Input/Output Configuration Program
| Industry | Computer software |
|---|---|
| Products | Software utility for IBM mainframe I/O configuration |
The Input/Output Configuration Program is a software utility designed for use on IBM mainframe systems.
History
In the initial designs of the S/360 and S/370 platforms, each CPU was equipped with dedicated I/O channels and identified devices using a 12-bit cuu address, composed of a 4-bit channel identifier and an 8-bit unit number. Device configuration and I/O request management were handled by the operating system, which maintained queues for each channel. When an I/O operation needed to be initiated on a channel belonging to another processor, it required sending an interrupt to that processor.
With the advent of the IBM 3081 and IBM 4381 models under the S/370-Extended Architecture, IBM transitioned the responsibility for channel scheduling from the operating system to the Channel Subsystem. This updated architecture introduced a 16-bit subchannel number, a 16-bit device number, and an 8-bit Channel Path Identifier (CHPID). The Channel Subsystem took charge of mapping subchannels to their corresponding channels and devices, managing I/O request queues, and selecting available communication paths.
The launch of the z990 brought the Logical Channel Subsystem (LCSS) into operation. In this design, each physical channel—up to 1024 in total—is assigned a unique Physical Channel Path Identifier (PCHPID). The LCSS handles the mapping between CHPIDs and PCHPIDs, while each Logical Partition (LPAR) is allocated its own LCSS.
Beginning with System z9 running z/OS version 1.7, IBM introduced support for multiple subchannel sets, enabling up to four distinct groups of 64 Ki subchannels each.
Organizations create Input/Output Configuration Data Sets (IOCDS), which can be selected during system power-on reset (POR). The Input/Output Configuration Program (IOCP) compiles configuration details pertaining to the Channel Subsystem and LPAR setups, optionally loading these into IOCDSs. IOCP accepts input formatted in MVS Configuration Program (MVSCP) syntax, removing the need for separate input files.
Today, direct interaction with IOCP and MVSCP has mostly been superseded by the use of Hardware Configuration Definition (HCD).