Chapter 3 - DEC OSF/1 Memory Management (II–B)

Abstract

This chapter discusses DEC OSF/1 memory management. Memory management provides the mechanism to map the active part of the virtual address space to the available physical address space. The operating system controls the virtual-to-physical address mapping tables and saves the inactive parts of the virtual address space on external storage media. Memory management is always enabled. Implementations must provide an environment for PALcode to service exceptions and to initialize and boot the processor. Memory protection is the function of validating whether a particular type of access is allowed to a specific page from a particular access mode. The processor uses the following to determine whether an intended access is allowed: (1) the virtual address, which is used to either select kseg mapping or provide the index into the page tables; (2) the intended access type; and (3) the current access mode base on processor mode. There are two processor modes, user and kernel. The access mode of a running process is stored in the processor status mode bit (PS<mode>). The Alpha AXP architecture allows a processor to optionally implement address space numbers to reduce the need for invalidation of cached address translations for process-specific addresses when a context switch occurs.