Jelena Mamčenko Operating Systems Lecture Notes on Operating Systems 67Labs used this initial "text processing system", made up of Unix, roff, and the editor, for text processing of patent applications. Roff soon evolved into troff, the first electronic publishing program with a full typesetting capability. The UNIX Programmer's Manualwas published on November 3, 1971. In 1973, Unix was rewritten in the C programming language, contrary to the general notion at the time "that something as complex as an operating system, which must deal with time-critical events, had to be written exclusively in assembly language" . The migration from assembly language to the higher-level language C resulted in much more portable software, requiring only a relatively small amount of machine-dependent code to be replaced when porting Unix to other computing platforms. AT&T made Unix available to universities and commercial firms, as well as the United States government under licenses. The licenses included all source code including the machine-dependent parts of the kernel, which were written in PDP-11 assembly code. Copies of the
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