122. 绝对,Y AND

添加 AND absolute,Y 操作码 ``0x39``。

第 122 / 356 · tests/chapter_01_cpu/test_122_AND_absolute_y.py

此步骤存在的原因

这为 absolute,X 提供了以 Y 索引的对应形式,使 AND 能够使用任一索引寄存器,而无需重复其累加器与标志位语义。

在此步骤中,在第 121 课的 absolute,X 之后,仅需在 `emulator/cpu/opcodes.py` 中添加以下处理程序及注册代码:

def and_absolute_y(cpu: CPU):
    addr = absolute_y(cpu)
    value = cpu.bus.read(addr)
    and_a(cpu, value)

OPCODE_TABLE = {
    ...
    0x39: and_absolute_y,
}

`emulator/cpu/addressing_modes.py::absolute_y 先获取小端序的基址字,再加上 cpu.y。解析出的内存值被传入已存在的 instructions.and_a`;A 与 Z/N 会改变,而 Y、Carry/Overflow 及内存保持不变。操作码与字共消耗三个字节。

常见误解:absolute,Y 并不是 zero-page,Y,也不会将索引范围限制在零页内。不在本步骤范围内:AND (indirect,X)/(indirect),Y(第 123-124 课)以及第 125-145 课中新增的 ORA/EOR/BIT。

运行本课

uv run pytest tests/chapter_01_cpu/test_122_AND_absolute_y.py -v

示例实现

此步骤存在的原因

def and_absolute_y(cpu: CPU):
    addr = absolute_y(cpu)
    value = cpu.bus.read(addr)
    and_a(cpu, value)

OPCODE_TABLE = {
    ...
    0x39: and_absolute_y,
}

查看测试代码test_122_AND_absolute_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_absolute_y_handler_exists_and_is_in_opcode_table():
    """Objective: create and_absolute_y(cpu) and add 0x39 to OPCODE_TABLE."""
    assert hasattr(opcodes, "and_absolute_y")
    assert callable(opcodes.and_absolute_y)
    assert list(inspect.signature(opcodes.and_absolute_y).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0x39] is opcodes.and_absolute_y


def test_opcode_39_and_absolute_y_reads_indexed_memory_value():
    """Objective: 39 00 02 with Y=0x04 reads RAM[$0204]."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x39)
    rom.write(0x0001, 0x00)
    rom.write(0x0002, 0x02)
    bus.write(0x0204, 0x0F)

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

    assert cpu.a == 0x03
    assert cpu.pc == 0x8003

测试函数 (2)

test_and_absolute_y_handler_exists_and_is_in_opcode_table
test_opcode_39_and_absolute_y_reads_indexed_memory_value