Editing Atari 8-bit computers (section)

===Built-in operating system===
[[File:Atari Computer Memo Pad.png|thumb|When no other program is available, the Atari 400 and 800 boot into Memo Pad: a fullscreen text editor which can't save or load.]]

The Atari 8-bit computers have an operating system built into the [[read-only memory|ROM]]. The Atari 400 and 800 have two versions:
*OS Rev. A – 10 KB ROM (3 chips) early machines
*OS Rev. B – 10 KB ROM (3 chips) most common

The XL/XE all have OS revisions, which created compatibility issues with certain software. Atari responded with the Translator Disk, a floppy disk which loads the older 400 and 800 Rev. 'B' or Rev. 'A' OS into the XL/XE computers.
*OS Rev. 10 – 16 KB ROM (2 chips) for 1200XL Rev A
*OS Rev. 11 – 16 KB ROM (2 chips) for 1200XL Rev B (bug fixes)
*OS Rev. 1 – 16 KB ROM for 600XL
*OS Rev. 2 – 16 KB ROM for 800XL
*OS Rev. 3 – 16 KB ROM for 800XE/130XE
*OS Rev. 4 – 32 KB ROM (16 KB OS + 8 KB BASIC + 8 KB Missile Command) for XEGS

The XL/XE models that followed the 1200XL also have the [[Atari BASIC]] ROM built-in, which can be disabled at startup by holding down the silver <code>OPTION</code> key. Originally this was revision B, which has some serious bugs. Later models have revision C.