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To Invent the Future, You Must Understand the Past
"[...] The first significant wave of venture capital firms hit Silicon Valley in the 1970s. Both Sequoia Capital and Kleiner Perkins Caufield and Byers were founded by Fairchild alumni in 1972. Between them, these venture firms would go on to fund Amazon, Apple, Cisco, Dropbox, Electronic Arts, Facebook, Genentech, Google, Instagram, Intuit, and LinkedIn — and that is just the first half of the alphabet."
In 1972, 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" (although Unix was not the first OS, written in high-level language, it was Burroughs B5000 from 1961). C language was created by Ritchie as an improved version of B language, created by Thompson as a translation of BCPL from Martin Richards. 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 annotated Unix kernel sources circulated widely in the late 1970s in the form of a much-copied book, which led to considerable use of Unix as an educational example. At some point, ARPA (Advanced Research Projects Agency) adopted Unix as a standard language for the Arpanet (the predecessor of Internet) community.
During the late 1970s and early 1980s, the influence of Unix in academic circles led to large-scale adoption of Unix (particularly of the BSD version, originating from the University of California, Berkeley) by many commercial startups, for example Solaris, HP-UX and AIX. Today, in addition to certified Unix systems such as those already mentioned, Unix-like operating systems such as Linux and BSD descendants (FreeBSD, NetBSD, and OpenBSD) are commonly encountered.