操作系统可以快速执行和响应程序，对计算机用户来说，这个执行过程难以被注意到的(因为太快了)。这也让我们觉得，操作系统好像同时执行了多道程序。

The operating system can allow execution of multiple programs so quickly that it is virtually unnoticeable to a human user, making it seem as all the programs are running at the same time.

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