Chapter 3 - OpenVMS Memory Management (II)

Abstract

This chapter reviews OpenVMS Memory Management. Memory management consists of the hardware and software that control the allocation and use of physical memory. Typically, in a multiprogramming system, several processes may reside in physical memory at the same time. OpenVMS Alpha uses memory protection and multiple address spaces to ensure that one process will not affect either other processes or the operating system. To improve further software reliability, four hierarchical access modes provide memory access control: (1) kernel, (2) executive, (3) supervisor, and (4) user. Protection is specified at the individual page level, where a page may be inaccessible, read-only, or read/write for each of the four access modes. Accessible pages can be restricted to have only data or instruction access. A program uses virtual addresses to access its data and instructions. However, before these virtual addresses can be used to access memory, they must be translated into physical addresses. Memory management software maintains tables of mapping information (page tables) that keep track of where each virtual page is located in physical memory. The processor utilizes this mapping information when it translates virtual addresses to physical addresses. Therefore, memory management provides both memory protection and memory mapping mechanisms.