The UNIX Time-Sharing System

T h e r e h a v e b e e n f o u r versions of t h e U N I X t i m e - s h a r i n g system. T h e earliest (circa 1969-70) ran on the Digital E q u i p m e n t C o r p o r a tion PDP-7 and -9 c o m p u t e r s . T h e second version ran on the u n p r o t Copyright 1974, Association for Computing Machinery, Inc., reprinted by permission. This is a revised version of an article that appeared in Communications of the ACM, 7 7, No. 7 (July 1974), pp. 365-375. That article was a revised version of a paper presented at the Fourth ACM Symposium on Operating Systems Principles, IBM Thomas J. Watson Research Center, Yorktown Heights, New York, October 15-17, 1973. * UNIX is a trademark of Bell Laboratories. 1905 tected PDP-11/20 c o m p u t e r . T h e third i n c o r p o r a t e d m u l t i p r o g r a m m i n g a n d ran o n t h e PDP-11/34, / 4 0 , / 4 5 , / 6 0 , a n d / 7 0 c o m p u t e r s ; it is t h e o n e described in t h e previously p u b l i s h e d version of this paper, a n d is also t h e m o s t widely u s e d today. T h i s paper describes only t h e f o u r t h , c u r r e n t s y s t e m that r u n s o n t h e PDP-11/70 a n d t h e I n t e r d a t a 8 / 3 2 c o m p u t e r s . I n fact, t h e d i f f e r e n c e s a m o n g t h e vario u s s y s t e m s is r a t h e r small; m o s t of t h e revisions m a d e to t h e originally p u b l i s h e d version of this paper, aside f r o m t h o s e c o n c e r n e d with style, h a d to do with details of t h e i m p l e m e n t a t i o n of t h e file system.