返回的是最终的 16 位目标地址，必须读取该地址处的内容。

范围之外

(间接),Y 的 SBC 包装器
对间接寻址或 SBC 算术的更改
周期时序

运行本课

uv run pytest tests/chapter_01_cpu/test_069_SBC_indirect_x.py -v

示例实现

完整示例实现

# emulator/cpu/opcodes.py
def sbc_indirect_x(cpu: CPU):
    addr = indirect_x(cpu)
    value = cpu.bus.read(addr)
    sbc(cpu, value)

OPCODE_TABLE = {
    # ... existing entries ...
    0xE1: sbc_indirect_x,
}

查看测试代码test_069_SBC_indirect_x.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


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_sbc_indirect_x_handler_exists_and_is_in_opcode_table():
    """Objective: create sbc_indirect_x(cpu) and add 0xE1 to OPCODE_TABLE."""
    assert hasattr(opcodes, "sbc_indirect_x")
    assert callable(opcodes.sbc_indirect_x)
    assert list(inspect.signature(opcodes.sbc_indirect_x).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0xE1] is opcodes.sbc_indirect_x


def test_opcode_E1_sbc_indirect_x_subtracts_value_from_final_address():
    """Objective: E1 20 with X=0x04 uses pointer at $0024 and subtracts final memory value."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0xE1)
    rom.write(0x0001, 0x20)
    bus.write(0x0024, 0x00)
    bus.write(0x0025, 0x02)
    bus.write(0x0200, 0x01)

    cpu.reset()
    cpu.a = 0x10
    cpu.x = 0x04
    cpu.flags.set_carry_flag(True)
    cpu.step()

    assert cpu.a == 0x0F
    assert cpu.pc == 0x8002

测试函数 (2)

test_sbc_indirect_x_handler_exists_and_is_in_opcode_table
test_opcode_E1_sbc_indirect_x_subtracts_value_from_final_address