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The very first operating system used on the earliest IBM PCs was called simply the Disk Operating System, abbreviated DOS. There are few PC users who have not heard of DOS; at one time it was used on pretty much every PC, and it is still around today in many different forms. Microsoft's version of DOS is the most common one, and is called MS-DOS--with the "MS" abbreviation being rather self-explanatory. :^) For a while IBM Corporation was producing a competing product called PC-DOS, and there are other companies' alternatives around as well.

As the name "DOS" implies, the use of disks is an inherent part of the DOS operating system. The FAT file system has been an essential component of DOS since the beginning, and generally speaking, DOS uses only various versions of FAT for managing files. Different flavors of FAT (sorry :^) ) are supported by different versions of DOS, as follows:

• DOS 1.x and 2.x: These ancient DOS versions support only the FAT12 file system, used today primarily for floppy disks. If you're still using DOS 1.x or 2.x, man do you need a new computer! :^)
• DOS 3.x through 6.x: These are more common versions of DOS for older PCs running either straight DOS or Windows 3.x. DOS 6.x was especially popular; millions of copies of these operating systems were sold. DOS 3.x through 6.x support the older FAT12 and the newer FAT16, which was the file system standard for many years in the PC world.
• DOS 7.0: MS-DOS 6.22 was the last "standalone" version of DOS sold by Microsoft. After DOS6.22, Microsoft sold DOS only as the underpinnings of other operating systems, such as Windows 95/98/ME. The first of these was MS-DOS 7.0, which runs "underneath" the first Windows 95 version (Windows 95A). It supports FAT12, FAT16 and VFAT, the enhanced version of FAT that includes support for long file names.
• DOS 7.1 and later: These versions of DOS underlie Windows versions from Windows 95 OEM Service Release 2 (Windows 95B) and later. They support FAT12, FAT16, VFAT and FAT32.

The most confusion is caused by the "appearance" of FAT32 support in the second release of Windows 95--corresponding to DOS 7.1--which was complicated by the fact that later versions of Windows that support FAT32 were not "officially" sold to the public. See the discussions of Windows 95A and Windows 95B and 95C for more information.

Tip: You can check the version of DOS your system is running by using the "VER" command from any DOS command line. Note that this may not work for DOS running under a Windows version, as Microsoft desperately wants people to think that Windows 95/98/ME do not run on the ancient DOS platform. ;^)

Next: Windows 3.x

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