An operating system is the piece of software which gets executed immediately after the BootStrap part of a PersonalComputer's (not sure about mainframes, but probably the case for them as well) startup sequence.

The most common variants of OperatingSystems? in general use are:

• One of the many Microsoft Windows version (3.1, 3.11, 95, 98, 98SE, NT3.51, NT4, 2000 (aka NT5), Millenium (aka ME), and XP)
• The antiquated MS-DOS or it's many clones and predecessors such as QDOS1?, and PC-DOS
• One of the many Linux variants such as RedHat, DebianLinux, Mandrake, OpenCaldera?, etc
• One of the many BSD versions, amongst which are FreeBSD and NetBSD
• A version of BeOS
• An original Unix, such as HP-UX, AIX, SCO-Unix, etc
• A Sun? varient SunOS or Solaris
• SiliconGraphics? Irix?
• Or even one of the many MacOS versions such as MacOSX
• There are still a few old VAXs and Alphas out there running VMS

There are many more listen in the CategoryOperatingSystem wiki, and for help with all the acronyms, check there.

Many others are undoubtly available to the persistent user, or someone may have been brave enough to have written their own2?.

From fortune(6)

There was once a programmer who was attached to the court of the

warlord of Wu. The warlord asked the programmer: "Which is easier to design: an accounting package or an operating system?"

"An operating system," replied the programmer. The warlord uttered an exclamation of disbelief. "Surely an

accounting package is trivial next to the complexity of an operating system," he said.

"Not so," said the programmer, "when designing an accounting package,

the programmer operates as a mediator between people having different ideas: how it must operate, how its reports must appear, and how it must conform to the tax laws. By contrast, an operating system is not limited by outside appearances. When designing an operating system, the programmer seeks the simplest harmony between machine and ideas. This is why an operating system is easier to design."

The warlord of Wu nodded and smiled. "That is all good and well, but

which is easier to debug?"

The programmer made no reply.

-- Geoffrey James, "The Tao of Programming"

Footnotes

1? QDOS is usually referenced in one of two ways. The first as "Quick Disk Operating System", the other as the correct "Quick and Dirty Operating System". 2? Writing your own operating system can be a lengthy and complicated process which will involve some form of AssemblyLanguage programming. However it gives a great sense of satisfaction at having completed such a difficult