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The I/O Manager provides asynchronous I/O support so that the originator of an I/O request (usually a user-mode application, but sometimes another driver) can continue executing, rather than waiting for its I/O request to be completed. Providing asynchronous I/O support improves overall system throughput, as well as the performance of any code that makes an I/O request.
As a consequence, kernel-mode drivers do not necessarily process I/O requests in the same order they were sent to the I/O Manager. The I/O Manager or a higher level driver can reorder I/O requests as they are received or can split a large data transfer request into smaller transfer requests. Moreover, a driver can overlap I/O request processing, particularly in a symmetric multiprocessor platform, as mentioned in Multiprocessor Safe.
Furthermore, a kernel-mode driver's processing of an individual I/O request is not necessarily serialized. That is, a driver does not process each IRP to completion before it starts processing the next incoming I/O request, as drivers naturally do in a single-tasking operating system written to support synchronous I/O in uniprocessor machines.
Instead, a driver responds to the current IRP as it is passed to the driver's standard routines by carrying out whatever routine-specific operations are necessary to eventually satisfy the current request. However, a lowest level driver is required to help the I/O Manager and any higher level drivers processing the same IRPs to track the status of each request by setting the I/O status block in the IRP.
Drivers also can maintain state information about their current I/O operations in a special part of their device objects, called a device extension.
For more information, see Handling IRPs and Input/Output Techniques.