Timestamp timer problem

Hi ,

I am having some problems in using the timestamp timer. Basically , when i am measuring the time to perform a single instruction, i occasionally(but not always) get a negative answer.I have taken care to call alt_timestamp_start() whenever I use the timer, to ensure that it does not overflow.

Also when i don't change the program and run it again, each time i get a slightly different number for the number of cycles taken. What could be the reason?

My code is as follows :

//CODE

if(alt_timestamp_start() < 0)

{

printf("Timer init failed \n");

exit(0);

}

time1 = alt_timestamp();

time2 = alt_timestamp();

timer_overhead = time2 - time1;

printf("\n timer overhead is %d",timer_overhead);

.........

..........

.......

time1 = alt_timestamp();

*(peripheral_base ) = 0x210f1007

time2 = alt_timestamp();

printf("\n time to perform 1 write to hw is %d" ,(time2 - time1 - timer_overhead));

printf("\n time1 is %lu",time1);

printf("\n time2 is %lu",time2);

//END OF CODE

I get the following output :

//OUTPUT

timer overhead is 246

time to perform 1 write to hw is -12

time1 is 143

time2 is 377

//END OF OUTPUT

Any suggestions ? Am i missing out something ?

Thanks,

Karthik