Nios II Classic Processor Reference Guide

ID 683620
Date 10/28/2016
Document Table of Contents
Give Feedback Virtual Addressing

A virtual address is the address that software uses. A physical address is the address which the hardware outputs on the address lines of the Avalon® ® bus. The Nios II MMU divides virtual memory into 4-KB pages and physical memory into 4-KB frames.

The MMU contains a hardware translation lookaside buffer (TLB). The operating system is responsible for creating and maintaining a page table (or equivalent data structures) in memory. The hardware TLB acts as a software managed cache for the page table. The MMU does not perform any operations on the page table, such as hardware table walks. Therefore the operating system is free to implement its page table in any appropriate manner.

There is a 20 bit virtual page number (VPN) and a 12 bit page offset.

Table 7.  MMU Virtual Address Fields
Bit Fields
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16
Virtual Page Number
15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
Virtual Page Number Page Offset

As input, the TLB takes a VPN plus a process identifier (to guarantee uniqueness). As output, the TLB provides the corresponding physical frame number (PFN).

Distinct processes can use the same virtual address space. The process identifier, concatenated with the virtual address, distinguishes identical virtual addresses in separate processes. To determine the physical address, the Nios II MMU translates a VPN to a PFN and then concatenates the PFN with the page offset. The bits in the page offset are not translated.