Unusual QIOs That Require Complex Processing
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). The Files-11 ODS-1 file system on RSX-11 was provided by a process called F11ACP; originally, the Files-11 ODS-2 file system was provided by F11BACP on VMS, but the functionality of F11BACP was later rearchitected into the VMS kernel to save the overhead of process context switches and is now called an XQP (eXtended Qio Processor).
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