Virtual Memory
PETER J. DENNING

Princeton University,* Princeton, New Jersey \

The need for automatic storage allocation arises from desires for program modularity, machine independence, and resource sharing. Virtual memory is an elegant way of achieving these objectives. In a virtual memory, the addresses a program may use to identify information are distinguished from the addresses the memory system uses to identify physical storage sites, and program-generated addresses are translated automatically to the corresponding machine addresses. Two principal methods for implementing virtual memory, segmentation and paging, are compared and contrasted. Many contemporary implementations have experienced one or more of these problems: poor utilization of storage, thrashing, and high costs associated with loading information into memory. These and subsidiary problems are studied from a theoretic view, and are shown to be controllable by a proper combination of hardware and memory management policies.

Key words and phrases: virtual memory, one-level store, memory allocation, storage allocation, dynamic storage allocation, segmentation, paging, replacement algorithms, storage fragmentation, thrashing, working set CR categories: 4.3, 6.2

INTRODUCTION

F r o m the earliest days of electronic computing it has been recognized that, because fast-access storage is so expensive, computer memories of very large overall capacity must be organized hierarchically, comprising at least two levels, " m a i n m e m o r y " and "auxiliary m e m o r y . " A program's information (i.e. instruction code and data) can be referenced only when it resides in main m e m o r y ; thus, information having immediate likelihood of being referenced must reside in main memory, and all other information in auxiliary memory. The storage allocation problem is t h a t of determining, at each m o m e n t of time, how information shall be distributed among the levels of memory. During the early