DOS stands for Disk Operating System that is mostly for historical interest only. It was originally developed for the IBM PC. Eventually DOS ran on a wide range of IBM compatible clones and some other operating systems provided a DOS environment so that they could run DOS compatible software. Originally an extension to DOS, Microsoft Windows has turned the tables on DOS. Now windows is the basic layer and DOS is emulated within it. Occasionally it can be useful to access the computer through its DOS interface, particularly when investigating a software fault.