DOS stands for “disk operating system,” a program that controls the computer’s transfer of data to and from a hard or floppy disk. Frequently, it is combined with the main operating system. The operating system was originally developed at Seattle Computer Products as SCP-DOS. When IBM decided to build a personal computer and needed an operating system, it chose the SCP-DOS after reaching an agreement with the Microsoft Corporation to produce the actual operating system. Under Microsoft, SCP-DOS became MS-DOS, which IBM referred to as PC-DOS (personal computer), which everyone eventually simply called DOS.