Nios II Classic Processor Reference Guide

ID 683620
Date 10/28/2016
Document Table of Contents The PID Field

PID is a unique identifier for the current process that effectively extends the virtual address. The process identifier can be less than 14 bits. Any unused upper bits must be zero.

tlbmisc.PID contains the PID field from a TLB tag. The operating system must set the PID field when switching processes, and before each TLB write operation.

Note: Use of the process identifier is optional. To implement memory management without process identifiers, clear tlbmisc.PID to zero. Without a process identifier, all processes share the same virtual address space.

The MMU sets tlbmisc.PID on a TLB read operation. When the software triggers a TLB read, by setting tlbmisc.RD to one with the wrctl instruction, the PID value read from the TLB has priority over the value written by the wrctl instruction.

The size of the PID field is configured in Platform Designer at system generation, and can be from 8 to 14 bits. If system software defines a process identifier smaller than the PID field, unused upper bits must be written as zero.