microsoft/ms-dos

This repository contains the original source code for MS-DOS versions 1.25, 2.0, and 4.0, the early operating systems that ran on IBM-compatible personal computers starting in the early 1980s. MS-DOS (Microsoft Disk Operating System) was the dominant PC operating system before graphical interfaces like Windows became widespread. It provided the foundational software layer that let programs run on a computer, managing the processor, memory, disk storage, and input/output devices through a text-based command-line interface. The code here is for historical reference only, it is not a living project. Microsoft originally released the v1.25 and v2.0 sources to the Computer History Museum in 2014, and this repository makes those files easier to find and study. The v4.0 source was added later and was jointly developed by IBM and Microsoft. None of these files are intended to be modified or used as the basis for new software; the README explicitly asks people not to submit changes to the source files, though forking and experimenting locally is encouraged. The code is written in Assembly language, which is a very low-level programming language that works almost directly with the processor's instruction set. Assembly programs are highly efficient but extremely difficult to read and write compared to modern languages, understanding this code gives genuine insight into how computers worked at the hardware level in the 1980s. You would visit this repository if you are a computer historian, a student of operating system design, a programmer curious about how early PC software was built, or simply someone with nostalgia for or interest in computing history. All files are released under the MIT License.