MULTITASKING

          Although Max-FORTH does not directly support multitasking, multi-
          tasking is possible.   The user may add on a multitasking program
          with  relative  ease  since  Max-FORTH  was written to be totally
          preemptable.   When Max-FORTH was  created,  user  variable  were
          reserved to specify each task's priority, record the task's state
          and allow for a double linked list of tasks to be created.

          Using  these variables and interrupts,  the user can create round
          robin or priority multitaskers,  or combinations thereof.    Pos-
          sibly the simplest structured multitasker is the round robin time
          sliced scheme.   The following example demonstarts such a system.
          For simplicity sake only one task is open to communicate with the
          terminal.  (The other tasks' USER variables, KEY-BC-PTR and EMIT-
          BC-PTR,  could have been modified to allow them to talk to  their
          own terminal.)

          (In  order  to  find  the value of NEXT2 in your revision of Max-
          FORTH,  tick the address of COLD and DUMP the following  100  hex
          bytes.  The address of NEXT2 is held in the two bytes immediately
          following the first 7E op-code in the that listing,  about 90 hex
          bytes down.)

          COLD

          HEX
          FORGET TASK

          ( MAKE NEW    )
          ( USER AREA   )
          ( FOR TASK #1 )
          0000 C000 100
          CMOVE
          C000 C00E +!
          C000 C010 +!
          C100 C01C !
          50 C01E !
          C160 C022 !
          C180 C02C !
          C000 C034 +!
          C000 C040 +!
          C000 C046 +!
          C000 C06A +!
          C000 C06C +!
          D006 C006 !
          C806 C008 !

          ( MAKE NEW    )
          ( USER AREA   )
          ( FOR TASK #2 )
          0000 C800 100

                                          1












          CMOVE
          C800 C80E +!
          C800 C810 +!
          C900 C81C !
          50 C81E !
          C960 C822 !
          C980 C82C !
          C800 C834 +!
          C800 C840 +!
          C800 C846 +!
          C800 C86A +!
          C800 C86C +!
          C006 C806 !
          D006 C808 !

          ( MAKE NEW    )
          ( USER AREA   )
          ( FOR TASK #3 )
          0000 D000 100
          CMOVE
          D000 D00E +!
          D000 D010 +!
          D100 D01C !
          50 D01E !
          D160 D022 !
          D180 D02C !
          D000 D034 +!
          D000 D040 +!
          D000 D046 +!
          D000 D06A +!
          D000 D06C +!
          C806 D006 !
          C006 D008 !

          C006 04  !
          ABORT
          ( Assumes three tasks and 8K of RAM at $C000 )
          ( The first task at $C000 slowly counts up on PB )
          ( The second task at $C800 acts as a thermostat: reads AN0 output PA6 )
          ( The third task at $D000 runs the normal Max-FORTH outer interpreter )
          (   the task will also be master task, starting & handling of others  )

          ( Begin with simple task first )
          : TASK ;
          : WAIT 1000 0 DO LOOP ;
          : RUN-TASK-1
            0
            BEGIN
              1+
              DUP
              B004 C!
              DUP

                                          2












              C00A ! ( THIS LINE IS ONLY TO LEAVE A TRACE IN MEM )
              WAIT
            AGAIN
          ;

          ' RUN-TASK-1 CFA C002 ! ( SAVE CFA TO RUN IN AUTOSTART POS FOR LATER )


          ( Switch to define the second task )
          C806 04  !
          ABORT
          : TASK ;

          VARIABLE LO-SET-POINT
          VARIABLE HI-SET-POINT
          VARIABLE HEATER-STATE
          VARIABLE TEMP

          : READ-A/D
            0 B030 C! ( Start conversion for PE0 )
            BEGIN B030 C@ 80 AND UNTIL ( Wait for conversion complete )
            B031 C@ TEMP !
          ;

          : CHK-LO?
            TEMP C@ LO-SET-POINT C@ <
          ;

          : HEATER-ON
            1 HEATER-STATE !
            FF B000 !
          ;

          : CHK-HI?
            TEMP C@ HI-SET-POINT C@ >
          ;

          : HEATER-OFF
            0 HEATER-STATE !
            BF B000 !
          ;

          : RUN-TASK-2
            70 LO-SET-POINT !
            74 HI-SET-POINT !
            BEGIN
              READ-A/D
              HEATER-STATE @ C80A ! ( THIS LINE IS ONLY TO LEAVE A TRACE IN MEM )
              CHK-LO?
              IF
                HEATER-ON
              THEN

                                          3












              CHK-HI?
              IF
                HEATER-OFF
              THEN
            AGAIN
          ;

          ' RUN-TASK-2 CFA C802 ! ( SAVE CFA TO RUN IN AUTOSTART POS FOR LATER )

          ( Switch to define the third task )
          D006 04  !
          ABORT
          : TASK ;

          CODE IRQRTN
            86 C, 40 C,  ( LDAA # $40        CLEAR THE INTERRUPT SOURCE )
            B7 C, B025 , ( STAA $B025        BY KNOCKING DOWN BIT TFLG2 )
            DE C, 02 C,  ( LDX IP            SAVE IP ON CONTEXT'S STACK )
            3C C,        ( PSHX                                         )
            DE C, 00 C,  ( LDX W             SAVE W  ON CONTEXT'S STACK )
            3C C,        ( PSHX                                         )
            DE C, 04 C,  ( LDX UP            GET THIS TASK PTR AT $0004 )
            AF C, 06 C,  ( STS RPSAVE,X      SAVE OLD CONTEXT )
            EE C, 02 C,  ( LDX UPLINK,X      FIND NEXT TASK )
            DF C, 04 C,  ( STX UP            AND SAVE FOR NXT IRQ )
            AE C, 06 C,  ( LDS RPSAVE,X      SWITCH CONTEXT )
            38 C,        ( PULX                                         )
            DF C, 00 C,  ( STX W             RECOVER W  CONTEXT'S STACK )
            38 C,        ( PULX                                         )
            DF C, 02 C,  ( STX IP            RECOVER IP CONTEXT'S STACK )
            3B C,        ( RTI )
          END-CODE

          CODE-SUB CLEAR-CC-MASKS
            86 C, 00 C, ( LDAA # 0 )
            06 C, ( TAP )
            39 C, ( RTS )
          END-CODE

          : ENABLE-MULTITASKING
            ( INSTALL THE REAL-TIME-INTERRUPT JUMP TABLE ENTRY )
            7E B7E6 EEC! ( JMP OP-CODE )
            [ ' IRQRTN @ >< FF AND ] LITERAL B7E7 EEC! ( HI BYTE )
            [ ' IRQRTN @    FF AND ] LITERAL B7E8 EEC! ( LO BYTE )

            ( CREATE CONTEXTS FOR THE OTHER TASKS ON THEIR STACKS )
            ( TASK 1 START UP CONTEXT )
            FE54   C0FE  ! ( PROGRAM COUNTER = NEXT2 )
            C010 @ C0FC  ! ( Y REGISTER = XXXX )
            C002 @ C0FA  ! ( X REGISTER = CFA TO RUN )
            0000   C0F8  ! ( B&A REGISTERS )
            00     C0F7 C! ( CC REGISTER )

                                          4












            0000   C0F5  ! ( IP )
            0000   C0F3  ! ( W )
            C0F2   C00C  ! ( RPSAVE )
            ( TASK 2 START UP CONTEXT )
            FE54   C8FE  ! ( PROGRAM COUNTER = NEXT2 )
            C810 @ C8FC  ! ( Y REGISTER = XXXX )
            C802 @ C8FA  ! ( X REGISTER = CFA TO RUN )
            0000   C8F8  ! ( B&A REGISTERS )
            00     C8F7 C! ( CC REGISTER )
            0000   C8F5  ! ( IP )
            0000   C8F3  ! ( W )
            C8F2   C80C  ! ( RPSAVE )

            ( ENABLE THE HARDWARE TO GIVE REAL TIME IRQ'S )
            40 B024 C! ( RTII SET )
            CLEAR-CC-MASKS
          ;

          : TASK1? BEGIN C00A @ U. ?TERMINAL UNTIL ;
          : TASK2? BEGIN C80A @ U. ?TERMINAL UNTIL ;

































                                          5