056. Adc 零页,X

将 ADC 零页,X 寻址（操作码 $75）接入 CPU 分发。

第 56 / 356 · tests/chapter_01_cpu/test_056_ADC_zero_page_x.py

要更新的文件

emulator/cpu/opcodes.py

要创建/更新的符号

opcodes.adc_zero_page_x
OPCODE_TABLE[$75]

为什么需要这一步

该模式复用已有的 zero_page_x 解析器，包括它的零页环绕行为。操作码处理函数随后读取解析出的位置，并把该处的值传给 adc。

完整示例实现

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

OPCODE_TABLE = {
    # ...existing entries...
    0x75: adc_zero_page_x,
}

重要不变量

X 由 zero_page_x 负责相加，而不是由处理函数再加一次
(operand + X) & 0xFF 使有效地址保持在零页之内
传给 adc 的是有效地址处的字节，而不是地址本身
PC 总共前进两个字节

常见误区

不要对变址使用普通的 16 位加法；$FF + $01 必须解析为 $0000，而不是 $0100。

范围之外

绝对变址与间接的 ADC 寻址模式
跨页时序
对现有寻址辅助函数的修改

运行本课

uv run pytest tests/chapter_01_cpu/test_056_ADC_zero_page_x.py -v

示例实现

完整示例实现

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

OPCODE_TABLE = {
    # ...existing entries...
    0x75: adc_zero_page_x,
}

查看测试代码test_056_ADC_zero_page_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_zero_page_x_handler_exists_and_is_in_opcode_table():
    """Objective: create adc_zero_page_x(cpu) and add 0x75 to OPCODE_TABLE."""
    assert hasattr(opcodes, "adc_zero_page_x")
    assert callable(opcodes.adc_zero_page_x)
    assert list(inspect.signature(opcodes.adc_zero_page_x).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0x75] is opcodes.adc_zero_page_x


def test_opcode_75_adc_zero_page_x_adds_indexed_value():
    """Objective: 75 10 with X=0x03 adds RAM[$0013] to A."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x75)
    rom.write(0x0001, 0x10)
    bus.write(0x0013, 0x05)

    cpu.reset()
    cpu.a = 0x10
    cpu.x = 0x03
    cpu.flags.set_carry_flag(False)
    cpu.step()

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


def test_opcode_75_adc_zero_page_x_wraps_inside_page_zero():
    """Objective: Zero Page,X wraps before reading the ADC value."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x75)
    rom.write(0x0001, 0xFF)
    bus.write(0x0000, 0x05)

    cpu.reset()
    cpu.a = 0x10
    cpu.x = 0x01
    cpu.flags.set_carry_flag(False)
    cpu.step()

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

测试函数 (3)

test_adc_zero_page_x_handler_exists_and_is_in_opcode_table
test_opcode_75_adc_zero_page_x_adds_indexed_value
test_opcode_75_adc_zero_page_x_wraps_inside_page_zero