实际上应该复制这些设备的寄存器的值。
如果这样做，counter的值必须先复制到寄存器，递减，然后对其内存更新。
If you do, the value of the counter must first be copied on to a register, decremented, and then updated to its memory.
PowerPC 使用了加载/存储（也成为 RISC）指令集，这意味着访问主存的惟一 时机就是将内存加载到寄存器或将寄存器中的内容复制到内存中时。
The PowerPC uses a load/store (also called RISC) instruction set, which means that the only time it accesses main memory is for loading into registers or copying a register to memory.
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