091. Asl 零页寻址

添加 ASL 零页寻址模式。

第 91 / 356 · tests/chapter_01_cpu/test_091_ASL_zero_page.py

在这一步中，添加第一个带地址的 ASL 形式。测试 088-090 是内存与累加器行为以及累加器分派的前置条件。

文件与符号

emulator/cpu/opcodes.py: asl_zero_page, OPCODE_TABLE[0x06]

为什么需要这一步

测试 088-090 已经建立了 instructions.asl、instructions.asl_a 以及累加器操作码。这次改动通过第一个带地址的形式暴露内存实现，而不在处理函数中重复移位或标志位行为。

本步骤的建议实现

# emulator/cpu/opcodes.py
def asl_zero_page(cpu: CPU):
    addr = zero_page(cpu)
    asl(cpu, addr)

OPCODE_TABLE = {
    # existing entries unchanged
    0x06: asl_zero_page,
}

重要不变量

zero_page(cpu) 消耗一个操作数字节，因此整条指令共两字节
该操作数是一个地址；asl 在该地址上执行读取/修改/写回
原第 7 位设置 Carry；Zero 和 Negative 由掩码后的结果决定
A 保持不变

常见误解

对于 06 10，0x10 并不会被直接移位，它选定的是 RAM[$0010]。

本步骤不涉及

测试 092-094 中的 ASL Zero Page,X 以及绝对寻址形式
从测试 095 开始的 LSR
周期时序以及读取/修改/写回总线周期的精确性

运行本课

uv run pytest tests/chapter_01_cpu/test_091_ASL_zero_page.py -v

示例实现

本步骤的建议实现

# emulator/cpu/opcodes.py
def asl_zero_page(cpu: CPU):
    addr = zero_page(cpu)
    asl(cpu, addr)

OPCODE_TABLE = {
    # existing entries unchanged
    0x06: asl_zero_page,
}

查看测试代码test_091_ASL_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
from tests.helpers import NEGATIVE_FLAG, ZERO_FLAG


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_asl_zero_page_handler_exists_and_is_in_opcode_table():
    """Objective: create asl_zero_page(cpu) and add 0x06 to OPCODE_TABLE."""
    assert hasattr(opcodes, "asl_zero_page")
    assert callable(opcodes.asl_zero_page)
    assert list(inspect.signature(opcodes.asl_zero_page).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0x06] is opcodes.asl_zero_page


def test_opcode_06_asl_zero_page_shifts_memory_value():
    """Objective: 06 10 means ASL $10, so RAM[$0010] is shifted left."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x06)
    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_06_asl_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, 0x06)
    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


def test_opcode_06_asl_zero_page_updates_zero_flag():
    """Objective: 0x80 shifts to 0x00 and sets Zero flag."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x06)
    rom.write(0x0001, 0x10)
    bus.write(0x0010, 0x80)

    cpu.reset()
    cpu.step()

    assert bus.read(0x0010) == 0x00
    assert (cpu.p & ZERO_FLAG) != 0
    assert (cpu.p & NEGATIVE_FLAG) == 0


def test_opcode_06_asl_zero_page_updates_negative_flag():
    """Objective: 0x40 shifts to 0x80 and sets Negative flag."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x06)
    rom.write(0x0001, 0x10)
    bus.write(0x0010, 0x40)

    cpu.reset()
    cpu.step()

    assert bus.read(0x0010) == 0x80
    assert (cpu.p & NEGATIVE_FLAG) != 0
    assert (cpu.p & ZERO_FLAG) == 0

测试函数 (5)

test_asl_zero_page_handler_exists_and_is_in_opcode_table
test_opcode_06_asl_zero_page_shifts_memory_value
test_opcode_06_asl_zero_page_sets_carry_from_old_bit_7
test_opcode_06_asl_zero_page_updates_zero_flag
test_opcode_06_asl_zero_page_updates_negative_flag