i read that ther's smthin called a scheduler that picks a process and gives it to the cpu for execution.....but scheduler is also a some code running?
so who runs this scheduler thing? cpu? what if ther's a single processor ? how can cpu run two set of code at the same time?

Question

i read that ther's smthin called a scheduler that picks a process and gives it to the cpu for execution.....but scheduler is also a some code running?
so who runs this scheduler thing? cpu? what if ther's a single processor ? how can cpu run two set of code at the same time?

anonymous · Answer

@ktobah 
@nick67

nick67 · Answer

@A.Avinash_Goutham

in the past I used homemade scheduler on microcontrollers, where many times there in no room for an operative system or a simple kernel, too. In that case I used a well known trick: you use an hardware interrupt from an internal clock in order to switch from one task to another. The single processor just jumps to an ISR (Interrupt Service Routine) where you save current task context (stopping it) and load next task context (running it), up to the next timer interrupt.

anonymous · Answer

like after a process completes execution.....it generates a system call and next one in ihe queue is loaded for execution....is it this way sir?

nick67 · Answer

yes, Avinash, you're right. May I suggest you this reading: http://www.freertos.org/implementation/main.html It's an open source Real Time Operative System well documented and simple to use. I think it's useful for students, too.

anonymous · Answer

sir....what about modern operating systems?
sm1 has to choose a process among several processes bases on priority?

nick67 · Answer

On most systems there is a priority scheme for the scheduler, that allows each task to consume some of the CPU time

nick67 · Answer

but, if there's any need, a task can also raise a flag (let's say an interrupt) to signal it has an urgent call to serve

anonymous · Answer

So ther's s no physical code that requires cpu attention all the time
it is just a priority queue that deques it's elements regularly

nick67 · Answer

one processor (single core) can execute one instruction each clock tick; the program counter tells it what is the next instruction, except that in hardware interrupts, that load the program counter with a preset address pointing to ISR

anonymous · Answer

oh ok i get it

anonymous · Answer

thank you sir! :)

nick67 · Answer

you're welcome, but again I suggest you reading the Free RTOS docs