Preemptive Multitasking Scheduler for AVR

1/*
2 * main.c
3 *
4 * Created: 27-06-2016 09:29:09 PM
5 * Author  : Akash Kollipara
6 */ 
7
8uint8_t R=0, currentTaskId=0, nextTaskId=0;
9
10typedef  enum status{t_ready, t_busy} status;
11
12typedef struct TaskControlBlock		//defines  and controls a task for this scheduler
13{
14	uint8_t task_id;
15	enum status  state;
16	unsigned int stack_pointer_begin;
17	unsigned int stack_pointer_end;
18	void  (*fnctpt)(void);
19} tcb;
20tcb task[maxTask];
21
22int main(void)
23{
24    	cli();
25	//initialization 
26	timer_init();				//initializes timer  for scheduler
27	Neutron_kernel_init();		//initializes the kernel
28	
29	//setting  up first task parameters
30	StackPointer=task[0].stack_pointer_begin;
31	currentTaskId=task[0].task_id;
32	if(task[0].state==t_ready)
33		task[0].state=t_busy;
34	sei();
35	//running  first task
36	task[0].fnctpt();
37	
38    	while (1);
39}
40
41ISR(TIMER0_COMPA_vect)
42{
43	uint8_t  q=0;
44	PORTB^=(1<<PORTB5);
45	cli();
46	//getting the termination pointer  of individual stacks
47	task[currentTaskId].stack_pointer_end=StackPointer;
48	for(q=0;  q<maxTask; q++)
49	{
50		if(task[q].state==t_ready)
51		{
52			//setting  up task parameters 
53			task[q].state=t_busy;
54			currentTaskId=task[q].task_id;		//sets  task_id for saving context in corresponding task's stack
55			StackPointer=task[q].stack_pointer_begin;
56			sei();
57			task[q].fnctpt();		//execute  task if not priorly executed
58		}
59		R=q;
60	}
61	if(R==maxTask-1)	//checks  if all the tasks are in queue
62	{
63		if(nextTaskId==maxTask) nextTaskId=0;		//resets  the the task_id if it exceeds maxTask
64		if(task[nextTaskId].state==t_busy)
65		{
66			//pointing  stack to start context restoring
67			StackPointer=task[nextTaskId].stack_pointer_end;
68			currentTaskId=nextTaskId;				//when  next interrupt occurs, this var is used as the task_id for getting stack end pointer
69			nextTaskId=task[nextTaskId].task_id+1;	//increments  the task_id for next call of scheduler
70		}
71	}
72}
73