We must prevent a user program from getting stuck in an infinite loop or not calling system services, and never returning control to the operating system.

• 和内存的保护,我们必须明确要由操作系统来维护控制。我们必须要阻止用户进程困在死循环中或不调用系统服务,以及永远不能将控制交还给操作系统。