How Bill Gates’ 4K Altair BASIC Shaped the Birth of Modern Programming

Introduction: Microsoft co‑founder shares the 4K Altair BASIC source code ahead of the company’s 50th anniversary.

Microsoft co‑founder Bill Gates recently shared the source code of Altair BASIC from 1975.

The source code is the foundation of Microsoft.

Before Windows and Office existed, Microsoft carefully designed a BASIC interpreter to run on the limited resources of the popular Altair 8800 computer.

Altair 8800 computer

Why use an interpreter? Gates explained that line‑by‑line interpretation helps novice programmers by providing immediate feedback so they can fix errors on the fly.

Gates and fellow co‑founder Paul Allen discovered the Altair on the cover of the January 1975 issue of Popular Electronics and believed a PC revolution was imminent, prompting them to seize the market opportunity.

Gates and Paul Allen

Gates and Allen told MITS founder Ed Roberts that the BASIC software was ready; they spent two months developing it, compressing the code to 4 KB so Altair users could run programs without extra memory.

Gates added, “The cost of an extra memory board for the Altair could easily exceed the price of the computer itself.”

They wrote the program without an Intel 8080 machine; the Altair also ran on a cheap chip. Paul wrote a simulator on a PDP‑10 to test the software, and Gates’ friend Monte Davidoff created a math package.

By today’s standards, this 4K BASIC version had many limitations, such as minimal string handling, which were later expanded in the 8K BASIC release.

Altair BASIC interface

Later, MITS obtained a software license from Gates’ new company.

Gates said this was a turning point; Altair BASIC became their first product and led them to name the company Microsoft (Micro‑Soft).

The source code is provided as a 157‑page scanned PDF rather than a convenient repository file.

Developers can find the commented disassembly of Altair BASIC 3.2 on GitHub.

https://github.com/option8/Altair-BASIC

“Even after all these years, seeing it still excites me,” Gates said. “Programming has made huge progress, but I’m still very proud of its development.”