154. CMP (indirect),Y

添加 CMP (indirect),Y 操作码 ``0xD1``。

第 154 / 356 · tests/chapter_01_cpu/test_154_CMP_indirect_y.py

此步骤存在的原因

(indirect),Y 形式通过对已解析的零页指针本身(而非指针所在位置)进行索引,完成了 CMP 的寻址方式。

在此步骤中,通过向 `emulator/cpu/opcodes.py` 添加以下内容,完成 CMP 操作码序列:

def cmp_indirect_y(cpu: CPU):
    addr = indirect_y(cpu)
    value = cpu.bus.read(addr)
    cmp(cpu, value)

OPCODE_TABLE = {
    ...
    0xD1: cmp_indirect_y,
}

`emulator/cpu/addressing_modes.py::indirect_y` 从零页读取一个小端序指针(指针高字节查找会回绕),然后将 Y 加到该 16 位地址上。只有 C/Z/N 会改变;A、Y、指针字节、目标内存和溢出标志保持不变;操作码加操作数使 PC 前进两个字节。

常见误解:与 (indirect,X) 不同,Y 是在指针解引用之后才被加上的,而不是加到零页操作数上。超出范围:课程 155-162 中的 CPX 和 CPY。

运行本课

uv run pytest tests/chapter_01_cpu/test_154_CMP_indirect_y.py -v

示例实现

此步骤存在的原因

def cmp_indirect_y(cpu: CPU):
    addr = indirect_y(cpu)
    value = cpu.bus.read(addr)
    cmp(cpu, value)

OPCODE_TABLE = {
    ...
    0xD1: cmp_indirect_y,
}

查看测试代码test_154_CMP_indirect_y.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_cmp_indirect_y_handler_exists_and_is_in_opcode_table():
    """Objective: create cmp_indirect_y(cpu) and add 0xD1 to OPCODE_TABLE."""
    assert hasattr(opcodes, "cmp_indirect_y")
    assert callable(opcodes.cmp_indirect_y)
    assert list(inspect.signature(opcodes.cmp_indirect_y).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0xD1] is opcodes.cmp_indirect_y


def test_opcode_D1_cmp_indirect_y_reads_pointed_indexed_memory_value():
    """Objective: D1 20 reads pointer at $20/$21, then adds Y."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0xD1)
    rom.write(0x0001, 0x20)
    bus.write(0x0020, 0x00)
    bus.write(0x0021, 0x02)
    bus.write(0x0204, 0x10)

    cpu.reset()
    cpu.y = 0x04
    cpu.a = 0x20
    cpu.step()

    assert (cpu.p & CARRY_FLAG) != 0
    assert cpu.pc == 0x8002

测试函数 (2)

test_cmp_indirect_y_handler_exists_and_is_in_opcode_table
test_opcode_D1_cmp_indirect_y_reads_pointed_indexed_memory_value