112. 零页 ROR

添加 ROR 零页操作码 ``0x66``。

第 112 / 356 · tests/chapter_01_cpu/test_112_ROR_zero_page.py

在这一步中，在第 111 课的基础上，把 `ror 加入 emulator/cpu/opcodes.py` 的指令导入，并添加零页处理函数和表条目。

为什么需要这一步

这一步把共享的 ROR 内存操作与第一种紧凑寻址形式连接起来，验证编码出的零页地址确实用于选定要旋转的字节。

建议实现

def ror_zero_page(cpu: CPU):
    addr = zero_page(cpu)
    ror(cpu, addr)

OPCODE_TABLE = {
    ...
    0x66: ror_zero_page,
}

`zero_page 已存在于 emulator/cpu/addressing_modes.py 中，会取一个字节。ror 已存在于 emulator/cpu/instructions.py 中；它执行总线的读/改/写，连同旧的 Carry 一起旋转，用旧的第 0 位设置 Carry，并根据结果设置 Zero/Negative。因此 66 10 修改的是 cpu.bus[$0010]`，而不是 A，PC 总共前进两个字节。

误解澄清：取到的 `0x10 是一个地址，而不是要传给 ror` 的值。本课不涉及的内容：变址和绝对寻址的 ROR 模式属于第 113-115 课；不要在这个操作码处理函数中重复实现寻址或旋转逻辑。

运行本课

uv run pytest tests/chapter_01_cpu/test_112_ROR_zero_page.py -v

示例实现

建议实现

def ror_zero_page(cpu: CPU):
    addr = zero_page(cpu)
    ror(cpu, addr)

OPCODE_TABLE = {
    ...
    0x66: ror_zero_page,
}

查看测试代码test_112_ROR_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_ror_zero_page_handler_exists_and_is_in_opcode_table():
    """Objective: create ror_zero_page(cpu) and add 0x66 to OPCODE_TABLE."""
    assert hasattr(opcodes, "ror_zero_page")
    assert callable(opcodes.ror_zero_page)
    assert list(inspect.signature(opcodes.ror_zero_page).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0x66] is opcodes.ror_zero_page


def test_opcode_66_ror_zero_page_rotates_memory_value():
    """Objective: 66 10 means ROR $10, so RAM[$0010] is rotated right."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x66)
    rom.write(0x0001, 0x10)
    bus.write(0x0010, 0b0000_0110)

    cpu.reset()
    cpu.step()

    assert bus.read(0x0010) == 0b0000_0011
    assert cpu.pc == 0x8002


def test_opcode_66_ror_zero_page_uses_old_carry_as_new_bit_7():
    """Objective: old Carry=1 is inserted into memory result bit 7."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x66)
    rom.write(0x0001, 0x10)
    bus.write(0x0010, 0b0000_0010)

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

    assert bus.read(0x0010) == 0b1000_0001
    assert (cpu.p & CARRY_FLAG) == 0


def test_opcode_66_ror_zero_page_sets_carry_from_old_bit_0():
    """Objective: old memory bit 0 becomes Carry."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x66)
    rom.write(0x0001, 0x10)
    bus.write(0x0010, 0b0000_0011)

    cpu.reset()
    cpu.step()

    assert bus.read(0x0010) == 0b0000_0001
    assert (cpu.p & CARRY_FLAG) != 0

测试函数 (4)

test_ror_zero_page_handler_exists_and_is_in_opcode_table
test_opcode_66_ror_zero_page_rotates_memory_value
test_opcode_66_ror_zero_page_uses_old_carry_as_new_bit_7
test_opcode_66_ror_zero_page_sets_carry_from_old_bit_0