Take a look at the MS-DOS source code

Technology News |
By eeNews Europe

Microsoft has donated the code to the Computer History Museum and it can be downloaded here.
The Museum is providing 7 assembler code files for MS-DOS v1.0 and an explanatory email from author Tim Paterson as well as 27 files including some binary programs and some sample programs. For v2.0 there are 118 text files, mostly assembler code and some documentation and 38 object files, some binary and some documentation.
These are available under a non-commercial license that does not give the right to license it to third parties by posting copies elsewhere on the web.
The birth of MS-DOS came from the unusual decision of IBM to use industry standard commercial parts for the IBM PC in 1981, including 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.
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, and the rest is industry history.

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