128. 零页,X ORA

添加 ORA zero-page,X 操作码 ``0x15``。

第 128 / 356 · tests/chapter_01_cpu/test_128_ORA_zero_page_x.py

此步骤存在的原因

这实现了紧凑的索引 ORA 访问,并验证在将选定字节与 A 组合之前,加上 X 的操作遵循零页环绕规则。

在此步骤中,紧接第 127 课之后,仅需在 `emulator/cpu/opcodes.py` 中添加以下处理程序及表项:

def ora_zero_page_x(cpu: CPU):
    addr = zero_page_x(cpu)
    value = cpu.bus.read(addr)
    or_a(cpu, value)

OPCODE_TABLE = {
    ...
    0x15: ora_zero_page_x,
}

`emulator/cpu/addressing_modes.py::zero_page_x 在总线读取之前计算 (base + cpu.x) & 0xFF。解析出的值被传给 instructions.or_a;A 与 Z/N 会改变,X、Carry/Overflow 及内存不会改变,PC 前进两个字节。因此基址 0xFE 加 X 0x03 会读取 $0001`。

常见误解:零页索引会环绕到 `$0001,而不是 $0101`;X 不会改变已加载的值。不在本步骤范围内:absolute 与 indirect 的 ORA 寻址模式(第 129-133 课)。

运行本课

uv run pytest tests/chapter_01_cpu/test_128_ORA_zero_page_x.py -v

示例实现

此步骤存在的原因

def ora_zero_page_x(cpu: CPU):
    addr = zero_page_x(cpu)
    value = cpu.bus.read(addr)
    or_a(cpu, value)

OPCODE_TABLE = {
    ...
    0x15: ora_zero_page_x,
}

查看测试代码test_128_ORA_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_ora_zero_page_x_handler_exists_and_is_in_opcode_table():
    """Objective: create ora_zero_page_x(cpu) and add 0x15 to OPCODE_TABLE."""
    assert hasattr(opcodes, "ora_zero_page_x")
    assert callable(opcodes.ora_zero_page_x)
    assert list(inspect.signature(opcodes.ora_zero_page_x).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0x15] is opcodes.ora_zero_page_x


def test_opcode_15_ora_zero_page_x_reads_indexed_memory_value():
    """Objective: 15 20 with X=0x04 reads RAM[$0024]."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x15)
    rom.write(0x0001, 0x20)
    bus.write(0x0024, 0x0F)

    cpu.reset()
    cpu.x = 0x04
    cpu.a = 0xF0
    cpu.step()

    assert cpu.a == 0xFF
    assert cpu.pc == 0x8002


def test_opcode_15_ora_zero_page_x_wraps_zero_page_address():
    """Objective: base=0xFE and X=0x03 reads RAM[$0001]."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x15)
    rom.write(0x0001, 0xFE)
    bus.write(0x0001, 0x0F)

    cpu.reset()
    cpu.x = 0x03
    cpu.a = 0xF0
    cpu.step()

    assert cpu.a == 0xFF
    assert cpu.pc == 0x8002

测试函数 (3)

test_ora_zero_page_x_handler_exists_and_is_in_opcode_table
test_opcode_15_ora_zero_page_x_reads_indexed_memory_value
test_opcode_15_ora_zero_page_x_wraps_zero_page_address