The External Interrupt Routine is called from the first level Ext Interrupt handler in low memory when an external interrupt occurs.
000000 8 TXXEXTIR CSECT ,
000000 18CF 9 LR R12,R15
00000 10 USING TXXEXTIR,R12 BASE REG
11 *
00000 12 USING @TCB,R4
13 *
000002 9180 401B 0001B 14 TM TCBPSW+3,X'80' TIMER INTERRUPT ?
000006 4710 C016 00016 15 BO TIMER YES - BRANCH
16 *
00000A 9140 401B 0001B 17 TM TCBPSW+3,X'40' EXT KEY PRESSED ?
00000E 4710 C026 00026 18 BO EXTKEY YES - BRANCH
19 *
000012 8200 C038 00038 20 LPSW EXTWPSW GO INTO WAIT STATE
We begin by examining the PSW to determine what type of external interruption occurred. The two conditions we are interested in are the Interval Timer expired and an operator generated interruption. If neither of these types of interruptions is indicated a wait state PSW is loaded.
000016 26 TIMER DS 0H
000016 5810 C040 00040 27 L R1,=A(X'80') RESTART TIMER
00001A 5010 0050 00050 28 ST R1,ITIMER-@LOWCORE
29 *
00001E 9610 4004 00004 30 OI TCBFLGS,X'10' INDICATE TIME EXPIRED
000022 47F0 C02A 0002A 31 B EXIT AND EXIT
If the interruption is the result of the Interval Timer expiring then it means the executing task is hogging the CPU. We set a bit in the TCB flags indicating the task was interrupted by the timer. This will cause the task to not be dispatched until all other tasks have been given the opportunity to run.
000026 37 EXTKEY DS 0H 000026 47F0 C02A 0002A 38 B EXIT IGNORE FOR NOW 00002A 44 EXIT DS 0H 00002A 58F0 0014 00014 45 L R15,20 GET CVT ADDRESS 00002E 58F0 F004 00004 46 L R15,MVTDISP-@MVT(,R15) GET DISPATCHER EP 000032 07FF 47 BR R15 BRANCH TO DISPATCHER 000038 51 DS 0D 000038 0002000000EE0002 52 EXTWPSW DC AL1(0,2,0,0),A(@E@EXT) DISABLED WAIT PSW
For now we will just ignore an external interrupt caused by the operator. Exit is simply branching to the Dispatcher routine.
[Next – TXXDISP – Dispatcher Routine]