According to PowerPC reference manual it says PID is used during address translation (virtual to physical). My question is - Is this PID(8bit) register in PowerPC contains Process ID of user space Processes? If yes who does it and when it is filled?

Following statement created confusion -
"... simply because the contents of PID register is different for each task. (Note: the PID register is only 8 bits and has no direct connection to the OS concept of a "process ID".) "

From - http://kvm.et.redhat.com/page/PowerPC_Book_E_MMU

Any reference for the purpose of this register, usecase nad how this register is used will be very helpful.

Thanks

Moved: This thread is more suitable in <GENERAL> and has been moved accordingly to help your thread/question get the exposure it deserves.

Maybe it's not "Process ID", but "Pointer ID"...? I don't know, I'm just taking a wild guess (hell, I don't even really know anything about the PPC architecture ).

It seems to be explained in the link you posted: it gives the OS some extra bits so that addresses in the TLB from one process will never match addresses from another process. It's not directly related to OS processes in that the hardware doesn't care about which process addresses come from, you could (theoretically) have an OS without any processes that makes use of this feature. Also since the PID is only 8 bits, it's unlikely that there will be a 1-to-1 mapping from OS pids to PID register values (most OS's will want to have more than 256 processes).