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.

The value of a computer system to its users is greatly enhanced if a user can, in a simple and general way, build his work upon procedures developed by others. The attainment of this essential generality requires that a computer system possess the features of equipment-independent addressing, an effectively infinite virtual memory, and provision for the dynamic linking of shared procedure and data objects. The paper explains how these features are realized in the Multics system.