THE OPERATING SYSTEM MACHINE LEVEL - Structured Computer Organization - page 482

elegant, we will begin with it. Also, UNIX was designed and implemented first and
had a major influence on Windows 7, so this order makes more sense than the re-
verse.
6.5.1 Introduction
In this section we will give a brief introduction to our two example operating
systems, UNIX and Windows 7, focusing on the history, structure, and system calls.
UNIX
UNIX was developed at Bell Labs in the early 1970s. The first version was
written by Ken Thompson in assembler for the PDP-7 minicomputer. This was
soon followed by a version for the PDP-11, written in a new language called C that
was devised and implemented by Dennis Ritchie. In 1974, Ritchie and his col-
league Ken Thompson published a landmark paper about UNIX (Ritchie and
Thompson, 1974). For the work described in this paper they were later given the
prestigious ACM Turing Award (Ritchie, 1984, Thompson, 1984). The publication
of this paper stimulated many universities to ask Bell Labs for a copy of UNIX .
Since Bell Labs' parent company, AT&T, was a regulated monopoly at the time
and was not permitted to be in the computer business, it had no objection to licens-
ing UNIX to universities for a modest fee.
In one of those coincidences that often shape history, the PDP-11 was the com-
puter of choice at nearly all university computer science departments, and the oper-
ating systems that came with the PDP-11 were widely regarded as being dreadful
by professors and students alike. UNIX quickly filled the void, not in the least be-
cause it was supplied with the complete source code, so people could, and did, tin-
ker with it endlessly.
One of the many universities that acquired UNIX early on was the University of
California at Berkeley. Because the complete source code was available, Berkeley
was able to modify the system substantially. Foremost among the changes was a
port to the VAX minicomputer and the addition of paged virtual memory, the ex-
tension of file names from 14 characters to 255 characters, and the inclusion of the
TCP/IP networking protocol, which is now used on the Internet (largely due to the
fact that it was in Berkeley UNIX ).
While Berkeley was making all these changes, AT&T itself continued to
develop UNIX , leading to System III in 1982 and then System V in 1984. By the
late 1980s, two different, and quite incompatible, versions of UNIX were in wide-
spread use: Berkeley UNIX and System V. This split in the UNIX world, together
with the fact that there were no standards for binary program formats, greatly
inhibited the commercial success of UNIX because it was impossible for software
vendors to write and package UNIX programs with the expectation that they would
run on any UNIX system (as was routinely done then with MS-DOS ). After much