Code associated with "Multitasking Mysteries Revealed" Bill Lamie February 1997

Listing 1: Control loop method for an application performing high-priority serial input.

main()
{
   /*Look for high-priority serial input. */
   poll_for_serial_input();

   /*Some other program task Òa.Ó */
   program_task_a();

   /*Some other program task Òb.Ó */
   program_task_b();

   /*Some other program task Òc.Ó */
   program_task_(c);
}

Listing 2:  Context switching on a Motorola 68000 target

_save_task_context:
				; Assume that interrupt stack frame
				;  is present at the top of the stack
movem.l		d0-d7/a0-a6,-(a7)	; Save 68000 registers
ori.w		#$0700,sr		; Lockout interrupts
movea.l		_current_task,a0	; Get current task pointer
move.l		a7,(a0)		; Save taskÕs stack pointer
clr.l		_current_task	; Clear current task pointer
movea.l		_system_stack,a7	; Switch to the system stack
jmp		_scheduler		; Return to the scheduler

.......

_scheduler:

move.l		_next_task,d0	; Pickup the next task to execute
beq		_scheduler		; If nothing to do, just keep checking!
jmp		_restore_task_context; Otherwise, transfer control to task

.......

_restore_task_context:
ori.w		#$0700,sr		; Lockout interrupts
move.l		d0,_current_task	; Setup current task pointer
movea.l		d0,a0		; Prepare to use task pointer
movea.l		(a0),a7		; Switch to taskÕs stack
movem.l		(a7)+,d0-d7/a0-a6	; Recover taskÕs registers
rte				; Recover taskÕs SR and return to it!

Listing 3: Polling calls to check for the presence of high-priority
main()
{

   /*Look for high-priority serial input. */
   poll_for_serial_input();

   /*Some other program task Òa.Ó */
   program_task_a();

   /*Look for high-priority serial input again. */
   poll_for_serial_input();

   /*Some other program task Òb.Ó */
   program_task_b();

   /*Look for high-priority serial input. */
   poll_for_serial_input();

   /*Some other program task Òc.Ó */
   program_task_c();
}

Listing 4:  code segments for tasks_1, 2, and 3.

task_a_b_and_c()
{

  /*Loop forever. */
  while(1)
  {

   /*Wait for message from queue. */
   my_queue_pend();

   /*Send message to next taskÕs queue. */
   send_to_next_task();
  }
}