Task vs Service

To execute a program in MVS, one can call the ATTACH macro which create a TCB. The ATTACH macro is expanded to a SVC call (42) which trigger interrupt handling. If the task was a performing very short procedure, the overhead of creating a task is too expensive. 

MVS provide a SRB mechanism to allow a subsystem or address space to perform a procedure with less overhead than via creating a task. SRB is invoked via a SCHEDULE macro which does not expand to a SVC instruction. The macro will put the SRB on the appropriate queue and awaiting execution when the address space is picked by dispatcher as the next highest priority to run