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MS-DOS is an acronym that stands for Microsoft Disk Operating System. MS-DOS oversees system functions like disk input and output, monitor support, keyboard control and internal functions related to program execution and file maintenance.

MS-DOS is a single-tasking system, which means that it is capable of performing only one function at a time. For example, if a program is saved using MS-DOS no other function can be performed until the save is completed. MS-DOS is also a single user operating system, which means that the software can only accommodate one person at a time. The entire MS-DOS code is small enough to be contained entirely on a single 3.5 or 5.25 inch diskette.

MS-DOS is a simple operating system that is text oriented and command-based. The MS-DOS command-line interface was designed solely for keyboard input, as opposed to a mouse-based graphical user interface (GUI), like the Macintosh, where the user chooses commands by pointing and clicking on onscreen objects.

What later became known as MS-DOS was first written by Tim Paterson, a 24 year old computer programmer from Seattle, Washington. The company he was working for, Seattle Computer Products needed an operating system for its new 16-bit computer that was based on the Intel 8086 processor. Seattle Computer Products was waiting on the 16-bit version of Digital Research's CP/M-86 which was the most popular 8-bit operating system at the time.

In April 1980, Seattle Computer decided to stop waiting and had Paterson write his own operating system, by July he had named his project QDOS. The name stood for "quick and dirty operating system" because it was written in less than two months. Paterson used Digital Research's 8-bit CP/M operating system as a basis for his design of QDOS.

The initial version of QDOS contained all of the basic utilities for assembly-language development except for an editor. Paterson hastily threw together a primitive editor in a single week. It was named EDLIN (for "editor of lines") and was supposed to be a stopgap measure that would be available for only six months until it could be rewritten. EDLIN suddenly became more permanent once the operating system was sold and acquired the name "MS-DOS."

After August 1980, Seattle Computer Products began shipping QDOS v0.10. Paterson then immediately began working on an improved updated version of the system. In December 1980, Seattle Computer Products released it and with the name 86-DOS v0.30. In April 1981, after further refining, Seattle Computer Products released 86-DOS version 1.00 which had, by then, evolved into what is essentially MS-DOS.

At this time, Microsoft was also looking for an operating system that would run on the Intel 8086 16-bit processor which was to power IBM's first personal computer. Seattle Computer was a hardware company, it did not specialize in software, so the rights to the 86-DOS were sold to Bill Gates and Microsoft outright for $50,000 on July 27, 1981.

In April 1981, Paterson quit Seattle Computer to work at Microsoft full time. Microsoft then undertook Project Chess, a highly secretive attempt to clean up and debug 86-DOS, which consisted of about 4000 lines of assembler source code. After incorporating the numerous changes that IBM insisted upon (like CP/M prompts), the team finished its work on 86-DOS in July. By this point its name had been changed again, this time to MS-DOS, incorporating the Microsoft name.

After debugging, [on August 12, 1981] IBM integrated MS-DOS as its main operating system for its new PC line (personal computer). Paterson later quit Microsoft and returned to Seattle Computer Products after completing the first update, MS-DOS version 1.1.

One of the reasons for the initial success of the DOS (Disk Operating System) was the fact that it carried a retail price of only $39.95. This was significantly cheaper than any other operating system at the time. MS-DOS became the standard operating system on the IBM PC and its long list of clone systems for years to come. More copies of DOS have been sold than any other computer program in history.