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 …showed first 75 words of 566 total…

…showed last 75 words of 566 total…protected mode routines for accessing the disk, instead of using the standard 16-bit DOS routines. In fact, this is really the first, partial implementation of the VFAT file system used by Windows 95, although not all of the VFAT features are included--only the use of 32-bit access routines. The only thing different here is how the disk is accessed; the file system structures are still "plain" FAT, so special features like long filenames are not included.