SVC FLIH

Upon an interrupt, the system will automatically save the PSW in PSA and invoke the SVC FLIH (first level interrupt handler). 

FLIH firstly save the state including registers into LCCA. FLIH is executed with interrupt disable so that the registers will not be change before the save is completed. Next FLIH checks various pre-requisites for SVC call before it invoke the actual SVC module to process the request. This include:

- is the SVC called from another SVC? If yes, SVC violation and abend 

- is is called from SRB mode? It f yes, abend

- test the issuer holding any locks  if yes, abend

- check SVC old PSW if it is in disabled state. If yes, abend

When the tests are passed, FLIH get the TCB added from PSATOLD. Get the RB adddress from TcB. Save the states (PSE, ILC, interrupt code) into RB. 

Finally FLIH looks up the SVC handler address from the SVC table and perform one more check - to check if the SVC needs AFP authorised and that the caller indeed satisfy this crirteria. Depending on the SVC type, FLIH will perform some more processing before calling the SBC handler 

As FLIH is executed disabled, it cannot call ABEND SVC.  To abend the caller when above check fails, it uses CALLRTM macro which will resolved into a branch instruction to RTM1 which in turn jump to RTM2 abend processing.