以下关于特权指令的叙述中，错误的是（）。
- A.特权指令集是计算机指令集的一个子集
- B.特权指令与系统资源的操纵和控制有关
- C.当计算机处于系统态运行时，它可以执行特权指令
- D.当计算机运行在用户态时，它可以执行特权指令
正确答案及解析
正确答案
解析
本题考查计算机状态和特权指令概念。计算机运行时的状态可以分为系统态(或称管态)和用户态(或称目态)两种。当计算机处于系统态运行时，它可以执行特权指令，而处于用户态运行时，则不能执行特权指令，如果此时程序中出现特权指令，机器将会发出特权指令使用错误的中断。所谓特权指令集是计算机指令集的一个子集，特权指令通常与系统资源的操纵和控制有关，例如，访外指令用于通道启动通道；时钟控制指令用于取、置时钟寄存器的值；程序状态字控制指令用于取、置程序状态字；通道控制指令用于访问通道状态字；中断控制指令则用于访问中断字等。
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