MSR is called by supervisory functions such as ASCBCHAP, lock release, DEQ etc of which new work is made available. MSR compares the priority of the new work va the existing active address space and if higher, update PSAANEW with the address of ASCB of the new work.
In multiprocessor system, MSR compares the priority against the address spaces in all CPU and update one of them if the condition matches. It then generated a SIGP interrupt to the CPU and forced an entry into the dispatcher even if the CPU may be in Wait state with no outstanding dispatch able works.
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