From "Multics—The first seven years" by F. J. Corbató, J. H. Saltzer, and C. T. Clingen in the Proceedings of the Spring Joint Computer Conference, 1972, AFIPS.Page 578:
Program execution environment
Another set of interfaces is embodied in the implementation
environment seen by PL/I programmers.
This environment consists of a directly addressable
virtual memory containing the entire hierarchy of online
information, a dynamic linking facility which
searches this hierarchy to bind procedure references, a
device-independent input/output system,* and program
debugging and metering facilities.
* The Michigan Terminal System has a similar device-independent input/output system.
By now it has become accepted lore in the computer
system field that the use of automatic management
algorithms for memory systems constructed of several
levels with different access times can provide a significant
reduction of user programming effort. Examples of
such automatic management strategies include the
buffer memories of the IBM system 370 models 155,
165, and 195 and the demand paging virtual memories
of Multics, IBM's CP-67 and the Michigan Terminal
 M T ALEXANDER
Organization and features of the Michigan terminal system
AFIPS Conf Proc 40 1972 SJCC AFIPS Press (This