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DOS was one of the three operating systems that could be run on the original IBM PCs (the other two being CP/M86 and USCD-P). Where DOS differed from the other two is that the PCs were supplied with DOS preinstalled. IBM had originally wanted to put CP/M on the PC as it was the most popular microcomputer operating system at the time but Digital Research refused to sign certain agreements prior to discussing the deal which therefore never took place. IBM then approached Microsoft who were supplying the programming language (BASIC) and asked if they could supply an operating system as well. Microsoft purchased a product called QDOS for $US50,000 and it became the basis for the operating system that has been the most popular operating system on PCs ever since they were introduced (Microsoft have sold DOS 7 as Windows 95, 98, SE, and ME depending on which version of their GUI interface it is sold with).
Ever since Windows 95 Microsoft has been attempting to shift people from their DOS based operating system onto Windows NT. Each upgrade since then has been announced to be the last. With Windows 2000 Microsoft renamed NT to use the same naming conventions perhaps in the hope that people will assume that it is the upgrade for DOS. With the introduction of Windows XP home edition, Microsoft say that they have finally abandoned the DOS operating system all together.
DOS itself (the kernel) consists of only three files. These are: COMMAND.COM, IO.SYS, and MSDOS.SYS and you can run the DOS operating system with just those three files. The rest of what you get with the operating system are utility programs that perform additional useful tasks and, since Windows 95, an API library that application programs can use to perform common tasks (such as opening and saving files) and which provides a common look and feel to all of the programs that use the API.