The multics virtual memory

ABSTRACT

As experience with use of on-line operating systems has grown, the need to share information among system users has become increasingly apparent. Many contemporary systems permit some degree of sharing. Usually, sharing is accomplished by allowing several users to share data via input and output of information stored in files kept in secondary storage. Through the use of segmentation, however, Multics provides direct hardware addressing by user and system programs of all information, independent of its physical storage location. Information is stored in segments each of which is potentially sharable and carries its own independent attributes of size and access privilege.Here, the design and implementation considerations of segmentation and sharing in Multics are first discussed under the assumption that all information resides in a large, segmented main memory. Since the size of main memory on contemporary systems is rather limited, it is then shown how the Multics software achieves the effect of a large segmented main memory through the use of the GE 645 segmentation and paging hardware.

REFERENCES

• 1: BELADY, L. A. A Study of Replacement Algorithms for a Virtual-Storage Computer. <u>IBM Systems J.</u> 5, 2 (1966), 78--101.
• 2: COMFORT, W. T. A Computing System Design For User Service. Proc. AFIPS 1965 Fall Joint Computer Conference Vol. 27, Pt. 1, Spartan Books, New York, pp. 619--628.
• 3: CORBATÓ, F. J., and VYSSOTSKY, V. A. Introduction and Overview of the Multics System. Proc. AFIPS 1965 Fall Joint Computer Conference, Vol. 27, Part 1. Spartan Books, New York, pp. 185--196.
• 4: CORBATÓ, F. J. A Paging Experiment with the Multics System. To be included in a Festschrift to be published in honor of Prof. P. M. Morse.
• 5: CRISMAN, P. A. ed. <u>The Compatible Time-Sharing System: A Programmer's Guide</u>, 2nd ed., MIT Press, Cambridge, Mass., 1965.
• 6: DALEY, R. C., and NEUMANN, P. G. A General - Purpose File System for Secondary Storage. Proc. AFIPS 1965 Fall Joint Computer Conf., Vol. 27, Part 1. Spartan Books, New York, pp. 213--229.
• 7: Robert C. Daley , Jack B. Dennis, Virtual memory, processes, and sharing in MULTICS, Communications of the ACM, v.11 n.5, p.306-312, May 1968 [doi>10.1145/363095.363139]
• 8: Peter J. Denning, The working set model for program behavior, Communications of the ACM, v.11 n.5, p.323-333, May 1968 [doi>10.1145/363095.363141]
• 9: Jack B. Dennis, Segmentation and the Design of Multiprogrammed Computer Systems, Journal of the ACM (JACM), v.12 n.4, p.589-602, Oct. 1965 [doi>10.1145/321296.321310]
• 10: John Fotheringham, Dynamic storage allocation in the Atlas computer, including an automatic use of a backing store, Communications of the ACM, v.4 n.10, p.435-436, Oct. 1961 [doi>10.1145/366786.366800]
• 11: GLASER, E. L., COULEUR, J. F., and OLIVER, G. A. System Design of a Computer for Time Sharing Applications. Proc. AFIPS 1965 Fall Joint Computer Conference, Vol. 27, Part 1. Spartan Books, New York, pp. 197--202.
• 12: Robert M. Graham, Protection in an information processing utility, Communications of the ACM, v.11 n.5, p.365-369, May 1968 [doi>10.1145/363095.363146]
• 13: J. H. Saltzer, TRAFFIC CONTROL IN A MULTIPLEXED COMPUTER, Massachusetts Institute of Technology, Cambridge, MA, 1966
• 14: The Descriptor -- A definition of the B5000 Information Processing System. Burroughs Corp., Detroit, Mich., 1961.