Introducing: tiny msp430 preemptive multitasking system

Other Parts Discussed in Thread: MSP430G2955

Tested and working on a G2553 Launchpad with IAR.
No real limit on the number of virtual cores/threads,
though you will run out of ram and recommend msp430G2955 that comes with 4K of RAM

To add: __system_delay() where it skips a task for a number of cycles, though how to estimate time if other task also skips is something to ponder on

#include "msp430.h"
#include "common.h"

//=========================(C) Tony Philipsson 2016 =======================
funcpnt const taskpnt[]={ task1, task2, task3,  // <- PUT YOUR TASKS HERE
}; 
const int stacksize[tasks] = {28};              // nonfilled values defaults to 24 stack words
//=========================================================================
int taskstackpnt[tasks];
unsigned int taskdelay[tasks];
char taskrun;

int main( void )
{
  WDTCTL = WDTPW + WDTHOLD;                     // Stop watchdog timer 
  if (CALBC1_8MHZ != 0xff){			            // erased by mistake?
    BCSCTL1 = CALBC1_8MHZ;		                // Set DCO to factory calibrate 8MHz  
    DCOCTL = CALDCO_8MHZ;
  } 
  int* multistack = (int*) __get_SP_register();
  int i=0; while(i<tasks-1){
    int j = stacksize[i]; if (!j) j = 24;
    multistack -= j;  
    *(multistack) = (int) taskpnt[++i];         // prefill in PC 
    *(multistack-1) = GIE;                      // prefill in SR
    taskstackpnt[i] = (int) multistack-26;      // needs 12 dummy push words
  }
  WDTCTL = WDTPW+WDTTMSEL+WDTCNTCL;             // 4ms interval at 8MHz smclk
  IE1 |= WDTIE;
  __bis_SR_register(GIE);
  asm ("br &taskpnt");                          // indirect jmp to first task
}

//============= TASK SWITCHER ISR =============
#pragma vector = WDT_VECTOR 
__raw __interrupt void taskswitcher(void)
{
  asm ("push R15\n push R14\n push R13\n push R12\n"
       "push R11\n push R10\n push R9\n push R8\n"
       "push R7\n push R6\n push R5\n push R4");

  taskstackpnt[taskrun] = __get_SP_register();
  if (++taskrun == tasks) taskrun = 0;
  __set_SP_register(taskstackpnt[taskrun]);
  
  asm ("pop R4\n pop R5\n pop R6\n pop R7\n"
       "pop R8\n pop R9\n pop R10\n pop R11\n"
       "pop R12\n pop R13\n pop R14\n pop R15");
} 

Example of three tasks plus common.h file

#include "msp430.h"
#include "common.h"

__task void task1(void){
  P1DIR |= BIT0;
  while(1){
    __delay_cycles(800000);
    P1OUT |= BIT0;
    __delay_cycles(800000);
    P1OUT &=~BIT0;        
  }
}

#include "msp430.h"
#include "common.h"

__task void task2(void){
  P1DIR |= BIT6;
  while(1){
    __delay_cycles(1200000);
    P1OUT |= BIT6;
    __delay_cycles(1200000);
    P1OUT &=~BIT6;        
  }
}

#include "msp430.h"
#include "common.h"
unsigned int fibo(int);

__task void task3(void){
  int temp = 0;
  while(1){
    fibo(++temp);
  }
}
unsigned int fibo(int n){
  if (n < 2)
    return n;
  else
    return (fibo(n-1) + fibo(n-2));
}

#ifndef COMMON_H_
#define COMMON_H_
#define  tasks (sizeof(taskpnt)/2)
__task void task1(void);
__task void task2(void);
__task void task3(void);
typedef __task void (*funcpnt)(void);

#endif