上下文交换
上下文交换(英语：context switch)，又称环境切换，电脑术语，是一个储存和重建CPU的状态 (内文)，因此令多个进程(process)可以分享单一CPU资源的计算过程。要交换CPU上的进程时，必需先行储存目前进程的状态，再将欲执行的进程之状态读回CPU中
In computing, a context switch is the process of storing the state of a process or of a thread, so that it can be restored and execution resumed from the same point later. This allows multiple processes to share a single CPU, and is an essential feature of a multitasking operating system.
When to switch?
Multitasking
Most commonly, within some scheduling scheme, one process must be switched out of the CPU so another process can run. This context switch can be triggered by the process making itself unrunnable, such as by waiting for an I/O or synchronization operation to complete. On a pre-emptive multitasking system, the scheduler may also switch out processes which are still runnable. To prevent other processes from being starved of CPU time, preemptive schedulers often configure a timer interrupt to fire when a process exceeds its time slice. This interrupt ensures that the scheduler will gain control to perform a context switch.
Interrupt handling
Modern architectures areinterruptdriven. This means that if the CPU requests data from a disk, for example, it does not need tobusy-waituntil the read is over; it can issue the request and continue with some other execution. When the read is over, the CPU can beinterruptedand presented with the read. For interrupts, a program called aninterrupt handleris installed, and it is the interrupt handler that handles the interrupt from the disk.
When an interrupt occurs, the hardware automatically switches a part of the context (at least enough to allow the handler to return to the interrupted code). The handler may save additional context, depending on details of the particular hardware and software designs. Often only a minimal part of the context is changed in order to minimize the amount of time spent handling the interrupt. Thekerneldoes not spawn or schedule a special process to handle interrupts, but instead the handler executes in the (often partial) context established at the beginning of interrupt handling. Once interrupt servicing is complete, the context in effect before the interrupt occurred is restored so that the interrupted process can resume execution in its proper state.
User and kernel mode switching
When a transition between user mode and kernel mode is required in an operating system, a context switch is not necessary; a mode transition is not by itself a context switch. However, depending on the operating system, a context switch may also take place at this time.