023. Sta 零页X寻址

添加零页,X的STA（$95）。

第 23 / 356 · tests/chapter_01_cpu/test_023_STA_zero_page_x.py

需要更新的文件

emulator/cpu/opcodes.py

位置

opcodes.sta_zero_page_x
opcodes.OPCODE_TABLE[$95]

为什么需要这一步

本课复用了测试022中的 sta 指令，以及测试017中带回绕的 zero_page_x 寻址方式，说明一个存储操作码处理函数只需组合已有的地址计算与已有的写入操作即可。

完整示例实现

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


OPCODE_TABLE = {
    # Preserve existing entries.
    0x95: sta_zero_page_x,
}

重要不变量

$95 消耗一个操作数字节
基址加X会在 $00 页内回绕
写入的值是cpu.a
STA不修改Zero或Negative标志位

常见误解

零页索引不会进位到 $01 页；当X=$01时，$FF,X 指向的是 $0000，而不是 $0100。

不在本课范围内

对zero_page_x或sta的修改
绝对寻址和间接寻址的STA操作码
周期时序

运行本课

uv run pytest tests/chapter_01_cpu/test_023_STA_zero_page_x.py -v

示例实现

完整示例实现

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


OPCODE_TABLE = {
    # Preserve existing entries.
    0x95: sta_zero_page_x,
}

查看测试代码test_023_STA_zero_page_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
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_zero_page_x_handler_exists_and_is_in_opcode_table():
    """
    Objective:
    Create in opcodes.py:
        def sta_zero_page_x(cpu):
            addr = zero_page_x(cpu)
            sta(cpu, addr)

    Then add:
        0x95: sta_zero_page_x
    """
    assert hasattr(opcodes, "sta_zero_page_x")
    assert callable(opcodes.sta_zero_page_x)
    assert list(inspect.signature(opcodes.sta_zero_page_x).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0x95] is opcodes.sta_zero_page_x


def test_opcode_95_sta_zero_page_x_stores_register_a():
    """
    Objective:
    95 10 means STA $10,X.
    If X is 0x03, store A into RAM $0013.
    """
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x95)
    rom.write(0x0001, 0x10)

    cpu.reset()
    cpu.a = 0x42
    cpu.x = 0x03
    cpu.step()

    assert bus.read(0x0013) == 0x42
    assert cpu.pc == 0x8002


def test_opcode_95_sta_zero_page_x_wraps_and_does_not_change_flags():
    """
    Objective:
    Zero Page,X wraps inside page $00.
    STA must not change Zero or Negative flags.
    """
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x95)
    rom.write(0x0001, 0xFF)

    cpu.reset()
    cpu.a = 0x00
    cpu.x = 0x01
    cpu.p = NEGATIVE_FLAG
    cpu.step()

    assert bus.read(0x0000) == 0x00
    assert (cpu.p & ZERO_FLAG) == 0
    assert (cpu.p & NEGATIVE_FLAG) != 0

测试函数 (3)

test_sta_zero_page_x_handler_exists_and_is_in_opcode_table
test_opcode_95_sta_zero_page_x_stores_register_a
test_opcode_95_sta_zero_page_x_wraps_and_does_not_change_flags