Bill Gates’ early BASIC code for MOS 6502 released as open source

Microsoft has released the source code for the BASIC version it developed in 1976 for the MOS 6502 processor, a central component of many early home computers, The Register reports.

As far back as 1975, Bill Gates and Paul Allen developed Microsoft’s first product: BASIC for the Altair 8800 with the Intel 8080 processor. The following year, Gates and Ric Weiland ported the language to MOS 6502. In 1977, Commodore licensed the code for $25,000 and used it in its PET, VIC-20, and Commodore 64 computers. Millions of copies of these computers were sold, helping to popularize the home computer.

The now-published version 1.1 of the code consists of 6,955 lines of assembler and is available on GitHub under an MIT license. It contains a full BASIC implementation with support for floating-point arithmetic, string and array handling, math functions, input and output, and efficient memory usage for 8-bit systems.

The code also supports several classic computers, including the Apple II, Commodore PET, Ohio Scientific, and MOS KIM-1. “It even contains a playful Bill Gates Easter egg, hidden in the labels STORDO and STORD0,” according to a Microsoft blog post.