Input/Output Configuration Program

Input/Output Configuration Program
Original author(s)	IBM
Developer(s)	IBM
Operating system	stand alone, z/OS, z/VM, z/VSE
Platform	IBM System/370-Extended Architecture
License	Proprietary

The Input/Output Configuration Program is a program on IBM mainframes.^[when?]

History

In the original S/360 and 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 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^[1] mode, IBM changed the I/O architecture to allow the Channel Subsystem to handle the channel scheduling that the operating system handled in S/370 mode. The new I/O architecture used a 16-bit subchannel number, a 16-bit device number and an 8-bit Channel Path Identifier (Channel Path Id or CHPID); the Channel Subsystem was responsible for mapping the subchannel number to the channel and device numbers, for queuing I/O requests and for selecting from the available paths.

Starting with z990, IBM introduced the concept of a Logical Channel Subsystem^[2] (LCSS); each physical channel (of 1024) has a unique Physical Channel Path Identifier (PCHPID), each LCSS has a CHPID to PCHPID mapping and each LPAR is assigned to one LCSS.

Starting with System z9 running z/OS 1.7, IBM offered the multiple-subchannel set facility, which allowed up to four independent sets of 64 Ki subchannels.^[3]

The installation was responsible for defining the Input/Output Configuration Data Sets (IOCDS's), and the operator could select a specific IOCDS as part of a power on reset (POR). Input/Output Configuration Program (IOCP)^[4]^[5]^[6] is a program for IBM mainframes that compiles a description of the Channel Subsystem and LPAR^[7] configuration, optionally loading it into an Input/Output Configuration Data Set (IOCDS); it recognizes the syntax of MVS Configuration Program (MVSCP)^[8] input, and there is no need to maintain separate input files.