Service Request Block

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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",[1] 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).

When employed by the Start Input/Output interface, an SRB is always paired with an Input/Output Supervisor Block (IOSB).

When otherwise employed, an SRB facilitates inter-address-space communication in general, and inter-application communication in particular. An SRB may also be employed for intra-address-space processes, where the highest possible performance is required.


With the introduction of MVS/370 and successor systems, a whole new environment was introduced: the Service Request Block (SRB), which generally has a higher priority than any Task Control Block (TCB), and, indeed, which itself has two distinct priorities: a Global SRB (priority over all local address space SRBs and TCBs) and a Local SRB (priority over only the local address space TCBs); and MVS's dispatcher must manage all of these with absolute consistency across as many as two processors (MVS/370) and as many as sixteen processors (successor systems).

An SRB is a highly-optimized instance of a Process control block in these MVS/370 and successor systems. A TCB is a general-purpose instance of a Process control block in these OS/360 and successor systems.


  1. ^ Sprinkle, Tommy. "SRB – Service Request Block Overview". MVS/System Programming. Retrieved Nov 11, 2013. 

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