Task Executive. Gary J. Minden February 9, 2017

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1 Task Executive Gary J. Minden February 9,

2 Task Executive Motivation Implementation 2

3 Current Approach Each task has a Task_X subroutine Each task has X_NextExecute and X_DeltaExecute variables Each task has common code A check if it is time to execute the task A increment to the X_NextExecute variable A mainline that initializes each task and then polls each task 3

4 Proposed Approach Pull common task information into a data structure, a Task Control Block Organize TCBs into a structure Write common code to check for execution and increment X_NextExecute 4

5 Task Control Blocks (TCBs) typedef void TaskSubroutine(); struct TaskControlBlock { Task* TaskSubroutine; uint32_t ; uint32_t ; }; typedef struct TaskControlBlock TaskControlBlock; TaskControlBlock Task0 = { &Task_0, 0, ((SysTickFrequency * 500) / 10000) }; A function pointer always points to a function with a specific signature! Thus all functions, you want to use with the same function pointer, must have the same parameters and return-type! 5

6 Array of TCBs TaskControlBlock Tasks[] = { { &Task_0, 0, ((SysTickFrequency * 500) / 10000) }, { &Task_1, 0, ((SysTickFrequency * 50) / 10000) }, }; { &Task_2, 0, ((SysTickFrequency * 5000) / 10000) } 6

7 TCB TaskSubroutine 7

8 TCB Array Tasks[0] TaskSubroutine Tasks[1] TaskSubroutine Tasks[2] TaskSubroutine 8

9 Setting up Task Management Variables TaskControlBlock* curtask; // Pointer to current task int NbrTasks = ( sizeof( Tasks ) / sizeof( TaskControlBlock ) ); int TaskIdx; 9

10 Initializing Tasks for ( TaskIdx = 0; TaskIdx < NbrTasks; TaskIdx++ ) { curtask = &Tasks[TaskIdx]; curtask->tasksubroutine(); curtask-> = g_ultickcount + curtask->; }; 10

11 Executing Tasks while (1) { for ( TaskIdx = 0; TaskIdx < NbrTasks; TaskIdx++ ) { curtask = &Tasks[TaskIdx]; }; }; if ( (curtask->) <= g_ultickcount ) { curtask->tasksubroutine(); curtask-> += curtask->; }; 11

12 Problems Continue to poll each task to check if it is ready to execute No priority among tasks... All are equal No dynamic added or deleting tasks No accounting for execution time 12

13 Dynamic Adding and Deleting Tasks Extend TCB with forward and backward links Forms a double ended queue, a deque TaskSubroutine 13

14 Dynamic Adding and Deleting Tasks Extend TCB with forward and backward links Forms a double ended queue, a deque End of deque indicated by pointer to deque head TaskList

15 Dynamic Adding and Deleting Tasks Extend TCB with forward and backward links Forms a double ended queue, a deque Easy to insert or delete TCBs TaskList

16 Multiple Task Lists Replicate the Task List HighPriorityTaskList LowPriorityTaskList 16

17 Multiple Task Lists Each Task List has a purpose Ready to execute With multiple priorities Waiting for I/O Waiting for a specific time 17

18 Multiple Task Lists Completion of I/O causes task to change lists HighPriorityTaskList LowPriorityTaskList 18

19 Time Expired Computation intensive tasks moved to deque end HighPriorityTaskList 19 16

20 Multiple Task Lists Computation intensive tasks moved to high compute list HighPriorityTaskList ComputeTasks 20

21 Additional Task State Extend TCB CPU State (Registers and Status) TaskSubroutine R0-R15 StatusRegister OtherState 21

22 Additional Task State Simplify TCB Current PC is R15 Next time to execute, task can determine delta delay (sleep) TaskMain R0-R15 StatusRegister MemoryLow MemoryHigh StackLow StackHigh 22

23 Task Pattern void Task_X () { <Initialization Code> while (1) { Sleep( NSec ); <Execute Code> }; }; Sleep routine returns to Task Executive Saves Task_X state Puts Task_X on waiting list 23

24 Task Pattern void Task_X () { <Initialization Code> while (1) { ReadNoBlock( UART1, IEV ); <Continue Execution> }; }; WaitFor( IEV ); GetBlock( UART1 ); Introduce Event Variables Continue executing Wait For IEV to be set Saves Task_X state Puts Task_X on waiting list Execution continues when I/O is completed 24

25 Timer Interrupt Timer interrupts current task Save state of current task Move to end of ready deque or to compute intensive deque Select next task ready to execute Restore state of next task Branch to current PC of next task 25

Interrupts (Exceptions) (From LM3S1968) Gary J. Minden August 29, 2016

Interrupts (Exceptions) (From LM3S1968) Gary J. Minden August 29, 2016 1 Interrupts Motivation Implementation Material from Stellaris LM3S1968 Micro-controller Datasheet Sections 2.5 and 2.6 2 Motivation