The original IBM PC.
Rather than using IBM proprietary components developed for their many other computers, the IBM PC used industry standard commercial parts. That included adopting the Intel 8088 microprocessor as the heart of the computer.
This “outsourcing” attitude extended to the software as well. Although IBM had prodigious internal software development resources, for the new PC they supported only operating systems that they did not themselves write, like CP/M-86 from Digital Research in Pacific Grove CA, and the Pascal-based P-System from the University of California in San Diego. But their favored OS was the newly-written PC DOS, commissioned by IBM from the five-year-old Seattle-based software company Microsoft.
When Microsoft signed the contract with IBM in November 1980, they had no such operating system. They too outsourced it, by first licensing then purchasing an operating system from Seattle Computer Products variously called QDOS (Quick and Dirty Operating System”) and 86-DOS.
A set of floppy disks for MS-DOS 2.0.
PC DOS version 1.0, which supported only floppy disks, was shipped when IBM first released their PC in August 1981. Microsoft then substantially rewrote the software to support subdirectories and hard disks; version 2.0 was released with the IBM PC-XT in March of 1983.
Microsoft retained the rights to the operating system and licensed it to other computer manufacturers, calling it MS-DOS. With the permission of Microsoft Corporation, the Computer History Museum is pleased to make available the source and object code to Microsoft’s MS-DOS operating system versions 1.1 and 2.0, for non-commercial use.
The zip file contains four subdirectories:
To access this material, you must agree to the terms of the license displayed here, which permits only non-commercial use and does not give you the right to license it to third parties by posting copies elsewhere on the web.
Download Microsoft DOS V1.1 and V2.0 Source Code
September 2018 update: Microsoft has just released the same code on GitHub. The announcement is here and the repository is here. Note the request to kindly not submit Pull Requests. It will not, of course, be an actively maintained project!