048. Sty 零页,X

添加 STY 零页,X 寻址（$94）。

第 48 / 356 · tests/chapter_01_cpu/test_048_STY_zero_page_x.py

要更新的文件

emulator/cpu/opcodes.py

位置

opcodes imports of zero_page_x and sty
opcodes.sty_zero_page_x
opcodes.OPCODE_TABLE[$94]

为什么需要这一步

这种编码将 STY 面向地址的指令边界与已有的零页,X 辅助函数结合在一起，包括页 $00 内的环绕行为。

完整示例实现

# emulator/cpu/opcodes.py
from emulator.cpu.addressing_modes import zero_page_x
from emulator.cpu.instructions import sty


def sty_zero_page_x(cpu: CPU):
    addr = zero_page_x(cpu)
    sty(cpu, addr)


OPCODE_TABLE = {
    # Preserve existing entries.
    0x94: sty_zero_page_x,
}

重要不变量

$94 映射到 sty_zero_page_x 并消耗一个操作数字节
X 对操作数进行变址，有效地址在 8 位范围内环绕
传给 sty 的是有效地址，而不是其当前内容
本次存储不会改变 Y 和处理器标志位

常见误区

当 X 等于 $01 时，$FF,X 存储到的地址是 $0000，而不是 $0100；零页寻址不能超出页 $00 的范围。

范围之外

绝对寻址的 STY
修改 zero_page_x
周期时序

运行本课

uv run pytest tests/chapter_01_cpu/test_048_STY_zero_page_x.py -v

示例实现

完整示例实现

# emulator/cpu/opcodes.py
from emulator.cpu.addressing_modes import zero_page_x
from emulator.cpu.instructions import sty


def sty_zero_page_x(cpu: CPU):
    addr = zero_page_x(cpu)
    sty(cpu, addr)


OPCODE_TABLE = {
    # Preserve existing entries.
    0x94: sty_zero_page_x,
}

查看测试代码test_048_STY_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


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_sty_zero_page_x_handler_exists_and_is_in_opcode_table():
    """Objective: create sty_zero_page_x(cpu) and add 0x94 to OPCODE_TABLE."""
    assert hasattr(opcodes, "sty_zero_page_x")
    assert callable(opcodes.sty_zero_page_x)
    assert list(inspect.signature(opcodes.sty_zero_page_x).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0x94] is opcodes.sty_zero_page_x


def test_opcode_94_sty_zero_page_x_stores_register_y():
    """Objective: 94 10 with X=0x03 stores Y into RAM[$0013]."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x94)
    rom.write(0x0001, 0x10)

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

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


def test_opcode_94_sty_zero_page_x_wraps():
    """Objective: Zero Page,X wraps inside page $00."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x94)
    rom.write(0x0001, 0xFF)

    cpu.reset()
    cpu.y = 0x37
    cpu.x = 0x01
    cpu.step()

    assert bus.read(0x0000) == 0x37
    assert cpu.pc == 0x8002

测试函数 (3)

test_sty_zero_page_x_handler_exists_and_is_in_opcode_table
test_opcode_94_sty_zero_page_x_stores_register_y
test_opcode_94_sty_zero_page_x_wraps