098. Lsr 零页寻址

添加 LSR Zero Page。

第 98 / 356 · tests/chapter_01_cpu/test_098_LSR_zero_page.py

在这一步中，通过第一个带地址的操作码暴露测试 095 中的内存 lsr。

文件与符号

emulator/cpu/opcodes.py: imported lsr, lsr_zero_page, OPCODE_TABLE[0x46]

为什么需要这一步

由于测试 095 已经定义了内存 lsr，这次改动引入了它的第一个带地址操作码，并使操作数解码与指令行为保持分离。

本步骤的建议实现

# emulator/cpu/opcodes.py
from emulator.cpu.instructions import lsr

def lsr_zero_page(cpu: CPU):
    addr = zero_page(cpu)
    lsr(cpu, addr)

OPCODE_TABLE = {
    # existing entries unchanged
    0x46: lsr_zero_page,
}

重要不变量

zero_page(cpu) 消耗一个操作数，并生成 $0000-$00FF 范围内的地址
lsr 执行读取/修改/写回，并替换 C/Z/N
这条两字节指令使 PC 前进两字节
A 保持不变

常见误解

对于 46 10，操作数 0x10 指定的是 RAM[$0010]；它不是被移位的值。

本步骤不涉及

测试 099-101 中的变址与绝对寻址 LSR 形式
对 zero_page 或 lsr 的改动
周期精确的总线行为

运行本课

uv run pytest tests/chapter_01_cpu/test_098_LSR_zero_page.py -v

示例实现

本步骤的建议实现

# emulator/cpu/opcodes.py
from emulator.cpu.instructions import lsr

def lsr_zero_page(cpu: CPU):
    addr = zero_page(cpu)
    lsr(cpu, addr)

OPCODE_TABLE = {
    # existing entries unchanged
    0x46: lsr_zero_page,
}

查看测试代码test_098_LSR_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_lsr_zero_page_handler_exists_and_is_in_opcode_table():
    """Objective: create lsr_zero_page(cpu) and add 0x46 to OPCODE_TABLE."""
    assert hasattr(opcodes, "lsr_zero_page")
    assert callable(opcodes.lsr_zero_page)
    assert list(inspect.signature(opcodes.lsr_zero_page).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0x46] is opcodes.lsr_zero_page


def test_opcode_46_lsr_zero_page_shifts_memory_value():
    """Objective: 46 10 means LSR $10, so RAM[$0010] is shifted right."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x46)
    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_46_lsr_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, 0x46)
    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 (3)

test_lsr_zero_page_handler_exists_and_is_in_opcode_table
test_opcode_46_lsr_zero_page_shifts_memory_value
test_opcode_46_lsr_zero_page_sets_carry_from_old_bit_0