028. Sta 间接Y寻址

添加间接索引的STA（$91，写作 `(d),Y`）。

第 28 / 356 · tests/chapter_01_cpu/test_028_STA_indirect_y.py

需要更新的文件

emulator/cpu/opcodes.py

位置

opcodes.sta_indirect_y
opcodes.OPCODE_TABLE[$91]

为什么需要这一步

本课复用测试021的 indirect_y 地址解析方式，完成这一系列中的STA寻址方式。处理函数得到最终的Y索引目标地址，并把写入操作委托给 sta。

完整示例实现

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


OPCODE_TABLE = {
    # Preserve existing entries.
    0x91: sta_indirect_y,
}

重要不变量

先读取零页指针，再加上Y
指针高字节的读取会从 $00FF 回绕到 $0000
Y对组装出的16位目标地址进行索引
$91 存储A而不改变标志位

常见误解

(d),Y 不会对零页指针位置本身进行索引。应先在 d 处读取指针，再把Y加到得到的地址上。

不在本课范围内

对indirect_y或sta的修改
跨页周期行为
把STA的各处理函数重构为通用辅助函数

运行本课

uv run pytest tests/chapter_01_cpu/test_028_STA_indirect_y.py -v

示例实现

完整示例实现

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


OPCODE_TABLE = {
    # Preserve existing entries.
    0x91: sta_indirect_y,
}

查看测试代码test_028_STA_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_sta_indirect_y_handler_exists_and_is_in_opcode_table():
    """
    Objective:
    Create sta_indirect_y(cpu) and add 0x91 to OPCODE_TABLE.
    """
    assert hasattr(opcodes, "sta_indirect_y")
    assert callable(opcodes.sta_indirect_y)
    assert list(inspect.signature(opcodes.sta_indirect_y).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0x91] is opcodes.sta_indirect_y


def test_opcode_91_sta_indirect_y_stores_register_a():
    """
    Objective:
    91 20 means STA ($20),Y.
    If RAM[$0020-$0021] points to $0200 and Y is 0x04,
    store A into RAM $0204.
    """
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x91)
    rom.write(0x0001, 0x20)
    bus.write(0x0020, 0x00)
    bus.write(0x0021, 0x02)

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

    assert bus.read(0x0204) == 0x42
    assert cpu.pc == 0x8002

测试函数 (2)

test_sta_indirect_y_handler_exists_and_is_in_opcode_table
test_opcode_91_sta_indirect_y_stores_register_a