TXXDISP – Dispatcher Routine

The Dispatcher looks at the list of TCB’s on the work queue and selects one to become the active task.  If there are no TCB’s ready to dispatch an enabled wait will be entered.

000000                                6 TXXDISP  CSECT ,
000000 18CF                           7          LR    R12,R15            ESTABLISH
                            00000     8          USING TXXDISP,R12                 BASE REG
                                      9 *
000002 1F11                          10          SLR   R1,R1              CLEAR TIME EXPIRED SWITCH
                                     11 *
000004 58B0 0010      00010          12          L     R11,16             GET CVT ADDRESS
                            00000    13          USING @CVT,R11
                                     14 *
000008 5840 B004      00004          15          L     R4,CVTTASKQ        START AT TOP OF TASK QUEUE
                                     16 *
                            00000    17          USING @TCB,R4
                                     18 *
00000C                               19 DISP010  DS    0H
00000C 91D0 4004      00004          20          TM    TCBFLGS,TCBFWAIT+TCBFNDSP+TCBFETS  DISPATCHABLE?
000010 4780 C05C      0005C          21          BZ    DISP050             YES - GO RUN IT
                                     22 *
000014 9110 4004      00004          23          TM    TCBFLGS,TCBFETS    TIME EXPIRED FOR THIS TASK?
000018 47E0 C020      00020          24          BNO   DISP020            NO - BRANCH
                                     25 *
00001C 4110 0001      00001          26          LA    R1,1               SET EXPIRED SWITCH
000020                               27 DISP020  DS    0H
000020 5840 4000      00000          28          L     R4,TCBNEXT         POINT TO NEXT TCB
000024 1244                          29          LTR   R4,R4              END OF CHAIN
000026 4770 C00C      0000C          30          BNZ   DISP010            LOOP BACK
                                     31 *
00002A 1211                          32          LTR   R1,R1              ANY TIME EXPIRED TASKS
00002C 4780 C06A      0006A          33          BZ    DISP060              NO - GO INTO WAIT

The Dispatcher starts at the top of the TCB queue.  It looks for a TCB that is eligible to be dispatched.  If a TCB is found it becomes the current TCB and control is passed to that task.  While scanning the queue a switch is set if any TCB on the queue is not dispatchable because it was interrupted by the timer task.  If no any TCB’s were found that were not dispatched because of the time exceeded flag, the dispatcher will clear these bits and allows the TCB’s to be dispatched.

000030 5840 B004      00004          35          L     R4,CVTTASKQ        LOAD TOP OF TASK QUEUE
000034                               36 DISP030  DS    0H
000034 94FE 4004      00004          37          NI    TCBFLGS,X'FF'-TCBFLONG  CLEAR LONG RUNNING FLAG
                                     38 *
000038 9110 4004      00004          39          TM    TCBFLGS,TCBFETS    TIME EXPIRED FOR THIS TASK?
00003C 47E0 C048      00048          40          BNO   DISP040            NO - BRANCH
                                     41 *
000040 94EF 4004      00004          42          NI    TCBFLGS,X'FF'-TCBFETS    CLEAR FLAG
000044 9601 4004      00004          43          OI    TCBFLGS,TCBFLONG         SET LONG RUNNING FLAG
                                     44 *
000048                               45 DISP040  DS    0H
000048 5840 4000      00000          46          L     R4,TCBNEXT               NEXT TCB ON CHAIN
00004C 1244                          47          LTR   R4,R4                    END OF CHAIN
00004E 4770 C034      00034          48          BNZ   DISP030                   NO - LOOP BACK
                                     49 *
000052 5840 B004      00004          50          L     R4,CVTTASKQ              POINT TO TOP OF QUEUE
000056 1F11                          51          SLR   R1,R1                    CLEAR SWITCH
000058 47F0 C020      00020          52          B     DISP020                  LOOK AGAIN FOR WORK

Here the Dispatcher loops through the TCB queue resetting time exceeded bit.  If the TCBFETS flag is reset in a TCB, the TCBFLONG bit is set.  This bit will not make the task non-dispatchable.  It only indicates the TCB had previously been prohibited from dispatch.  Once the entire queue has been scanned the main dispatcher loop is once again entered to select a task for dispatch.

00005C                               58 DISP050  DS    0H
00005C 5040 B000      00000          59          ST    R4,CVTCTCB
000060 58F0 0014      00014          60          L     R15,20                   MVT ADDRESS
000064 58F0 F008      00008          61          L     R15,MVTLLDSP-@MVT(,R15)  LLDISP ROUTINE
000068 07FF                          62          BR    R15

Once a TCB is selected for dispatch the address of the TCB is stored in the CVT at CVTCTCB  (current TCB).  Control is then passed the the Low Core Dispatch routine in low memory to restore the registers and PSW from the TCB.

00006A                               68 DISP060  DS    0H
00006A 5840 B004      00004          69          L     R4,CVTTASKQ              WAIT TASK
00006E 5040 B000      00000          70          ST    R4,CVTCTCB                 MAKE IT CURRENT TASK
000072 8200 C078      00078          71          LPSW  DISPWPSW           ENABLED WAIT
000078                               75          DS    0D
000078 FF0200000000FEAD              76 DISPWPSW DC    AL1(255,2,0,0),A(X'FEAD')  ENABLED WAIT PSW

If no TCB is available for dispatch the dummy Wait Task TCB is made the current TCB and an enabled wait is entered.  The dummy Wait Task TCB is needed so that when an interruption occurs while in the disabled wait there will be a TCB for the interrupt handler to store registers and the PSW.  Although this information is saved at the interrupt, it is never used since the Wait Task is always associated with the enabled wait PSW.

[Next – TXXDINIT – Dispatcher Initialization ]

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