SVC EXIT PROLOG

 After SBC completed processing, it passed control to EXIT PROLOG via a BR 14. EXIT PROLOG may pass control to the dispatcher to f the force switch is on. Done SVC like POST or ENQ will caused new work to be ready and thus letting the dispatcher to chose the highest priority work to run next. If the force switch is ofF, EXIT PROLOG may pass control back to the caller. T first determine the type of SVC to work out how to restore the state before returning. Type 1 saved the register in TCB and the other type saves in SVRB. 

EXIT PROLOG would turn off the type 1 flag in ASCB and releases all locks used by SVC. For other SVC types, it acquired the local lock so it can manipulate the RB chain, restored the registers and disabled the io and external interrupt to make it same condition at the time of interrupt. It uses a LPSW instruction to pass control back to the caller.