105. Rol zero page 零页寻址

接入 ROL Zero Page(操作码 ``0x26``)。

第 105 / 356 · tests/chapter_01_cpu/test_105_ROL_zero_page.py

在本步骤中,在第 104 课完成累加器接入之后,只添加零页适配器和表项。

生产代码位置中的完整示例实现

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

def rol_zero_page(cpu: CPU):
    addr = zero_page(cpu)
    rol(cpu, addr)

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

0x26: rol_zero_page,

本步骤存在的原因

操作码层负责解析操作数;第 102 课的原语负责读/改/写以及标志位。

不变量:`zero_page(cpu) 消耗一个操作数字节,并返回 $0000-$00FF 范围内的地址;26 10 旋转 RAM 中的 $0010`,最终 PC 为起始位置+2。A 保持不变,ROL 的 C/Z/N 规则得以保留。

常见误解:`$10` 是待旋转字节的地址,而不是字节值本身;不要旋转指令流中的操作数字节。

不在本步骤范围内:变址零页是第 106 课,绝对寻址模式是第 107-108 课,ROR 从第 109 课开始,精确的周期时序是之后的工作。

运行本课

uv run pytest tests/chapter_01_cpu/test_105_ROL_zero_page.py -v

示例实现

生产代码位置中的完整示例实现

def rol_zero_page(cpu: CPU):
    addr = zero_page(cpu)
    rol(cpu, addr)

查看测试代码test_105_ROL_zero_page.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


CARRY_FLAG = 1 << 0


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_handler_exists_and_is_in_opcode_table():
    """Objective: create rol_zero_page(cpu) and add 0x26 to OPCODE_TABLE."""
    assert hasattr(opcodes, "rol_zero_page")
    assert callable(opcodes.rol_zero_page)
    assert list(inspect.signature(opcodes.rol_zero_page).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0x26] is opcodes.rol_zero_page


def test_opcode_26_rol_zero_page_rotates_memory_value():
    """Objective: 26 10 means ROL $10, so RAM[$0010] is rotated left."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x26)
    rom.write(0x0001, 0x10)
    bus.write(0x0010, 0b0000_0011)

    cpu.reset()
    cpu.step()

    assert bus.read(0x0010) == 0b0000_0110
    assert cpu.pc == 0x8002


def test_opcode_26_rol_zero_page_uses_old_carry_as_new_bit_0():
    """Objective: old Carry=1 is inserted into memory result bit 0."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x26)
    rom.write(0x0001, 0x10)
    bus.write(0x0010, 0b0000_0010)

    cpu.reset()
    cpu.p |= CARRY_FLAG
    cpu.step()

    assert bus.read(0x0010) == 0b0000_0101
    assert (cpu.p & CARRY_FLAG) == 0


def test_opcode_26_rol_zero_page_sets_carry_from_old_bit_7():
    """Objective: old memory bit 7 becomes Carry."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x26)
    rom.write(0x0001, 0x10)
    bus.write(0x0010, 0b1000_0001)

    cpu.reset()
    cpu.step()

    assert bus.read(0x0010) == 0b0000_0010
    assert (cpu.p & CARRY_FLAG) != 0

测试函数 (4)

test_rol_zero_page_handler_exists_and_is_in_opcode_table
test_opcode_26_rol_zero_page_rotates_memory_value
test_opcode_26_rol_zero_page_uses_old_carry_as_new_bit_0
test_opcode_26_rol_zero_page_sets_carry_from_old_bit_7