Service Request Block
A Service Request Block (SRB) is a data structure employed mainly, but not exclusively, by the Start Input/Output interface of MVS/370 and successor versions of IBM mainframe operating systems.
When employed by the Start Input/Output interface, an SRB is always paired with an 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.
History
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).
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