Contains research objectives and report on status of research.Joint Services Electronics Programs (U. S. Army, U. S. Navy, and U. S. Air Force) under Contract DA 36-039-AMC-03200(E)National Science Foundation (Grant GP-2495)National Aeronautics and Space Administration (Grant NsG-496

Dennis, J. B.

Kerllenevich, N.

Rotenberg, L. J.

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XXIX. COMPUTER RESEARCHAcademic and Research StaffProf. J. B. DennisN. KerllenevichRachel J. LevyRESEARCH OBJECTIVESThe purpose of this group, which is operated jointly by the Research Laboratory ofElectronics, the Electronic Systems Laboratory, and the Department of Electrical Engi-neering, M. I. T. , is fourfold:1. To provide a flexible and readily accessible computation facility oriented towardthe Laboratory's research goals.2. To provide an educational facility where students may learn the principles ofautomatic computation, and undergraduate and graduate thesis projects may be carriedout.3. To develop computation techniques, especially methods that make program designand construction more convenient, and allow easier communication with the machine.4. To investigate new principles for organizing multiprogrammed computer systems.J. B. DennisSTATUS OF RESEARCH1. Computing EquipmentThe Computer Research Group operates two general-purpose machines. TheTX-0 computer was built by Lincoln Laboratory, M. I. T. , as an experimental machine,and was turned over to the Department of Electrical Engineering in 1958. The DigitalEquipment Corporation donated one of their PDP-1 machines in 1961. The PDP-I hasbecome the central processor of a time-shared multiuser system1 implemented by thegroup. Our activities have included modifications to the two computer systems toimprove their performance, as well as the programming of translators, editors,debugging programs, and executive routines suited to the on-line nature of the useof the two systems.2. Development of an Improved Multiuser SystemExperience obtained from operation of the PDP- 1 time-sharing system, togetherwith experience at Project MAC, has taught us much about desirable properties of time-shared computer systems, or, in more general terms, multiprogrammed computerThis work is supported principally by the Joint Services Electronics Programs(U. S. Army, U. S. Navy, and U. S. Air Force) under Contract DA 36-039-AMC-03200(E);and in part by the National Science Foundation (Grant GP-2495) and the National Aero-nautics and Space Administration (Grant NsG-496).QPR No. 80 255(XXIX. COMPUTER RESEARCH)systems. Features of a multiprogrammed computer system (MCS) are generally evidentin the services that an object program may obtain from the executive program by givingcommands that we call meta-instructions. From work done at Project MAC, Dennis andVan Horn 2 have published a set of meta-instructions and defined their meaning in termsof a program structure felt to be particularly suitable for an MCS. The principal ideasin this paper concern parallel processing, relations among computations, and the organ-ization and manipulation of directories of files, etc. , retained by the system on behalfof its users. Plans have been made to implement many of these ideas in the next incar-nation of our multiuser system. The new system involves hardware additions and modi-fications, as well as the preparation of a new executive system. Some of the moreimportant aspects of the system design are outlined here.a. File memory. A file memory system is being added to the PDP-1 installationin the form of miniature tape units (DecTape), so that a user may retrieve programsand data by command from his console. Consistent with the informal way in which thesystem is operated, a user will ordinarily bring a tape containing his programs withhim and mount it on a free unit at the beginning of a session of use, and take the(updated) tape with him when he leaves.b. Parallel programming. We have found the notion of process to be more funda-mental to the design of an MCS than the concept of program. A process is the sequentialexecution of the steps of a procedure. One process may by means of a fork meta-instruction create a second process which is thought of as running concurrently with thefirst. The use of a join meta-instruction allows a process to test whether two or moreasynchronous activities have been completed. Execution of a quit meta-instruction ter-minates a process. The fork, join, and quit meta-instructions are basic primitiveoperations of parallel programming. A particularly efficient implementation of theseprimitives through a combination of hardware and software techniques is a centralfeature of the new design. In the new system the use of these primitives to control con-current input/output activity replaces the interrupt feature that serves this function onessentially all present machines.c. Computations. We use the term computation to mean a collection of processesthat collectively accomplish a computing task. Each computation runs on behalf of auser of the system and has an associated sphere of protection that determines what files,services or directories the processes of the computation are allowed to access or use.In our revised system the sphere of protection is established by a 64-entry list, calleda capability list, 2 through which the processes of the computation execute input/outputoperations, or gain access to directories, files, and other services provided by theexecutive system. We permit one computation to run a second computation under itsQPR No. 80 256(XXIX. COMPUTER RESEARCH)surveillance. This is a valuable asset in program debugging: It allows a programmingsystem running as an object computation to monitor a second object computation undertest without the danger of being disabled if a process should run wild or addressincorrectly.d. Directory Structure. The system design includes a hierarchical directory struc-ture. Each item in a directory associates an alphanumeric name with a pointer to a file,an input/output function, an entry point giving access to some system service, or adirectory giving further associations of names with pointers. Processes may referenceitems in a subtree of the directory lattice by specifying a downward path from a directorythat is accessible through the capability list of the computation. The root directory of auser of the system is always accessible through the capability list of a computation oper-ating on his behalf. By means of meta-instructions similar to those defined elsewhere, 2processes are able to share files in a rather flexible way.J. B. Dennis, N. Kerllenevich, L. J. RotenbergReferences1. J. B. Dennis, "A Multiuser Computation Facility for Education and Research,"Communs. ACM, Vol. 7, No. 9 (September 1964)2. J. B. Dennis and E. C. Van Horn, "Programming Semantics for Multi-ProgrammedComputations," Technical Report MAC-TR-23, Project MAC, Massachusetts Insti-tute of Technology, December 1965 (to be published in Communs. ACM, March 1966),QPR No. 80 257

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