127. 零页 ORA

添加 ORA zero-page 操作码 ``0x05``。

第 127 / 356 · tests/chapter_01_cpu/test_127_ORA_zero_page.py

此步骤存在的原因

ORA 必须区分零页地址操作数与字面数据,先读取被寻址的字节,再应用已定义好的包含或行为。

在此步骤中,由于第 126 课已导入 `or_a,emulator/cpu/opcodes.py` 中完整的增量实现如下:

def ora_zero_page(cpu: CPU):
    addr = zero_page(cpu)
    value = cpu.bus.read(addr)
    or_a(cpu, value)

OPCODE_TABLE = {
    ...
    0x05: ora_zero_page,
}

`emulator/cpu/addressing_modes.py::zero_page 获取一字节地址,随后处理程序在调用 instructions.or_a 之前先读取其值。A 与 Z/N 会改变;Carry/Overflow 及内存保持不变;05 10 读取 $0010` 并使 PC 前进两个字节。

常见误解:应将存储在 `$0010 处的字节传给 or_a,而不是传地址 0x10`,并且不要将结果写回内存。不在本步骤范围内:从 zero-page,X 到 indirect,Y 的 ORA 寻址模式(第 128-133 课)。

运行本课

uv run pytest tests/chapter_01_cpu/test_127_ORA_zero_page.py -v

示例实现

此步骤存在的原因

def ora_zero_page(cpu: CPU):
    addr = zero_page(cpu)
    value = cpu.bus.read(addr)
    or_a(cpu, value)

OPCODE_TABLE = {
    ...
    0x05: ora_zero_page,
}

查看测试代码test_127_ORA_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


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_handler_exists_and_is_in_opcode_table():
    """Objective: create ora_zero_page(cpu) and add 0x05 to OPCODE_TABLE."""
    assert hasattr(opcodes, "ora_zero_page")
    assert callable(opcodes.ora_zero_page)
    assert list(inspect.signature(opcodes.ora_zero_page).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0x05] is opcodes.ora_zero_page


def test_opcode_05_ora_zero_page_reads_memory_value_and_updates_a():
    """Objective: 05 10 means ORA value at RAM[$0010]."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x05)
    rom.write(0x0001, 0x10)
    bus.write(0x0010, 0x0F)

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

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

测试函数 (2)

test_ora_zero_page_handler_exists_and_is_in_opcode_table
test_opcode_05_ora_zero_page_reads_memory_value_and_updates_a