The Dispatcher looks at the list of TCB’s on the work queue and selects one to become the active task. \u00a0If there are no TCB’s ready to dispatch an enabled wait will be entered.<\/p>\n
000000 6 TXXDISP CSECT ,\r\n000000 18CF 7 LR R12,R15 ESTABLISH\r\n 00000 8 USING TXXDISP,R12 BASE REG\r\n 9 *\r\n000002 1F11 10 SLR R1,R1 CLEAR TIME EXPIRED SWITCH\r\n 11 *\r\n000004 58B0 0010 00010 12 L R11,16 GET CVT ADDRESS\r\n 00000 13 USING @CVT,R11\r\n 14 *\r\n000008 5840 B004 00004 15 L R4,CVTTASKQ START AT TOP OF TASK QUEUE\r\n 16 *\r\n 00000 17 USING @TCB,R4\r\n 18 *\r\n00000C 19 DISP010 DS 0H\r\n00000C 91D0 4004 00004 20 TM TCBFLGS,TCBFWAIT+TCBFNDSP+TCBFETS DISPATCHABLE?\r\n000010 4780 C05C 0005C 21 BZ DISP050 YES - GO RUN IT\r\n 22 *\r\n000014 9110 4004 00004 23 TM TCBFLGS,TCBFETS TIME EXPIRED FOR THIS TASK?\r\n000018 47E0 C020 00020 24 BNO DISP020 NO - BRANCH\r\n 25 *\r\n00001C 4110 0001 00001 26 LA R1,1 SET EXPIRED SWITCH\r\n000020 27 DISP020 DS 0H\r\n000020 5840 4000 00000 28 L R4,TCBNEXT POINT TO NEXT TCB\r\n000024 1244 29 LTR R4,R4 END OF CHAIN\r\n000026 4770 C00C 0000C 30 BNZ DISP010 LOOP BACK\r\n 31 *\r\n00002A 1211 32 LTR R1,R1 ANY TIME EXPIRED TASKS\r\n00002C 4780 C06A 0006A 33 BZ DISP060 NO - GO INTO WAIT<\/pre>\nThe Dispatcher starts at the top of the TCB queue. \u00a0It looks for a TCB that is\u00a0eligible\u00a0to be dispatched. \u00a0If a TCB is found it becomes the current TCB and control is passed to that task. \u00a0While scanning the queue a switch is set if any TCB on the queue is not dispatchable because it was interrupted by the timer task. \u00a0If 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.<\/p>\n
000030 5840 B004 00004 35 L R4,CVTTASKQ LOAD TOP OF TASK QUEUE\r\n000034 36 DISP030 DS 0H\r\n000034 94FE 4004 00004 37 NI TCBFLGS,X'FF'-TCBFLONG CLEAR LONG RUNNING FLAG\r\n 38 *\r\n000038 9110 4004 00004 39 TM TCBFLGS,TCBFETS TIME EXPIRED FOR THIS TASK?\r\n00003C 47E0 C048 00048 40 BNO DISP040 NO - BRANCH\r\n 41 *\r\n000040 94EF 4004 00004 42 NI TCBFLGS,X'FF'-TCBFETS CLEAR FLAG\r\n000044 9601 4004 00004 43 OI TCBFLGS,TCBFLONG SET LONG RUNNING FLAG\r\n 44 *\r\n000048 45 DISP040 DS 0H\r\n000048 5840 4000 00000 46 L R4,TCBNEXT NEXT TCB ON CHAIN\r\n00004C 1244 47 LTR R4,R4 END OF CHAIN\r\n00004E 4770 C034 00034 48 BNZ DISP030 NO - LOOP BACK\r\n 49 *\r\n000052 5840 B004 00004 50 L R4,CVTTASKQ POINT TO TOP OF QUEUE\r\n000056 1F11 51 SLR R1,R1 CLEAR SWITCH\r\n000058 47F0 C020 00020 52 B DISP020 LOOK AGAIN FOR WORK<\/pre>\nHere the Dispatcher loops through the TCB queue resetting time exceeded bit. \u00a0If the TCBFETS flag is reset in a TCB, the TCBFLONG bit is set. \u00a0This bit will not make the task non-dispatchable. \u00a0It only indicates the TCB had previously been prohibited from dispatch. \u00a0Once the entire queue has been scanned the main dispatcher loop is once again entered to select a task for dispatch.<\/p>\n
00005C 58 DISP050 DS 0H\r\n00005C 5040 B000 00000 59 ST R4,CVTCTCB\r\n000060 58F0 0014 00014 60 L R15,20 MVT ADDRESS\r\n000064 58F0 F008 00008 61 L R15,MVTLLDSP-@MVT(,R15) LLDISP ROUTINE\r\n000068 07FF 62 BR R15<\/pre>\nOnce a TCB is selected for dispatch the address of the TCB is stored in the CVT at CVTCTCB \u00a0(current TCB). \u00a0Control is then passed the the Low Core Dispatch routine in low memory to restore the registers and PSW from the TCB.<\/p>\n
00006A 68 DISP060 DS 0H\r\n00006A 5840 B004 00004 69 L R4,CVTTASKQ WAIT TASK\r\n00006E 5040 B000 00000 70 ST R4,CVTCTCB MAKE IT CURRENT TASK\r\n000072 8200 C078 00078 71 LPSW DISPWPSW ENABLED WAIT\r\n000078 75 DS 0D\r\n000078 FF0200000000FEAD 76 DISPWPSW DC AL1(255,2,0,0),A(X'FEAD') ENABLED WAIT PSW<\/pre>\nIf no TCB is\u00a0available\u00a0for dispatch the dummy Wait Task TCB is made the current TCB and an enabled wait is entered. \u00a0The 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. \u00a0Although this information is saved at the interrupt, it is never used since the Wait Task is always associated with the enabled wait PSW.<\/p>\n
[Next – TXXDINIT – Dispatcher Initialization<\/a>\u00a0]<\/p>\n","protected":false},"excerpt":{"rendered":"
The Dispatcher looks at the list of TCB’s on the work queue and selects one to become the active task. \u00a0If there are no TCB’s ready to dispatch an enabled wait will be entered. 000000 6 TXXDISP CSECT , 000000 … Continue reading