009. CPU 步进

执行第一个操作码：立即寻址 LDA ()。

第 9 / 356 · tests/chapter_01_cpu/test_009_cpu_step.py

需要更新的文件

emulator/cpu/cpu.py

位置

CPU.step

参考

https://www.nesdev.org/wiki/Instruction_reference#LDA

此步骤存在的原因

CPU.step 引入了最基本的取指-译码-执行循环。立即寻址 LDA 会直接读取操作码 $A9 后面的字节，并将其存入累加器 A。

完整示例实现

class CPU:
    # Keep the state, fetch, and reset behavior from earlier tests.

    def step(self) -> None:
        opcode = self.fetch_byte()

        if opcode == 0xA9:
            self.a = self.fetch_byte()
            return

        raise NotImplementedError(
            f"Opcode {opcode:02X} not implemented"
        )

执行时间线

PC=$8000 -> fetch $A9 -> PC=$8001
PC=$8001 -> fetch operand $42 -> PC=$8002 -> A=$42

重要不变量

每次 step 恰好执行一条指令
未知的操作码会明显地失败
立即寻址 LDA 总共消费两个字节

常见误解

操作数 $42 是一个数值，不是一个地址。立即寻址模式不会再用 $0042 去做一次总线查找。

范围之外

测试 010 中引入的 Zero 和 Negative 标志位更新
LDA 的其他寻址模式
操作码表和周期计数

运行本课

uv run pytest tests/chapter_01_cpu/test_009_cpu_step.py -v

示例实现

完整示例实现

class CPU:
    # Keep the state, fetch, and reset behavior from earlier tests.

    def step(self) -> None:
        opcode = self.fetch_byte()

        if opcode == 0xA9:
            self.a = self.fetch_byte()
            return

        raise NotImplementedError(
            f"Opcode {opcode:02X} not implemented"
        )

查看测试代码test_009_cpu_step.py

from emulator.cpu.cpu import CPU
from emulator.bus.cpu_bus import CpuBus
from emulator.memory.fake_rom import FakeROM


def test_lda_inmediate_without_flags():
    """This is yout first opcode resoultion.
    You should define a step function inside CPU
    cpu.step() and implement opcode LDA 0xA9
    It should: 
        - Fetch opcode [fetch_byte]
        - Decode opcode [if opcode == 0xA9 ... then ... return]
        - raise NotImplementedError if Opcode Not Implemented
    
    Baby steps:
    
    def step(self) -> None:
        opcode = self.fetch_byte()

        if opcode == 0xA9
            self.a = self.fetch_byte()
            return
        raise NotImplementedError(f"Opcode {opcode:02X} not implemented")

    LDA Reference:
    https://www.nesdev.org/wiki/Instruction_reference#LDA
    """
    
    # Current flow CPU Status: Reset -> Fetch -> Decode
    rom = FakeROM()
    
    # Reset Vector
    rom.write(0x7FFC, 0x00)
    rom.write(0x7FFD, 0x80)

    # LDA #$42
    rom.write(0x0000, 0xA9)
    rom.write(0x0001, 0x42)

    bus = CpuBus(program_rom=rom)
    cpu = CPU(bus)

    cpu.reset()
    cpu.step() # LDA should fetch_byte and put it on register A

    assert cpu.a == 0x42

def test_lda_flags():
    """Now it's time configure LDA flags
    If result == 0 -> Flag Zero is set 
    If result is negative -> Flag N is set 

    7  bit  0
    ---- ----
    NV1B DIZC
    |||| ||||
    |||| |||+- Carry
    |||| ||+-- Zero
    |||| |+--- Interrupt Disable
    |||| +---- Decimal
    |||+------ (No CPU effect; see: the B flag)
    ||+------- (No CPU effect; always pushed as 1)
    |+-------- Overflow
    +--------- Negative
    """
    rom = FakeROM()
    
    rom.write(0x7FFC, 0x00)
    rom.write(0x7FFD, 0x80)

    rom.write(0x0000, 0xA9)
    rom.write(0x0001, 0x42)

    bus = CpuBus(program_rom=rom)
    cpu = CPU(bus)

    cpu.reset()
    cpu.step() 

    assert cpu.a == 0x42

测试函数 (2)

test_lda_inmediate_without_flags
test_lda_flags