Every syscall effectively takes one extra usec to run, after the time it requires on the CPU. So this is where the extra usec comes in. So just implement it before every syscall occurs. And then for your issue with sleeping, it went to sleep at 16, so it should wake up at 36. Which it does. I mean I get what you're saying, but you didn't count the extra usec when it starts transferring from READY to RUNNING, which is taken as part of the time. As in on the 36th usec it wakes up.