constrained to be a member in a family of compatible computers, the series evolved into five basic designs (PDP-6, KA-10, KI-10, KL10, and KL20) with over 700 systems installed. The notions and need for com- patibility were neither understood at the initial design G. Bell, S. H. Fuller, and Computer time nor did we have adequate technology to undertake D Siewiorek, Editors Systems . such a task. Each successive implementation in the series has generally offered increased performance for The Evolution of the only slightly increased cost. Currently, the KLlO and KL20 systems span a 5 to 1 price range. DECsystem 10 TOPS-10, the major user software interface, devel- oped from a 6 Kword monitor for the PDP-6. A C. G. Bell, second user interface, TOPS-20, introduced in 1976 Digital Equipment Corporation, Maynard, with upgraded facilities is based on multiprocess oper- Mass. and Carnegie-Mellon University, ating systems advances. Pittsburgh, Pa. The paper is divided into seven sections. Section 2 provides a brief historical setting followed by a discus- A. Kotok, T. N. Hastings, and R. Hill sion of the initial project's goals, constraints, and basic Digital Equipment Corporation, Maynard, design decisions. The instruction set and system orga- Mass. nization are given in Sections 4 and 5 respectively. Section 6 discusses the operating system while Section The DECsystem 10, also known as the PDP-10, 7 presents the technological influences on the designs. evolved from the PDP-6 (circa 1963) over five Sections 4 to 7 begin with a presentation of the goals generations of implementations to presently include and constraints, proceed to the basic PDP-6 design, systems covering a price range of five to one. The and conclude with the evolution (and current state). origin and evolution of the hardware, operating We try to answer the often asked questions "Why did system, and languages are described in terms of you do . .?", by giving the contextual environment. technological change, user requirements, and user Figure 1 helps summarize this context in the form of a developments. The PDP-lo's contributions to time line that depicts the various hardwarelsoftware computing technology include: accelerating the technologies (above line) and when they were applied transition from batch oriented to time sharing (below line) to the DECsystem 10. computing systems; transferring hardware technology within DEC (and elsewhere) to minicomputer design and manufacturing; supporting minicomputer 2. Historical Setting hardware and software development; and serving as a model for single user and timeshared interactive The PDP-6 was designed for both a timeshared minicomputer/microcomputer systems. computational environment and real-time laboratory Key Words and Phrases: computer structures, use with straightforward interfacing capability. At the architecture, operating system, timesharing initiation of the project, three timeshared computers CR Categories: 4.32, 6.21, 6.3 were operational: A PDP-1 at Bolt, Beranek and Newman (BBN) which used a high-speed drum that could swap a 4 Kword core image in one 34-millisecond 1. Introduction revolution; an IBM 7090 system at MIT, called CTSS, which provided each of 32 users a 32 Kword environ- The project originating the PDP-6, DECsystem 10, ment; and an AN/FSQ-32V at SDC which could serve and DECsystem 20 series of scientific, timeshared 40 simultaneous users. computers began in the spring of 1963, and continued The Bell Laboratory's IBM 7094 Operating System with the delivery of a PDP-6 in the summer of 1964. was a model operating system for batch users. Bur- Initially, the PDP-6 was designed to extend DEC's roughs had implemented a multiprogrammed system line of 18-bit computers by providing more perform- on the B5000. Dartmouth was considering the design ance at increased price. Although the PDP-6 was not of a single language, timesharing system which subse- quently became Basic. The MIT Multics System, the Copyright 0 1977, Association for Computing Machinery, Inc. Berkeley SDS 940, the Stanford PDP-1 based time- General permission to republish, but not for profit, all or part of this material is granted provided that ACM's copyright notice is shared system for computer aided instruction, and the given and that reference is made to the publication, to its date of BBN Tenex System all contributed concepts to the issue, and to the fact that reprinting privileges were granted by DECsystem 10 evolution in the 1960's. permission of the Association for Computing Machinery. Author's address: Digital Equipment Corporation, 146 Main In architecture, the Manchester Atlas [3, ch. 231 Street, Maynard, MA 01754. was exemplary, not because it was a large machine Communications January 1978 of Volume 21 the ACM Number 1 Fig. 1. Time line diagram of DECsystem 10 evolution. 158 160 162 164 166 168 170 172 174 176 I I , 2nd Generation 3rd ene era ti on 4th 2 Proc. ..- ." inRn.--- A Earlyllate - I 3rd generation - 2 Proc. KllO 1060 - DECsystem 10 2040 family tree - 2 Proc. Earlyllate DECsystem-10 KAlO 2nd generation PDP-6 IAK I Semiconductors 1 Wirewrao (7090) 1 First IC's 1 TTL 1TTLIH. ECL 10K 1 TTLISchottky 1 ECL 100K tech./ I I Silicon translstors machines Germanium transistors J Semiconductors Macro (Bell Labs) COBOL 60 FORTRAN standards PASCAL (0" 704) LISP ALGOL 60 1 APL paper JOSS (RAND) SIMULA Johniac 4 p:F.6 @L/360L 4 Languages & Utilities Macro, FORTRAN II, + t ALGOL Editor, FORT. IV piP.periphera1 t~~~~(MIT)~~:.~:~$IC COBOL MU) Interchange Program SOS Editor (StaAford) (APL/SS-'DEC/APL) Strachey SDC, Q32V Begin multiprog. MITCTSS BEN BASICI erkeley 940 IBM 360167 TENEX TENEX paper + paper paper tsystem TSS Multics Oper. + paper ~- Operating Systems t2 t f Core resident t Swapping 2 Processor Virtual Memory TOPS-20 PDP-6 (begin TOPS-10) Gseries) Introduction op. sys. (TENEX base) that we would build, but because it illustrated a number Table I. DEC's 1964 computer products. of good design principles. Atlas was multiprogrammed Word with a well-defined interface between the user and Year In- Size Price operating system, had a very large address space, and Name troduced (Bits) 6K) Status introduced the notion of extra codes to extend the PDP- 1 1960 18 Marketed functionality of its instruction-set. Paging was a concept PDP-2 1960 24 Resewed for future implementation we just could not afford to implement without a fast, 36 Paper machine small memory. The IBM Channel concept was in use, 18 Marketed on their 7094, and was one we wanted to avoid since 12 Introduced Introduced our minicomputers (e.g. PDP-1) were generally smaller 36 than a single channel and could outperform the 7094 in terms of i/o concurrency and i/o programmability by didn't start until 1965. Generally there were no people a clean, simple interrupt mechanism. who directly represented the users within the company, The DEC product line in 1964 is summarized in although all the designers were computer users. A Table I. Corporation wide sales totaled $1 1 million at number of users in the Cambridge (Mass.) community that time and it was felt that computers had to be advised on the design (especially John McCarthy, offered in the $20,000 to $300,000 range. We were Marvin Minsky, and Peter Sampson at the MIT Artifi- sensitive to the problems encountered by not having cial Intelligence Laboratory). enough address bits, having watched DEC and IBM Although there was little consensus that Fortran machines exceed their addressing capacities. would be so important, it was clear our machine would On the software side, most programmers at DEC be used extensively to execute Fortran. The macroas- had been large machine (16 to 32 Kwords) users semblers, basically unchanged even today, were used although they had most recently programmed mini- in various laboratories and our first one for PDP-1 was computers where program size of 4 to 8 Kwords was done by MIT in 1961. We also felt the list languages, the main constraint. There was not a good understand- especially LISP for symbolic processing were important. ing of operating systems structure and design in either There was virtually no interest in business data process- academia or industry. For example, MIT's Multics ing although we had all looked at Cobol. Project was being formed and IBM's 360lTSS Project We did not understand the concept of technology 45 Communications January 1978 of Volume 21 the ACM Number 1 evolution very well, even though integrated circuits Data Types and Operators were both forecast and under development. Germa- Earlier DEC designs and the then current 6-bit nium transistors were available, and silicon transistors character standard forced a word length which was a were just on the market. IBM was using machine multiple of 6, 12, and 18 bits. Thus a 36-bit word was wirewrap technology, while DEC back panels were selected. handwired and soldered. The basic DEC logic circuits The language goals and constraints forced the inclu- were saturating transistor as distinct from the more sion of integer and real (floating point) variables. We expensive current mode used by IBM in the 7094 and chose two's complement integer representation rather Stretch computers. Production core memories of 2 than the sign-magnitude representation used on the microseconds were beginning to appear, and their 7090, or the one's complement representation on PDP- speed was improving. Our PDP-1 used a 5 microsecond 1. The floating point format was chosen to be the core. Hence, it was unclear what memory speed a same as the 7090, but with a format that permitted processor should support. comparison to be made on the number as an integer in The notions of compatibility and family range were order to speed up comparisons and only require a not appreciated even though SDS (eventually XDS single set of compare instructions.