Atari BASIC

Atari BASIC is a BASIC interpreter that shipped with the Atari 8-bit family of 6502-based home computers. The language was originally on an 8 KB ROM cartridge. On the XL/XE computers it is built-in and can be disabled by holding down the OPTION key while booting. The XEGS disables BASIC if powered without the keyboard attached. The complete commented source code and design specifications of Atari BASIC were published as a book in 1983.