060. Adc (间接,X)

将 ADC (间接,X) 寻址（操作码 $61）接入 CPU 分发。

第 60 / 356 · tests/chapter_01_cpu/test_060_ADC_indirect_x.py

要更新的文件

emulator/cpu/opcodes.py

要创建/更新的符号

opcodes.adc_indirect_x
OPCODE_TABLE[$61]

为什么需要这一步

indirect_x 先把 X 加到零页操作数上，从零页读取一个小端指针，并返回最终的有效地址。随后该处理函数必须从这个最终地址读取 ADC 所需的值。

完整示例实现

# emulator/cpu/opcodes.py
def adc_indirect_x(cpu):
    addr = indirect_x(cpu)
    value = cpu.bus.read(addr)
    adc(cpu, value)

OPCODE_TABLE = {
    # ...existing entries...
    0x61: adc_indirect_x,
}

重要不变量

X 在解引用之前先对零页指针的位置进行变址
指针字节的读取在零页内环绕
指针指向的是一个地址，因此最终读取由处理函数完成
PC 总共前进两个字节

常见误区

不要把 X 加到最终的 16 位目标地址上。那描述的是另一种操作；对于 (间接,X)，X 在指针被解引用之前先选定零页指针。

范围之外

ADC (间接),Y，由下一个编号测试引入
周期计数
对 indirect_x 或 adc 的修改

运行本课

uv run pytest tests/chapter_01_cpu/test_060_ADC_indirect_x.py -v

示例实现

完整示例实现

# emulator/cpu/opcodes.py
def adc_indirect_x(cpu):
    addr = indirect_x(cpu)
    value = cpu.bus.read(addr)
    adc(cpu, value)

OPCODE_TABLE = {
    # ...existing entries...
    0x61: adc_indirect_x,
}

查看测试代码test_060_ADC_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_adc_indirect_x_handler_exists_and_is_in_opcode_table():
    """Objective: create adc_indirect_x(cpu) and add 0x61 to OPCODE_TABLE."""
    assert hasattr(opcodes, "adc_indirect_x")
    assert callable(opcodes.adc_indirect_x)
    assert list(inspect.signature(opcodes.adc_indirect_x).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0x61] is opcodes.adc_indirect_x


def test_opcode_61_adc_indirect_x_adds_value_from_final_address():
    """Objective: 61 20 with X=0x04 uses pointer at $0024 and adds final memory value."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x61)
    rom.write(0x0001, 0x20)
    bus.write(0x0024, 0x00)
    bus.write(0x0025, 0x02)
    bus.write(0x0200, 0x05)

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

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

测试函数 (2)

test_adc_indirect_x_handler_exists_and_is_in_opcode_table
test_opcode_61_adc_indirect_x_adds_value_from_final_address