Altair BASIC was the BASIC interpreter that Bill Gates and Paul Allen wrote in 1975 for the MITS Altair 8800, and it was the first product of the partnership they called Micro-Soft. The Computer History Museum’s catalog record for the surviving paper tape (X507.84) dates the working software to March 2, 1975 and states plainly that writing it “was the beginning of Microsoft.” The original paper tape, later donated by Gates, is inscribed “Bill Gates Paul Allen MITS Altair 2 Mar 75.”
The technical feat was that the interpreter was written for a machine its authors had never used. Gates and Allen developed and debugged the code on a DEC PDP-10 time-sharing system, running a simulator of the Altair’s Intel microprocessor that Allen had written, then carried the result to MITS in Albuquerque on paper tape for its first run on real hardware. That it worked at all on the first serious try is one of the celebrated moments of the personal-computer story.
The 1975 MITS Altair BASIC Reference Manual is the primary record of the product. It is published by “MITS, Inc.” of Albuquerque, New Mexico, names “Bill Gates, Paul Allen and Monte Davidoff” as the “joint authors of the ALTAIR BASIC Interpreter,” and presents BASIC as “a programming language both easily understood and simple to use.” For the price of the software, an Altair owner gained a way to actually program the machine in an interactive, approachable language rather than toggling in raw machine code at the front panel.
The significance of Altair BASIC reaches well past one computer. It proved there was a market for software sold separately from hardware, it established the BASIC-in-microcomputers pattern that would spread across the industry, and it founded the company that came to dominate personal-computer software. It also set off an early and bitter fight over copying, when Gates responded to widespread unauthorized duplication of the interpreter with his 1976 Open Letter to Hobbyists.