Atari BASIC
 An Atari BASIC program ready to run
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| Original author(s) | Paul Laughton Kathleen O'Brien |
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| Developer(s) | Shepardson Microsystems |
| Initial release | 1979 |
| Stable release |
Revision C / 1983
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| Platform | Atari 8-bit family |
| Type | BASIC |
| License | Copyright © 1979 Atari Inc. Proprietary |
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.
The machines that would become the Atari 8-bit family had originally been developed as second-generation video game consoles intended to replace the Atari 2600. Ray Kassar, the new president of Atari, decided to challenge Apple Computer by building a home computer instead. This meant Atari needed the BASIC programming language, then the standard language for home computers.
Atari purchased the source code to the MOS 6502 version of Microsoft 8K BASIC. The original 8K BASIC referred to its memory footprint when compiled on the Intel 8080's instruction set. The lower code density of the 6502 expanded the code to about 9 kB. This was slightly larger than the natural 8 kB size of the Atari's ROM cartridges.
Atari felt that they also needed to expand the language to add better support for the specific hardware features of their computers, similar to what Apple had done with their Applesoft BASIC. This increased the size from 9 kB to around 11 kB. Atari had designed their ROM layout in 8 kB blocks, and paring down the code from 11 to 8 kB turned out to be a significant problem. Adding to the problem was the fact that the 6502 code supplied by Microsoft was undocumented.
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