in ULT, scheduling is performed at user level (without the kernel involvement). but if one user level thread leave CPU and another will be schedule, then we have to change the value of program counter, Stack pointer and CPU register. without the involvement of kernel, how we can access these registers (hardware)?
Start with https://en.wikipedia.org/wiki/GNU_Portable_Threads and dig into the resources linked there.
In principle, ULT requires the implementation of some scheduler on the user level to do the scheduling of your N user tasks. If you understand the GNU PT functions etc., you'll see how to achieve that.
The program counter changes with every instruction. As well as with every function call, jump etc. Thus, changing the PC is not the challenge.
Regards
You might want to check this out:
http://superuser.com/questions/669883/why-are-user-level-threads-faster-than-kernel-level-threads
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