UNIX was initially developed at Bell Labs and became operational on a PDP-7 in 1970. MIT’s Project MAC is project led to the development of first CTSS and then Multics. Nevertheless, UNIX incorporated many ideas from Multics.
The first notable milestone was porting the UNIX system from the PDP-7 to the PDP-11. This was the first hint that UNIX would be an OS for all computers. The next important milestone was the rewriting of UNIX in the programming language C. It was generally felt that something as complex as an OS, which must deal with time-critical events, had to be written exclusively in assembly language. Reasons for this attitude include the following: