Altair BASIC source code
 

On April 4, 1975, Bill Gates and Paul Allen founded Microsoft Corporation in Albuquerque, New Mexico. Today, half a century later, Microsoft has grown into a super-tech giant.

Recently, Bill Gates wrote about the birth of Microsoft and their first business, as well as sharing the Altair BASIC（before office,win95,xbox...） source code they wrote for the business in a 157-page PDF.

The blog:
https://www.gatesnotes.com/meet-bill/source-code/reader/microsoft-original-source-code

The souce code:
https://images.gatesnotes.com/12514eb8-7b51-008e-41a9-512542cf683b/34d561c8-cf5c-4e69-af47-3782ea11482e/Original-Microsoft-Source-Code.pdf