操作系统如何知道I/O操作何时完成执行？
考虑情况，您可以从光盘（I/O操作）中发出阅读。那么，操作系统用来了解操作是否已执行的确切机制是什么？
Consider the situation, where you issue a read from the disc (I/O operation). Then what is the exact mechanism that the OS uses to get to know whether the operation has been executed?
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确切的机制取决于特定的硬件（以及OS和方案）；但是通常，当设备完成某些操作时，设备会触发一个IRQ，该IRQ会导致CPU中断其所做的任何事情并切换到设备驱动程序的中断处理程序。
有时/经常设备驱动程序最终会保持待处理命令的队列或缓冲区；因此，当执行其中断处理程序（告诉它已经完成了一个命令）时，请采用下一个待处理命令并告诉设备启动它。有时/通常这还包括某种IO优先级方案，驾驶员可以要求设备更快地做更重要的工作（而不太重要的工作被推迟/仍在待处理中）。
设备驱动程序通常还以某种方式与调度程序绑定 - 用户空间中的正常线程可能（例如直接或间接 - 通过。文件系统）请求数据传输数据，并且将告知调度程序不给该线程CPU时间因为它被阻止/等待某物；然后，当传输完成后，设备驱动程序的中断处理程序告诉调度程序，请求线程可以继续，从而使其被调度程序未经阻止/可以通过调度程序给予CPU时间。
The exact mechanism depends on the specific hardware (and OS and scenario); but typically when a device finishes doing something the device triggers an IRQ that causes the CPU to interrupt whatever it was doing and switch to a device driver's interrupt handler.
Sometimes/often device driver ends up maintaining a queue or buffer of pending commands; so that when its interrupt handler is executed (telling it that a previous command has completed) it takes the next pending command and tells the device to start it. Sometimes/often this also includes some kind of IO priority scheme, where driver can ask device to do more important work sooner (while less important work is postponed/remains pending).
A device driver is typically also tied to scheduler in some way - a normal thread in user-space might (directly or indirectly - e.g. via. file system) request that data be transferred and the scheduler will be told to not give that thread CPU time because it's blocked/waiting for something; and then later when the transfer is completed the device driver's interrupt handler tells the scheduler that the requesting thread can continue, causing it to be unblocked/able to be given CPU time by scheduler again.