Method and system for superimposing, creating and altering I/O applications and controls within an I/O subsystem by using an I/O subchannel intercept field

An Internal I/O Facility (iIOF) having a general interface for executing I/O related applications in an Input/Ouput SubSystem (IOSS) of a computer system. IIOF applications are executed in the IOSS outside the scope of CPU applications executed under the operating system (OS) of the computer system. IIOF applications are allowed to use all facilities in the IOSS. An iIOF subchannel extension is provided for those subchannels which can execute an iIOF application. IIOF subchannel extensions contain interception control fields which determine if conventional processing for the subchannel by the IOSS is to be intercepted to execute an iIOF application. A front-end interception bit in a subchannel extension is used to initiate iIOF application execution when the subchannel is taken from the IOSS work request queue. A back-end interception bit in a subchannel extension is used to initiate iIOF application execution when the subchannel is to have status put on the IOSS interruption queue. The iIOF application is defined in a field in the iIOF subchannel extension. The IOSS may front-end intercept I/O requests made by the OS, and the IOSS may back-end intercept status received by the IOSS for I/O interruptions to be generated by the IOSS.