在操作系统中，由于进程总数多于处理机，它们必然竞争处理机。进程调度的功能就是按一定策略、动态地把处理机分配给处于就绪队列中的某一进程并使之执行。根据不同的系统设计目标，可有多种选择某一进程的策略。有两种基本的进程调度方式，即剥夺方式(preemptive mode)和非剥夺方式(non-preemptive mode)。前者指就绪队列中一旦有优先级高于现行进程优先级的进程出现时，系统便立即把处理机分配给高优先级的进程。当然，被剥夺了处理机的进程的有关状态和上下文都必须妥善保存以便今后恢复。后者是，一旦处理机分给了某进程，除非该进程的时间片已满或它主动放弃处理机引起进程调度的原因与操作系统的类型有关，大体可归结为以下几种：

(1) 进程运行完毕

(2) 进程提出I/O请求；

(3) 进程执行某种原语操作(如P操作)导致进程阻塞；

(4) 时间片已满；

(5) 可剥夺方式中，就绪队列中某进程的优先级变得高于现行进程；，系统不得以任何理由剥夺该现行进程的处理机。

-the operating system, due process than the total number of processor, and they will compete processor. Process scheduling function is to a certain strategy, Dynamic processor allocation of land to put in place a cohort of the implementation process of making it. According to various system design objectives, can have a wide choice of a process strategy. There are two basic process scheduling, that is deprived of means (preemptive mode) and non-denial mode (non-preempti 've mode). The former refers to ready queue once they have priority over the current process priority class process, the system will immediately processors allocated to high-priority process. Of course, the processors are deprived of the process of the state and context must be properly preserved for future restoration.