Input/Output Configuration Program: Difference between revisions

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In the original [[IBM System/360|S/360]] and [[IBM System/370|S/370]] architectures, each processor had its own set of I/O channels and addressed I/O devices with a 12-bit ''cuu'' address, containing a 4-bit channel number and an 8-bit unit (device) number to be sent on the channel bus in order to select the device; the operating system had to be configured to reflect the processor and ''cuu'' address for each device. The operating system had logic to queue pending I/O on each channel and to handle selection of alternate channels. Initiating an I/O to a channel on a different processor required causing a [[Shoulder tap (computer)|shoulder tap]] interrupt on the other processor so that it could initiate the I/O.

Starting with the [[IBM 3081]] and [[IBM 4381]] in [[S/370 Extended Architecture]]<ref name=SA22-708>{{cite manual

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}}</ref> mode, IBM changed the I/O architecture to useallow the [[Channel Subsystem]] to handle the channel scheduling that the operating system had to handle in S/370 mode. The new I/O architecture used a 16-bit '''Channel Path Id''' ('''CHPID'''); the [[Channel Subsystem]] was responsible for mapping the '''CHPID''' to the channel and device numbers, for queuing I/O requests and for selecting from the available paths. The installation was responsible for defining the [[Input/Output Configuration Data Set]]s ('''IOCDS''''s), and the operator could select a specific '''IOCDS''' as part of of a [[Power On Reset]] (POR). Input/Output Configuration Program (IOCP)<ref name=GC38-0097/><ref name=GC38-0401/><ref name=GC38-0456/> is a program for IBM mainframes that compiles a description of the [[Channel Subsystem]] and [[LPAR]]<ref name=GA22-7123/> configuration, optionally loading it into an [[Input/Output Configuration Data Set]] (IOCDS); it recognizes the syntax of [[MVS Configuration Program]] (MVSCP)<ref name=GC28-1615/> input, and there is no need to maintain separate input files.

The direct use of IOCP and [[MVS Configuration Program | MVSCP]] has been mostly supplanted by [[Hardware Configuration Definition | Hardware Configuration Definition (HCD)]]<ref name=GA22-7525/>