Service Request Block

A Service Request Block (SRB) 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 to be a highly optimized Task Control Block (TCB), a "lightweight task", 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 (which an SRB may not employ for any purpose other than abnormally terminating itself).