106. Rol zero page x 变址零页

接入 ROL Zero Page,X(操作码 ``0x36``)。

第 106 / 356 · tests/chapter_01_cpu/test_106_ROL_zero_page_x.py

在本步骤中,在第 105 课的基础上只扩展变址零页适配器和分派条目。

生产代码位置中的完整示例实现

`emulator/cpu/opcodes.py::rol_zero_page_x`::

def rol_zero_page_x(cpu: CPU):
    addr = zero_page_x(cpu)
    rol(cpu, addr)

`emulator/cpu/opcodes.py::OPCODE_TABLE`::

0x36: rol_zero_page_x,

本步骤存在的原因

复用已有的寻址辅助函数,避免在指令原语中重复实现回绕策略。

不变量:消耗一个操作数字节;有效地址为 `(operand + cpu.x) & 0xFF;PC 最终为起始位置+2;rol 只修改该字节,C/Z/N 行为与第 102 课一致。因此基址 0xFE 加上 X 0x03 指向 $0001`。

常见误解:零页变址不会溢出到 `$0101`,也不是 16 位的绝对变址;必须按 8 位回绕。

不在本步骤范围内:绝对寻址模式是第 107-108 课,ROR 从第 109 课开始,周期精确的变址读/改/写时序是之后的工作。

运行本课

uv run pytest tests/chapter_01_cpu/test_106_ROL_zero_page_x.py -v

示例实现

生产代码位置中的完整示例实现

def rol_zero_page_x(cpu: CPU):
    addr = zero_page_x(cpu)
    rol(cpu, addr)

查看测试代码test_106_ROL_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_rol_zero_page_x_handler_exists_and_is_in_opcode_table():
    """Objective: create rol_zero_page_x(cpu) and add 0x36 to OPCODE_TABLE."""
    assert hasattr(opcodes, "rol_zero_page_x")
    assert callable(opcodes.rol_zero_page_x)
    assert list(inspect.signature(opcodes.rol_zero_page_x).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0x36] is opcodes.rol_zero_page_x


def test_opcode_36_rol_zero_page_x_rotates_indexed_memory_value():
    """Objective: 36 20 with X=0x04 rotates RAM[$0024]."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x36)
    rom.write(0x0001, 0x20)
    bus.write(0x0024, 0b0000_0011)

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

    assert bus.read(0x0024) == 0b0000_0110
    assert cpu.pc == 0x8002


def test_opcode_36_rol_zero_page_x_wraps_zero_page_address():
    """Objective: zero-page indexed addresses wrap to 8 bits."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x36)
    rom.write(0x0001, 0xFE)
    bus.write(0x0001, 0x02)

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

    assert bus.read(0x0001) == 0x04
    assert cpu.pc == 0x8002

测试函数 (3)

test_rol_zero_page_x_handler_exists_and_is_in_opcode_table
test_opcode_36_rol_zero_page_x_rotates_indexed_memory_value
test_opcode_36_rol_zero_page_x_wraps_zero_page_address