004. CPU 初始化

创建基本的 CPU 状态和取值辅助方法。

第 4 / 356 · tests/chapter_01_cpu/test_004_cpu_init.py

需要更新的文件

emulator/cpu/cpu.py

位置

class CPU

参考

https://www.nesdev.org/wiki/CPU_registers

此步骤存在的原因

CPU 在能够取指或执行指令之前，需要显式的寄存器和一个总线依赖。取值操作会推进程序计数器，因为指令字节是按顺序消费的。

完整示例实现

from dataclasses import dataclass

from emulator.bus.cpu_bus import CpuBus


@dataclass
class CPU:
    bus: CpuBus
    a: int = 0
    x: int = 0
    y: int = 0
    pc: int = 0
    s: int = 0
    p: int = 0

    def fetch_byte(self) -> int:
        value = self.bus.read(self.pc)
        self.pc += 1
        return value

    def fetch_word(self) -> int:
        low = self.fetch_byte()
        high = self.fetch_byte()
        return low | (high << 8)

重要不变量

CPU 保留调用方提供的确切总线对象
fetch_byte 会将 PC 前移一个字节
fetch_word 先消费低字节，并将 PC 前移两个字节

常见误解

小端存储改变的是内存中的字节顺序，而不是最终的数值结果。字节 $34 之后是 $12，得到的整数是 $1234。

范围之外

复位向量行为
操作码解码
与硬件完全一致的上电寄存器值

运行本课

uv run pytest tests/chapter_01_cpu/test_004_cpu_init.py -v

示例实现

完整示例实现

from dataclasses import dataclass

from emulator.bus.cpu_bus import CpuBus


@dataclass
class CPU:
    bus: CpuBus
    a: int = 0
    x: int = 0
    y: int = 0
    pc: int = 0
    s: int = 0
    p: int = 0

    def fetch_byte(self) -> int:
        value = self.bus.read(self.pc)
        self.pc += 1
        return value

    def fetch_word(self) -> int:
        low = self.fetch_byte()
        high = self.fetch_byte()
        return low | (high << 8)

查看测试代码test_004_cpu_init.py

import pytest
from emulator.bus.cpu_bus import CpuBus
from emulator.cpu.cpu import CPU


@pytest.fixture(scope="session")
def cpu():
    bus = CpuBus()
    default_cpu = CPU(bus)
    return default_cpu

def test_cpu_can_be_created():
    bus = CpuBus()
    cpu = CPU(bus)
    assert cpu.bus is bus

def test_cpu_has_registers(cpu):
    assert hasattr(cpu, "a") # Accumulator register
    assert hasattr(cpu, "x") # Index register X
    assert hasattr(cpu, "y") # Index register Y
    assert hasattr(cpu, "pc") # program counter
    assert hasattr(cpu, "s") # stack register
    assert hasattr(cpu, "p") # status register (Flags)

def test_cpu_registers_are_initialized_to_0(cpu):
    """ Initial register values set to 0
    Later on future tests we will implement cpu.reset() that put the starting values
    """
    assert (cpu.a, cpu.x, cpu.y, cpu.pc, cpu.s, cpu.p) == (0, 0, 0, 0, 0, 0)

def test_cpu_fetch_byte(cpu):
    cpu.pc = 0 # Temporal PC value for testing
    """Get current value from register PC and then pc+=1"""
    cpu.bus.write(0x0000, 0x42)

    value = cpu.fetch_byte()

    assert value == 0x42
    assert cpu.pc == 1


def test_cpu_fetch_word(cpu):
    cpu.pc = 0 # Temporal PC value for testing
    cpu.bus.write(0x0000, 0x34)
    cpu.bus.write(0x0001, 0x12) # Write 0x34 , 0x12 (Little endian 0x1234)

    assert cpu.fetch_word() == 0x1234

测试函数 (5)

test_cpu_can_be_created
test_cpu_has_registers
test_cpu_registers_are_initialized_to_0
test_cpu_fetch_byte
test_cpu_fetch_word