wed all of the users to have the illusion of having
exclusive access to the machine; the Multics timesharing system was the most famous of a number
of new operating systems developed to take advantage of the concept.
Multics, particularly, was an inspiration to a number of operating systems developed in the
1970s, notably Unix. Another commercially-popular minicomputer operating system was VMS.
The first microcomputers did not have the capacity or need for the elaborate operating systems that
had been developed for mainframes and minis; minimalistic operating systems were developed.
One notable early operating system was CP/M, which was supported on many early
microcomputers and was largely cloned in creating MS-DOS, which became wildly popular as the
operating system chosen for the IBM PC (IBM's version of it was called IBM-DOS or PC-DOS), its
successors making Microsoft one of the world's most profitable companies. The major alternative
throughout the 1980s in the microcomputer market was Mac OS, tied intimately to the Apple
By the 1990s, the microcomputer had evolved to the point where, as well as extensive GUI
facilities, the robustness and flexibility of operating systems of larger computers became
increasingly desirable. Microsoft's response to this change was the development of Windows NT,
which served as the basis for Microsoft's entire operating system line starting in 1999. Apple rebuilt
their operating system on top of a Unix core as Mac OS X, released in 2001. Hobbyist-developed
reimplementations of Unix, assembled with the tools from the GNU project, also became popular;
versions based on the Linux kernel are by far the most popular, with the BSD derived UNIXes
holding a small portion of the server market.
The growing complexity of embedded devices has a growing trend to use embedded
operating systems on them.
Lecture Notes on Operating Systems
Command line interface (or CLI) operating systems can operate using only the keyboard for
input. Modern OS's use a mouse for input with a graphical user interface (GUI) sometimes
implemented as a shell. The appropriate OS may depend on the hardware architecture, specifically
the CPU, with only Linux and BSD running on almost any CPU. Windows NT has been ported to
other CPUs, most notably the Alpha, but not many. Since the early 1990s the choice for personal
computers has been largely limited to the Microsoft Windows family and the Unix-like family, of
which Linux and Mac OS X are becoming the major choices. Mainframe computers and embedded
systems use a variety of different operating systems, many with no direct connection to Windows or
Unix, but typically more similar to Unix than Windows.
• Personal computers
o IBM PC compatible - Microsoft Windows and smaller Unix-variants (like Linux and
o Apple Macintosh - Mac OS X, Windows, Linux and BSD
• Mainframes - A number of unique OS's, sometimes Linux and other Unix variants.
• Embedded systems - a variety of dedicated OS's, and limited versions of Linux or other OS's
The Unix-like family is a diverse group of operating systems, with several major subcategories including System V, BSD, and Linux. The name "Unix" is a trademark of The Open
Group which licenses it for use to any operating system that has been shown to conform to the
definitions that they have cooperatively developed. The name is commonly used to refer to the large
set of operating systems which resemble the original Unix.
Unix systems run on a wide variety of machine architectures. They are used heavily as
server systems in business, as well as workstations in academic and engineering environments. Free
software Unix variants, such as Linux and BSD, are increasingly popular. They are used in the
desktop market as well, for example Ubuntu, but mostly by hobbyists.
Some Unix variants like HP's HP-UX and IBM's AIX are designed to run only on that
vendor's proprietary hardware. Others, such as Solaris, can run on both proprietary hardware and on
commodity x86 PCs. Apple's Mac OS X, a microkernel BSD variant derived from NeXTSTEP,
Mach, and FreeBSD, has replaced Apple's earlier (non-Unix) Mac OS. Over the past several years,
free Unix systems have supplanted proprietary ones in most instances. For instance, scientific
modeling and computer animation were once the province of SGI's IRIX. Today, they are
dominated by Linux-based or Plan 9 clusters.
The team at Bell Labs who designed and developed Unix went on to develop Plan 9 and
Inferno, which were designed for modern distributed environment