Bill Gates helped develop Altair BASIC for the Altair 8800 before Microsoft was founded.
Gates and Paul Allen adapted the BASIC programming language for the Altair 8800 in 1975. They wrote and tested the interpreter on a simulated Altair environment before demonstrating it to Micro Instrumentation and Telemetry Systems, the computer’s manufacturer.
The software became one of the earliest commercially successful products for a personal computer. MITS licensed it, and Gates and Allen formed Microsoft partly to continue developing and selling software for emerging microcomputers.
Altair BASIC is sometimes confused with Microsoft BASIC, a broader family of later dialects. Altair BASIC was the specific version created for the Altair 8800 and is regarded as Microsoft’s first product.