Microsoft BASIC began as Altair BASIC, the interpreter Bill Gates and Paul Allen wrote in 1975 for the MITS Altair 8800. The 1975 MITS Altair BASIC Reference Manual names “Bill Gates, Paul Allen and Monte Davidoff” as the “joint authors of the ALTAIR BASIC Interpreter,” and the Computer History Museum’s catalog record for the surviving paper tape (X507.84) dates the software to March 2, 1975 and notes that writing it “was the beginning of Microsoft.” That first interpreter is documented in this library under the slug altair-basic.
What made Microsoft BASIC a business rather than a single product was portability. The interpreter was structured so that the bulk of the BASIC language could be reused while only a small machine-specific layer was rewritten for each new processor and computer. Through the late 1970s and 1980s, Microsoft licensed BASIC to a long list of manufacturers, and the language was burned into the read-only memory of machine after machine, so that turning on the computer dropped the user straight at a BASIC prompt.
The manual itself shows why the language travelled so well. It describes BASIC as “a programming language both easily understood and simple to use” that serves “as an excellent ‘tool’ for applications in areas such as business, science and education,” and it stresses the interactive style in which commands run immediately rather than waiting on a batch. That combination of small footprint and immediate feedback fit the constrained, single-user microcomputers of the era exactly.
Microsoft BASIC mattered far beyond any one machine. It established Microsoft’s core business model of licensing software to hardware makers, it made BASIC the de facto first language of home computing, and it set the pattern Gates and Allen would repeat on a much larger scale with the operating systems that followed. For a great many programmers, their introduction to writing code was a dialect of Microsoft BASIC waiting at the READY prompt.