 'A' OS into the XL/XE computers.The XL/XE models that followed the 1200XL also came with the Atari BASIC ROM built in, which could be disabled at startup by holding down the silver OPTION key to the right of the keyboard; the earlier-manufactured 1200XL required an Atari BASIC cartridge for that functionality.Early models with built-in BASIC came with the notoriously buggy revision B.