124. 间接,Y AND

添加 AND (indirect),Y 操作码 ``0x31``。

第 124 / 356 · tests/chapter_01_cpu/test_124_AND_indirect_y.py

此步骤存在的原因

此步骤通过“先获取零页指针、再进行 Y 索引”的方式补全了 AND 的寻址覆盖范围,同时复用了间接寻址辅助函数与通用的 AND 基元。

在此步骤中,通过在 `emulator/cpu/opcodes.py` 中添加以下内容,完成已编号的 AND 寻址模式:

def and_indirect_y(cpu: CPU):
    addr = indirect_y(cpu)
    value = cpu.bus.read(addr)
    and_a(cpu, value)

OPCODE_TABLE = {
    ...
    0x31: and_indirect_y,
}

`emulator/cpu/addressing_modes.py::indirect_y 获取零页指针位置,以对高字节做零页环绕处理的方式读取其小端序字,然后才加上 cpu.y。目标值被传给 instructions.and_a`。A 与 Z/N 会改变;Y、Carry/Overflow、指针字节及目标内存保持不变;PC 前进两个字节。

常见误解:(indirect),Y 并不会将 Y 加到零页指针位置上;那种顺序属于 (indirect,X)。不在本步骤范围内:从第 125 课开始的 ORA、EOR 及 BIT 相关工作。

运行本课

uv run pytest tests/chapter_01_cpu/test_124_AND_indirect_y.py -v

示例实现

此步骤存在的原因

def and_indirect_y(cpu: CPU):
    addr = indirect_y(cpu)
    value = cpu.bus.read(addr)
    and_a(cpu, value)

OPCODE_TABLE = {
    ...
    0x31: and_indirect_y,
}

查看测试代码test_124_AND_indirect_y.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_and_indirect_y_handler_exists_and_is_in_opcode_table():
    """Objective: create and_indirect_y(cpu) and add 0x31 to OPCODE_TABLE."""
    assert hasattr(opcodes, "and_indirect_y")
    assert callable(opcodes.and_indirect_y)
    assert list(inspect.signature(opcodes.and_indirect_y).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0x31] is opcodes.and_indirect_y


def test_opcode_31_and_indirect_y_reads_pointed_indexed_memory_value():
    """Objective: 31 20 reads pointer at $20/$21, then adds Y."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x31)
    rom.write(0x0001, 0x20)
    bus.write(0x0020, 0x00)
    bus.write(0x0021, 0x02)
    bus.write(0x0204, 0x0F)

    cpu.reset()
    cpu.y = 0x04
    cpu.a = 0xF3
    cpu.step()

    assert cpu.a == 0x03
    assert cpu.pc == 0x8002

测试函数 (2)

test_and_indirect_y_handler_exists_and_is_in_opcode_table
test_opcode_31_and_indirect_y_reads_pointed_indexed_memory_value