A simple cooperative multitasking system by Harold Hallikainen.
See "PIC 18C Microcontoller - Preemptive real-time multitasking kernel" /techref/microchip/18c/multitask-rtk-ba.htm for a more complex multutasker for the PIC18C. See "multitasking" /techref/method/multitask.htm for multitasking in general.
// main.c // SIMPLE COOPERATIVE MULTITASKING SYSTEM // Harold Hallikainen // harold@hallikainen.org // December 2006 // This is a test of a SIMPLE cooperative multitasker. For each task, the system keeps a copy of the // stack pointer and a copy of the stack. // Initialization - // Each task starts with a call of InitTask(n) where n is the task number. The task follows the InitTask // call with any other required initialization, then enters a loop. InitTask saves the return address into // the task in the SavedStack array. It then pops the return address off the stack so we return to main() // instead of the function that called InitTask(). This allows us to continue setting up other tasks in main(). // Running- // After each task has been initialized, main calls StartMultiTask(). This loads the stack with the values // stored for task 0 by InitTask(). // Task Switching- // Whenever a task is waiting for something (typically input, perhaps waiting for an output device to be ready), // the loop contains a call of NextTask(). NextTask saves the current stack, loads the stack for the next task, // then returns into that function where IT had called NextTask. Note that this return may be several // function call levels down. It need not be in the original task loop, but, instead, in a function called // by that loop. This is makes multitasking much simpler than use of state machines for each task and function // within that task. // Variables and Function Parameters // If a particular function contains a NextTask call, local variables and parameters to the function MUST // be static. This keeps them from being located on the software stack (as automatic variables would be). // Automatic variables and function parameters are lost during a task switch. In addition, the software stack // may undeflow or overflow if you do a task switch while automatic variables or parameters exist. However, // functions that do NOT include a task switch may use automatic parameters and functions since these are // destroyed before the NextTask is called. #include <p18f8627.h> #include "types.h" #define MaxTask 3 // Function Prototypes void InitTask(static uint8_t TaskNum); // Save return address for this function to continue, // then throw it out, dropping to main() void StartMultiTask(void); // loads stack with initial data for task 0, then returns to task 0. void NextTask(void); // save current stack and load stack for next task void task0(void); void task1(void); void task2(void); void task3(void); uint8_t Task3SubFunction(static uint8_t n); // testing a subfunction uint8_t Task3subSub(uint8_t n); #pragma udata MultiTask // generally requires more than 256 bytes or ram, so put in bigger section union{ uint24_t u24b; // access as 24 bit unsigned int uint8_t u8b[sizeof(uint24_t)]; // or array of bytes }SavedStack[MaxTask+1][31]; // Each stack entry is 3 bytes and there are 31 of them in each stack. // MaxTask is highest task number. Since they start at zero, dimension to MaxTask+1 static uint8_t SavedStackPointer[MaxTask+1]; // stack pointer saved for a particular task #pragma udata // let linker allocate rest of stuff void main(void){ task0(); task1(); task2(); task3(); // init each task StartMultiTask(); // go run multitask } void InitTask(static uint8_t TaskNum){ // Call from within a task. Puts the Saves the return address in SavedStack[TaskNum][1] so a later call // of NextTask will continue from this address. Uses static parameter to avoid use of parameter stack which // would be messed up when we do a return to main instead of calling function. INTCONbits.GIEH=0; // disable high priority interrupts INTCONbits.GIEL=0; // and low priority interrupts while messing with stack SavedStackPointer[TaskNum]=STKPTR; // save the current stack pointer SavedStack[TaskNum][STKPTR & 0b11111].u24b=TOS; // save current tos, which is where this function should start _asm POP _endasm; // remove return address so we drop back to main() INTCONbits.GIEL=1; // interrupts ok now INTCONbits.GIEH=1; } void StartMultiTask(void){ // loads stack with initial data for task 0, then returns to task 0. INTCONbits.GIEH=0; // disable high priority interrupts INTCONbits.GIEL=0; // and low priority interrupts while messing with stack STKPTR=0; // start with clear stack pointer while(STKPTR<=SavedStackPointer[0]){ //TOS=SavedStack[0][STKPTR & 0b11111]; // restore stack value - use below code to avoid movff to tos WREG=SavedStack[0][STKPTR & 0b11111].u8b[0]; TOSL=WREG; WREG=SavedStack[0][STKPTR & 0b11111].u8b[1]; TOSH=WREG; WREG=SavedStack[0][STKPTR & 0b11111].u8b[2]; TOSU=WREG; _asm PUSH _endasm; // and increment stack pointer } _asm POP _endasm; // get rid of extra stack pointer increment at end INTCONbits.GIEL=1; // interrupts ok now INTCONbits.GIEH=1; } void NextTask(void){ // save away the current stack, load the one for the next task, then exit static uint8_t TaskNum=0; INTCONbits.GIEH=0; // disable high priority interrupts INTCONbits.GIEL=0; // and low priority interrupts while messing with stack SavedStackPointer[TaskNum]=STKPTR; // save the current stack pointer while(STKPTR & 0b11111){ // while stuff still on the stack SavedStack[TaskNum][STKPTR & 0b11111].u24b=TOS; // save current tos _asm POP _endasm; // decrement the stack pointer } TaskNum++; // on to next task if(TaskNum>MaxTask) TaskNum=0; // roll over if past last while(STKPTR<=SavedStackPointer[TaskNum]){ //TOS=SavedStack[TaskNum][STKPTR & 0b11111]; // restore stack value - use below code to avoid movff to tos WREG=SavedStack[TaskNum][STKPTR & 0b11111].u8b[0]; TOSL=WREG; WREG=SavedStack[TaskNum][STKPTR & 0b11111].u8b[1]; TOSH=WREG; WREG=SavedStack[TaskNum][STKPTR & 0b11111].u8b[2]; TOSU=WREG; _asm PUSH _endasm; // and increment stack pointer } _asm POP _endasm; // get rid of extra stack pointer increment at end INTCONbits.GIEL=1; // interrupts ok now INTCONbits.GIEH=1; } void task0(void){ static uint8_t n=0; InitTask(0); // set up return address for here while(1){ // task loop ClrWdt(); n++; NextTask(); } // end while } // end function void task1(void){ static uint8_t n=0; InitTask(1); // set up return address for here while(1){ // task loop ClrWdt(); n++; NextTask(); } // end while } void task2(void){ static uint8_t n=0; InitTask(2); // set up return address for here while(1){ // task loop ClrWdt(); n++; NextTask(); } // end while } void task3(void){ static uint8_t n=0; InitTask(3); // set up return address for here while(1){ // task loop ClrWdt(); n++; n=Task3SubFunction(n); ClrWdt(); } // end while } uint8_t Task3SubFunction(static uint8_t n){ // testing a subfunction. Make parameters and local variables static on any functions that include NextTask. ClrWdt(); n=3*n; NextTask(); ClrWdt(); n=Task3subSub(n); return(n); } uint8_t Task3subSub(uint8_t n){ // since this function does not call NextTask, we don't need to use statics uint8_t y=3; uint8_t z; z=n*y; return(z); }
file: /Techref/microchip/language/c/MultiTask.c.htm, 13KB, , updated: 2007/1/24 18:19, local time: 2024/8/15 00:05,
owner: harold-hallikainen-,
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