152. CMP absolute,Y

添加 CMP absolute,Y 操作码 ``0xD9``。

第 152 / 356 · tests/chapter_01_cpu/test_152_CMP_absolute_y.py

此步骤存在的原因

Absolute,Y 提供了通过 Y 进行索引的对应比较,同时复用共享的 CMP 行为。

在此步骤中,课程 151 已经添加了 absolute,X。将以下内容准确地添加到 `emulator/cpu/opcodes.py`:

def cmp_absolute_y(cpu: CPU):
    addr = absolute_y(cpu)
    value = cpu.bus.read(addr)
    cmp(cpu, value)

OPCODE_TABLE = {
    ...
    0xD9: cmp_absolute_y,
}

`emulator/cpu/addressing_modes.py::absolute_y 读取小端序基址字,并在总线读取之前加上 Y。instructions.cmp` 只改变 C/Z/N;A、Y、内存和溢出标志保持不变,操作码加字使 PC 前进三个字节。

常见误解:Y 修改的是有效地址,而不是取到的值,且基址字节不是大端序。超出范围:CMP (indirect,X) 和 (indirect),Y(153-154),以及之后的 CPX/CPY(155-162)。

运行本课

uv run pytest tests/chapter_01_cpu/test_152_CMP_absolute_y.py -v

示例实现

此步骤存在的原因

def cmp_absolute_y(cpu: CPU):
    addr = absolute_y(cpu)
    value = cpu.bus.read(addr)
    cmp(cpu, value)

OPCODE_TABLE = {
    ...
    0xD9: cmp_absolute_y,
}

查看测试代码test_152_CMP_absolute_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_absolute_y_handler_exists_and_is_in_opcode_table():
    """Objective: create cmp_absolute_y(cpu) and add 0xD9 to OPCODE_TABLE."""
    assert hasattr(opcodes, "cmp_absolute_y")
    assert callable(opcodes.cmp_absolute_y)
    assert list(inspect.signature(opcodes.cmp_absolute_y).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0xD9] is opcodes.cmp_absolute_y


def test_opcode_D9_cmp_absolute_y_reads_indexed_memory_value():
    """Objective: D9 00 02 with Y=0x04 compares A with RAM[$0204]."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0xD9)
    rom.write(0x0001, 0x00)
    rom.write(0x0002, 0x02)
    bus.write(0x0204, 0x10)

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

    assert (cpu.p & CARRY_FLAG) != 0
    assert cpu.pc == 0x8003

测试函数 (2)

test_cmp_absolute_y_handler_exists_and_is_in_opcode_table
test_opcode_D9_cmp_absolute_y_reads_indexed_memory_value