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Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. High Quality Content by WIKIPEDIA articles! QIO (Queue I/O) is a term used in several computer operating systems designed by the former Digital Equipment Corporation (DEC) of Maynard, Massachusetts. I/O operations on these systems are initiated by issuing a QIO call to the kernel. The call returns immediately and if the I/O operation is successfully enqueued, it will be done asynchronously with its completion signaled by the raising of an event flag and-if requested-the issuance of an Asynchronous System Trap (AST) to the calling process/task. Optionally, the call may be issued as QIOW (Queue I/O and Wait for completion), allowing synchronous I/O. In this case, the wait-for-event-flag operation is combined so the call does not return until the I/O operation completes or fails. Simple QIOs, such as read or write requests, are either serviced by the kernel itself or by device drivers. Certain more complicated requests, specifically those involving tape drives and file-level operations, were originally executed by an Ancillary Control Processor (ACP) (a special purpose task with its own address mapping).

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