027. Sta 间接X寻址

添加变址间接的STA（$81，写作 `(d,X)`）。

第 27 / 356 · tests/chapter_01_cpu/test_027_STA_indirect_x.py

需要更新的文件

emulator/cpu/opcodes.py

位置

opcodes.sta_indirect_x
opcodes.OPCODE_TABLE[$81]

为什么需要这一步

测试020为加载指令建立了 (d,X) 指针解析方式。本课复用同样的最终地址计算作为存储目标，使指针机制保持在 sta 指令之外。

完整示例实现

# emulator/cpu/opcodes.py
def sta_indirect_x(cpu) -> None:
    address = indirect_x(cpu)
    sta(cpu, address)


OPCODE_TABLE = {
    # Preserve existing entries.
    0x81: sta_indirect_x,
}

重要不变量

在解引用之前，X先对零页指针位置进行索引
指针位置的选取以及其高字节的读取都在零页内回绕
指针得到的16位结果就是写入目标
$81 消耗一个操作数字节，且不更新标志位

常见误解

不要把X加到最终的指针值上。在 (d,X) 中，X选择的是指针的读取位置，而 sta 是把A写入该指针所指向的地址。

不在本课范围内

对indirect_x或sta的修改
间接,Y方式的STA
周期时序

运行本课

uv run pytest tests/chapter_01_cpu/test_027_STA_indirect_x.py -v

示例实现

完整示例实现

# emulator/cpu/opcodes.py
def sta_indirect_x(cpu) -> None:
    address = indirect_x(cpu)
    sta(cpu, address)


OPCODE_TABLE = {
    # Preserve existing entries.
    0x81: sta_indirect_x,
}

查看测试代码test_027_STA_indirect_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_sta_indirect_x_handler_exists_and_is_in_opcode_table():
    """
    Objective:
    Create sta_indirect_x(cpu) and add 0x81 to OPCODE_TABLE.
    """
    assert hasattr(opcodes, "sta_indirect_x")
    assert callable(opcodes.sta_indirect_x)
    assert list(inspect.signature(opcodes.sta_indirect_x).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0x81] is opcodes.sta_indirect_x


def test_opcode_81_sta_indirect_x_stores_register_a():
    """
    Objective:
    81 20 means STA ($20,X).
    If X is 0x04, pointer is at $0024.
    If RAM[$0024-$0025] points to $0200, store A into $0200.
    """
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x81)
    rom.write(0x0001, 0x20)
    bus.write(0x0024, 0x00)
    bus.write(0x0025, 0x02)

    cpu.reset()
    cpu.a = 0x42
    cpu.x = 0x04
    cpu.step()

    assert bus.read(0x0200) == 0x42
    assert cpu.pc == 0x8002

测试函数 (2)

test_sta_indirect_x_handler_exists_and_is_in_opcode_table
test_opcode_81_sta_indirect_x_stores_register_a