Editing QIO (section)

{{Short description|Term used in several computer operating systems}}
{{More citations needed|date=July 2009}}

'''QIO''' ('''Queue I/O''') is a term used in several computer [[operating system]]s designed by the former [[Digital Equipment Corporation]] ([[Digital Equipment Corporation|DEC]]) of [[Maynard, Massachusetts]].

I/O operations on these systems are initiated by issuing a QIO call to the [[Kernel (operating system)|kernel]].  There are two types of QIO - Queue I/O and Queue I/O with Wait.

For QIO without wait, the call returns immediately.  If the request is successfully enqueued, the actual operation occurs asynchronously.  On completion, status is returned in the QIO status doubleword.  The QIO request may also specify that completion set an event flag or issue an [[Asynchronous System Trap]] ([[Asynchronous System Trap|AST]]).

The call may also 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.

The following operating systems implement QIO(W):
* [[RSX-15]]
* [[RSX-11]] (including all of the variants)
* [[RSTS/E]] (synchronous only, emulated by the RSX [[runtime system]])
* [[OpenVMS]]<ref>[http://h30266.www3.hpe.com/odl/i64os/opsys/vmsos84/4527/4527pro_091.html#jun_423 HP OpenVMS System Services Reference Manual]</ref>