Hello, i stud actually protected mode and user task system, but i don't found short and simple asm example do simply user task. Can you give me very simple example with user task ?

I don't understand how to do that and how reserve memory for task ...

Does your OS have a memory manager? If not then you will probably need one.

Although it does depend upon how you have constructed your OS. If you use protected mode with paging then a simple memory pool allocator would probably be fine for a simple OS. If you are in real or unreal mode then it would be trickier to manage the fragmentation with a linear address space.

Oh i think i begin to understand ! I think it's the use of GDT, to define memory segment size and privilege type ?

And what use the use of pagination system ? I don't understand very well? To manage gdt segment ?

Where is it possible to find best suggestion to make securised system ?

OSes like Linux and Windows don't manipulate the GDT for tasks. They only use paging. It is the most flexible. Often the GDT is simply set to span the entire address space for each selector.

I have difficulty to understand the use of paging, i don't understand...

have you heard of American owned airliners dragging people out of planes? U.S. law allows airliners to overbook their planes. they sell to multiple people expecting some of them to not show up. this is also what paging is meant to do.

one example is when you cache a file you might not need all of it. also; as a general rule you won't need access to all of it at the same time. the OS stores parts of different files for multiple programs allowing them to effectively (or actually in a multiple core system) run in parallel.