OS stands for operating system.

In simple terms this is the main program the computer runs.  As far as any
computer's concerned, when it's doing anything, that's the operating system.
An operating system could be any program.  If you wrote an embedded computer
program to manage a thermostat for your home, that's an operating system.
It's what runs on the CPU.

With a regular OS, time is NOT as important.  It can take three seconds to
return a value from an extremely complicated function.  Five seconds to go
to the USB port and obtain data from an external device.  Real-Time OS's
have a far more constrained view.  Time is of the essence.  A real time OS
is commonly an embedded operating system, such as the above mentioned
thermostat.  When you push the button on the box, you want to see the
current temperature in the room NOW.  Not in 3 seconds.  Not after the
"loading" icon has flashed a few times.  Now.  Everything is based on time,
because when the program that determines that the radiation level in the
research facility's nuclear reactor has gone above the value expected, it
must respond immediately.  When your tires lock up in your Anti-Lock Brake
system, your computer in your car (yes, there is a car in the computer)
needs to react immediately.

For more information on real time operating systems, I recommend:
And other articles on the linuxdevices website.  Have fun!

----- Original Message -----
From: "Prasad Mehendale" <[EMAIL PROTECTED]>
Sent: Tuesday, October 26, 2004 10:17
Subject: [Freedos-user] What is OS ?

> Hallo all !
> I hope I am not asking an Off Topic question.
> I am a designer of microcontroller based systems, and prefer to write in
assembly for speed and small size.
> I never learned anything about an OS systematically though I used some
(dos & linux).
> My question is-
> Is it possible to describe an OS in short ? If yes, can any one do this
for me? You also may provide some link on net.
> I am interested in knowing the difference between the RTOS and the one we
frequently use.
> I have fair understanding of internal hardware and architecture of
micro-processor and controller. Is it sufficient to understand what an OS is
? One may describe all this with reference to Freedos.
> Thanks again.
> Prasad Mehendale
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