092. Asl 零页,X 寻址

添加 ASL Zero Page,X。

第 92 / 356 · tests/chapter_01_cpu/test_092_ASL_zero_page_x.py

在这一步中，为测试 091 中的零页形式扩展 X 变址。

文件与符号

emulator/cpu/opcodes.py: asl_zero_page_x, OPCODE_TABLE[0x16]

为什么需要这一步

在测试 091 接入普通零页 ASL 之后，这次改动复用了已有的 zero_page_x 解析器，使变址与零页环绕仍然只属于寻址层面的问题。

本步骤的建议实现

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

OPCODE_TABLE = {
    # existing entries unchanged
    0x16: asl_zero_page_x,
}

重要不变量

有效地址为 (operand + cpu.x) & 0xFF
消耗一个操作数字节，因此包含操作码在内 PC 前进两字节
asl 负责内存写入以及 Carry、Zero、Negative 的更新
变址改变的是目标地址，而不是传给 asl 的值

常见误解

Zero Page,X 不会溢出到第一页：基址 0xFE 加上 X=0x03 指向的是 $0001。

本步骤不涉及

测试 093-094 中的 ASL Absolute 与 Absolute,X
修改 zero_page_x 或 asl
周期时序

运行本课

uv run pytest tests/chapter_01_cpu/test_092_ASL_zero_page_x.py -v

示例实现

本步骤的建议实现

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

OPCODE_TABLE = {
    # existing entries unchanged
    0x16: asl_zero_page_x,
}

查看测试代码test_092_ASL_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_asl_zero_page_x_handler_exists_and_is_in_opcode_table():
    """Objective: create asl_zero_page_x(cpu) and add 0x16 to OPCODE_TABLE."""
    assert hasattr(opcodes, "asl_zero_page_x")
    assert callable(opcodes.asl_zero_page_x)
    assert list(inspect.signature(opcodes.asl_zero_page_x).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0x16] is opcodes.asl_zero_page_x


def test_opcode_16_asl_zero_page_x_shifts_indexed_memory_value():
    """Objective: 16 20 with X=0x04 shifts RAM[$0024]."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x16)
    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_16_asl_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, 0x16)
    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_asl_zero_page_x_handler_exists_and_is_in_opcode_table
test_opcode_16_asl_zero_page_x_shifts_indexed_memory_value
test_opcode_16_asl_zero_page_x_wraps_zero_page_address