h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

The primary reason for an operating system is a lot more basic than avoiding conflicts between applications. It is simly to avoid every program from having to directly control the hardware upon which it runs. It is unconceivable to think running a machine without an operating system, however the first computers didn't have one.

In fact it's even stranger than that. What is an operating system ? It's simply a program that was written to run on hardware without the need for an operating system. It's purpose - to run other programs.

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From Distant Shores

Quite - the fundamental function of an operating system is to provide a higher-level layer of abstraction around the physical hardware, allowing software to be written more easily in the first place, and also allowing software written on one machine to be to be ported with little or no modification to different machines running the same operating system, if such machines exist, (or may exist in the future)

Multitasking operating systems should also provide some protection to ensure one bad program doesn't interfere with the operation of other programs, but that doesn't necessarily mean they actually do so reliably (or at all) in practice.

There can be many levels of abstraction - an emulator which, for instance, might allow Windows programs to run on a Unix machine could be considered as an ordinary program from the perspective of the Unix operating system, or as an operating system from the perspective of the programs it allows to be run.

And as for the ability to run programs on different operating systems, books have been written on the subject of writing 'portable' code in languages like C. In fact, when C first came out the authors hoped that all code written in C would be portable, with a little care when the code was written. The problem comes when you have to deal with the hardware on the system. True, you can write software that just passes the hardware handling to something else, but you have to know how to access the 'something else', which will be different for each operating system. So it all falls apart!

Right, many OS didn't even have to cope with multiple programs concurrently - remember MS DOS. The main task for an OS is to provide an abstract access to the systems ressources like memory and IO devices (disks etc).