063. SBC 立即寻址

添加 SBC 立即寻址。

第 63 / 356 · tests/chapter_01_cpu/test_063_SBC_immediate.py

要更新的文件

emulator/cpu/opcodes.py

要添加/更新的符号

the `sbc` import, opcodes.sbc_immediate, and OPCODE_TABLE[0xE9]

为什么需要这一步

这是核心 sbc 指令的第一个操作码包装器。立即寻址模式已经返回操作数值，因此这个包装器只需解析并委托。

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, adc, sbc,
)

def sbc_immediate(cpu: CPU):
    value = immediate(cpu)
    sbc(cpu, value)

OPCODE_TABLE = {
    # ... existing entries ...
    0xE9: sbc_immediate,
}

重要不变量

操作码 $E9 映射到 sbc_immediate
immediate 恰好取出并返回一个操作数字节
该返回值被直接传给 sbc
执行这条两字节指令会使 PC 前进两个字节

常见误解

不要使用 immediate 的结果去读总线；与内存寻址辅助函数不同，它返回的是操作数值而不是地址。

范围之外

所有内存寻址的 SBC 操作码包装器
对核心 SBC 算术或标志的更改
周期时序和十进制模式

运行本课

uv run pytest tests/chapter_01_cpu/test_063_SBC_immediate.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, adc, sbc,
)

def sbc_immediate(cpu: CPU):
    value = immediate(cpu)
    sbc(cpu, value)

OPCODE_TABLE = {
    # ... existing entries ...
    0xE9: sbc_immediate,
}

查看测试代码test_063_SBC_immediate.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_immediate_handler_exists_and_is_in_opcode_table():
    """
    Objective:
    Create sbc_immediate(cpu) and add 0xE9 to OPCODE_TABLE.

    Important:
    immediate(cpu) returns the value directly.
    Do not read from cpu.bus again for immediate mode.
    """
    assert hasattr(opcodes, "sbc_immediate")
    assert callable(opcodes.sbc_immediate)
    assert list(inspect.signature(opcodes.sbc_immediate).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0xE9] is opcodes.sbc_immediate


def test_opcode_E9_sbc_immediate_subtracts_value_from_register_a():
    """
    Objective:
    E9 01 means SBC #$01.

    If Carry is set, A = A - value.
    """
    cpu, _, rom = make_cpu_with_rom()
    rom.write(0x0000, 0xE9)
    rom.write(0x0001, 0x01)

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

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


def test_opcode_E9_sbc_immediate_uses_carry_as_no_borrow():
    """
    Objective:
    If Carry is clear, SBC subtracts one extra.

    A = A - value - 1
    """
    cpu, _, rom = make_cpu_with_rom()
    rom.write(0x0000, 0xE9)
    rom.write(0x0001, 0x01)

    cpu.reset()
    cpu.a = 0x10
    cpu.flags.set_carry_flag(False)
    cpu.step()

    assert cpu.a == 0x0E
    assert cpu.pc == 0x8002

测试函数 (3)

test_sbc_immediate_handler_exists_and_is_in_opcode_table
test_opcode_E9_sbc_immediate_subtracts_value_from_register_a
test_opcode_E9_sbc_immediate_uses_carry_as_no_borrow