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A Service Request Block is a data structure of MVS/370 and successor versions of IBM mainframe operating systems employed mainly, but not exclusively, by the Start Input/Output interface.
An SRB may be considered, in the abstract, to be a highly optimized Task Control Block , one which has few, if any, associated resources other than access to the processor itself. All system resources which are utilized under an SRB must be accessed through the use of "branch entries", some of which are new entries to traditional system services which were formerly accessed exclusively using SVC instructions.
When employed by the Start Input/Output interface, an SRB is always paired with an Input/Output Supervisor Block.
When otherwise employed, an SRB facilitates inter-address-space communication in general, and inter-application communication in particular.