024. Sta 绝对寻址

添加绝对寻址的STA（$8D）。

第 24 / 356 · tests/chapter_01_cpu/test_024_STA_absolute.py

需要更新的文件

emulator/cpu/opcodes.py

位置

opcodes.sta_absolute
opcodes.OPCODE_TABLE[$8D]

为什么需要这一步

STA现在需要一个完整的16位目标地址。本课复用 absolute 和 sta，说明操作数宽度的变化应属于寻址逻辑，而不是存储指令本身。

完整示例实现

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


OPCODE_TABLE = {
    # Preserve existing entries.
    0x8D: sta_absolute,
}

重要不变量

absolute先消耗低字节，再消耗高字节
得到的16位地址会直接传给sta
$8D 使PC前进三个字节，包括操作码本身
存储A不改变Zero和Negative标志位

常见误解

这两个操作数字节编码的是一个小端目标地址，而不是两个要写入的值，不能按大端方式读取。

不在本课范围内

带索引的绝对寻址STA操作码
对absolute或sta的修改
周期时序

运行本课

uv run pytest tests/chapter_01_cpu/test_024_STA_absolute.py -v

示例实现

完整示例实现

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


OPCODE_TABLE = {
    # Preserve existing entries.
    0x8D: sta_absolute,
}

查看测试代码test_024_STA_absolute.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
from tests.helpers import NEGATIVE_FLAG, ZERO_FLAG


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_absolute_handler_exists_and_is_in_opcode_table():
    """
    Objective:
    Create in opcodes.py:
        def sta_absolute(cpu):
            addr = absolute(cpu)
            sta(cpu, addr)

    Then add:
        0x8D: sta_absolute
    """
    assert hasattr(opcodes, "sta_absolute")
    assert callable(opcodes.sta_absolute)
    assert list(inspect.signature(opcodes.sta_absolute).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0x8D] is opcodes.sta_absolute


def test_opcode_8D_sta_absolute_stores_register_a():
    """
    Objective:
    8D 00 02 means STA $0200.
    Store register A into RAM $0200.
    """
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x8D)
    rom.write(0x0001, 0x00)
    rom.write(0x0002, 0x02)

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

    assert bus.read(0x0200) == 0x42
    assert cpu.pc == 0x8003


def test_opcode_8D_sta_absolute_does_not_change_flags():
    """
    Objective:
    STA Absolute stores A but does not update flags.
    """
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x8D)
    rom.write(0x0001, 0x00)
    rom.write(0x0002, 0x02)

    cpu.reset()
    cpu.a = 0x00
    cpu.p = NEGATIVE_FLAG
    cpu.step()

    assert bus.read(0x0200) == 0x00
    assert (cpu.p & ZERO_FLAG) == 0
    assert (cpu.p & NEGATIVE_FLAG) != 0

测试函数 (3)

test_sta_absolute_handler_exists_and_is_in_opcode_table
test_opcode_8D_sta_absolute_stores_register_a
test_opcode_8D_sta_absolute_does_not_change_flags