133. 间接,Y ORA

添加 ORA (indirect),Y 操作码 ``0x11``。

第 133 / 356 · tests/chapter_01_cpu/test_133_ORA_indirect_y.py

此步骤存在的原因

此步骤通过先解析零页指针再加上 Y,完成了 ORA 的全部寻址形式,而不会重复实现运算结果或标志位逻辑。

在此步骤中,继第 125-132 课之后,通过在 `emulator/cpu/opcodes.py` 中添加以下内容来完成 ORA:

def ora_indirect_y(cpu: CPU):
    addr = indirect_y(cpu)
    value = cpu.bus.read(addr)
    or_a(cpu, value)

OPCODE_TABLE = {
    ...
    0x11: ora_indirect_y,
}

`emulator/cpu/addressing_modes.py::indirect_y 获取零页指针,以回绕方式查找其高字节来读取小端序目标地址,然后才加上 Y。处理函数读取该变址后的目标值,并调用 instructions.or_a`。A 与 Z/N 可能改变;Carry/Overflow、内存、X 和 Y 保持不变;操作码加操作数使 PC 前进两个字节。

常见误解:与 (indirect,X) 不同,Y 并不用于选择指针位置,而是对解析出的目标地址进行变址。不在本步骤范围内:EOR 和 BIT(第 134-145 课)。

运行本课

uv run pytest tests/chapter_01_cpu/test_133_ORA_indirect_y.py -v

示例实现

此步骤存在的原因

def ora_indirect_y(cpu: CPU):
    addr = indirect_y(cpu)
    value = cpu.bus.read(addr)
    or_a(cpu, value)

OPCODE_TABLE = {
    ...
    0x11: ora_indirect_y,
}

查看测试代码test_133_ORA_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_ora_indirect_y_handler_exists_and_is_in_opcode_table():
    """Objective: create ora_indirect_y(cpu) and add 0x11 to OPCODE_TABLE."""
    assert hasattr(opcodes, "ora_indirect_y")
    assert callable(opcodes.ora_indirect_y)
    assert list(inspect.signature(opcodes.ora_indirect_y).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0x11] is opcodes.ora_indirect_y


def test_opcode_11_ora_indirect_y_reads_pointed_indexed_memory_value():
    """Objective: 11 20 reads pointer at $20/$21, then adds Y."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x11)
    rom.write(0x0001, 0x20)
    bus.write(0x0020, 0x00)
    bus.write(0x0021, 0x02)
    bus.write(0x0204, 0x0F)

    cpu.reset()
    cpu.y = 0x04
    cpu.a = 0xF0
    cpu.step()

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

测试函数 (2)

test_ora_indirect_y_handler_exists_and_is_in_opcode_table
test_opcode_11_ora_indirect_y_reads_pointed_indexed_memory_value