054. Adc 立即数

将 ADC 立即数寻址（操作码 $69）接入 CPU 分发。

第 54 / 356 · tests/chapter_01_cpu/test_054_ADC_immediate.py

要更新的文件

emulator/cpu/opcodes.py

要创建/更新的符号

opcodes.adc_immediate
OPCODE_TABLE[$69]
the `adc` instruction import

为什么需要这一步

测试 053 提供的是作用于已解析值的算术运算。立即数寻址模式自行提供下一个程序字节，因此该处理函数把 immediate(cpu) 的返回值直接传给 adc。

完整示例实现

# emulator/cpu/opcodes.py
from emulator.cpu.instructions import ..., adc

def adc_immediate(cpu):
    value = immediate(cpu)
    adc(cpu, value)

OPCODE_TABLE = {
    # ...existing entries...
    0x69: adc_immediate,
}

重要不变量

该处理函数只接受 cpu
立即数寻址模式恰好消耗一个操作数字节
PC 总共前进两个字节：操作码加操作数
该处理函数把所有算术运算和标志位变更都委托给 adc

常见误区

不要把立即数字节当作地址再去读一次总线；immediate(cpu) 已经返回操作数的值。

范围之外

所有访问内存的 ADC 操作码
对 adc 或寻址辅助函数的修改
周期计数

运行本课

uv run pytest tests/chapter_01_cpu/test_054_ADC_immediate.py -v

示例实现

完整示例实现

# emulator/cpu/opcodes.py
from emulator.cpu.instructions import ..., adc

def adc_immediate(cpu):
    value = immediate(cpu)
    adc(cpu, value)

OPCODE_TABLE = {
    # ...existing entries...
    0x69: adc_immediate,
}

查看测试代码test_054_ADC_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_adc_immediate_handler_exists_and_is_in_opcode_table():
    """
    Objective:
    Create adc_immediate(cpu) and add 0x69 to OPCODE_TABLE.

    Important:
    immediate(cpu) returns the value directly.
    Do not read from cpu.bus again for immediate mode.
    """
    assert hasattr(opcodes, "adc_immediate")
    assert callable(opcodes.adc_immediate)
    assert list(inspect.signature(opcodes.adc_immediate).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0x69] is opcodes.adc_immediate


def test_opcode_69_adc_immediate_adds_value_to_register_a():
    """Objective: 69 05 means ADC #$05, so A = A + 0x05 + Carry."""
    cpu, _, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x69)
    rom.write(0x0001, 0x05)

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

    assert cpu.a == 0x15
    assert cpu.pc == 0x8002


def test_opcode_69_adc_immediate_uses_carry_flag():
    """Objective: ADC includes the old Carry flag in the addition."""
    cpu, _, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x69)
    rom.write(0x0001, 0x05)

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

    assert cpu.a == 0x16
    assert cpu.pc == 0x8002

测试函数 (3)

test_adc_immediate_handler_exists_and_is_in_opcode_table
test_opcode_69_adc_immediate_adds_value_to_register_a
test_opcode_69_adc_immediate_uses_carry_flag