DOS (Disk Operating System) is an operating system that runs from a hard disk drive. The term can also refer to a particular family of disk operating systems, most commonly MS-DOS (Microsoft Disk Operating System).
An operating system (OS) is the software that controls a computer's hardware and peripheral devices and allows other programs to function. Early computers did not have disk drives but were hard-wired to carry out specific computations. Later, computers were able to store instructions loaded into the computer's memory using punch cards and later magnetic tapes. Computer memory space was limited and when the instructions to control a computer were moved onto a disk drive, such as a floppy disk or internal hard drive, it was considered cutting-edge technology. Today, any modern operating system would be considered a disk operating system.
Disk operating system is also used to describe several very similar command line disk operating systems. PC-DOS (Personal Computer Disk Operating System) was the first widely-installed disk operating system used in personal computers running on Intel 8086 16-bit processors. It was developed for IBM by Microsoft Corporation, which also produced its own almost identical version called MS-DOS. Other computers at the time, such as the Commodore 64, Atari 800, and Apple II, all featured a disk operating system, CBM DOS, Atari DOS, and Apple DOS, respectively. (DOS/360 was an operating system for IBM mainframes which first appeared in 1966, but is unrelated to the 8086-based DOS of the 1980s.
These early operating systems did not multitask, as they were only able to run one program at a time. The command line interface, in which a user has to type in commands, required the user to remember commands to run programs or do other operating system tasks, making it difficult for novices to use. For example, typing the command "cd \directory_name" changed the current working directory to the named directory and typing the command "dir" listed the files in the current directory.
When Microsoft first introduced Windows as a graphical user interface (GUI) for MS-DOS, early users had to type "WIN" at the DOS prompt to launch the Windows program. Windows has since evolved from being a GUI program running under DOS to a full operating system taking over as the default OS, though it was not until Windows XP that consumer versions of Windows stopped relying on the DOS program win.com to bootstrap the Windows kernel.
The last retail version of MS-DOS was MS-DOS 6.22. After this release, MS-DOS was still bundled as part of Windows, but no longer required a separate license. It can still be run under Windows using the Command Prompt program. There is an open source version of DOS called FreeDOS which is based on and compatible with MS-DOS.
Note: The acronym DoS with a lowercase O is short for Denial of Service, a method of attacking a networked computer by sending it an abnormally high number of requests in order to exhaust its resources so that genuine users cannot gain access.