* Copyright 1974, Association for Computing Machinery, Inc., reprinted by permission. This is a revised version of an article that appeared in Communications of the ACM, 17, No. 7 (July 1974), pp. 365-375. That article was a revised version of a paper presented at the Fourth ACM Symposium on Operating Systems Principles, IBM Thomas J. Watson Research Center, Yorktown Heights, New York, October 15-17, 1973.