130. 绝对,X ORA

添加 ORA absolute,X 操作码 ``0x1D``。

第 130 / 356 · tests/chapter_01_cpu/test_130_ORA_absolute_x.py

此步骤存在的原因

这使得 ORA 能够对以 X 索引的完整地址表进行操作,同时使地址解析与该操作的累加器及标志位语义保持分离。

在此步骤中,紧接第 129 课中未索引的 absolute ORA 之后,仅需在 `emulator/cpu/opcodes.py` 中添加以下代码:

def ora_absolute_x(cpu: CPU):
    addr = absolute_x(cpu)
    value = cpu.bus.read(addr)
    or_a(cpu, value)

OPCODE_TABLE = {
    ...
    0x1D: ora_absolute_x,
}

`emulator/cpu/addressing_modes.py::absolute_x 先获取小端序的基址字,再加上 cpu.x。处理程序读取该解析出的地址,并将其值传递给 instructions.or_a`。A 与 Z/N 会改变;X、Carry/Overflow 及内存保持不变;操作码加字使 PC 前进三个字节。

常见误解:X 是对解码后的地址进行索引,而不是对操作数字节或从内存中读取的数据进行索引。不在本步骤范围内:ORA absolute,Y 与间接寻址模式(第 131-133 课),以及随后第 134-145 课中的 EOR 与 BIT。

运行本课

uv run pytest tests/chapter_01_cpu/test_130_ORA_absolute_x.py -v

示例实现

此步骤存在的原因

def ora_absolute_x(cpu: CPU):
    addr = absolute_x(cpu)
    value = cpu.bus.read(addr)
    or_a(cpu, value)

OPCODE_TABLE = {
    ...
    0x1D: ora_absolute_x,
}

查看测试代码test_130_ORA_absolute_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_ora_absolute_x_handler_exists_and_is_in_opcode_table():
    """Objective: create ora_absolute_x(cpu) and add 0x1D to OPCODE_TABLE."""
    assert hasattr(opcodes, "ora_absolute_x")
    assert callable(opcodes.ora_absolute_x)
    assert list(inspect.signature(opcodes.ora_absolute_x).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0x1D] is opcodes.ora_absolute_x


def test_opcode_1D_ora_absolute_x_reads_indexed_memory_value():
    """Objective: 1D 00 02 with X=0x04 reads RAM[$0204]."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x1D)
    rom.write(0x0001, 0x00)
    rom.write(0x0002, 0x02)
    bus.write(0x0204, 0x0F)

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

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

测试函数 (2)

test_ora_absolute_x_handler_exists_and_is_in_opcode_table
test_opcode_1D_ora_absolute_x_reads_indexed_memory_value