!!!How to find the OS Version of your computer
!! [Atari ROM Checker by JAC!|http://www.wudsn.com/productions/atari800/atariromchecker/help/AtariROMChecker.html]
!!For ATARI 400 and ATARI 800 Computers
In Atari BASIC type the following command if you have an 400/800
%%prettify 
{{{
PRINT PEEK(65528) 
}}}
/%
and press the RETURN key

||Value||OS Version||CRC 32||MD5||ROMs
|255|First 800 OS NTSC ; please further see [Rarity 10]|?|?|__we are still searching for this. Please help us ;-)__
|221|400/800 OS Rev. A NTSC|c1b3bb02|a3c1585b5d19719f8acfa2b093bea75f|CO12499A, CO14599A, CO12399B
|214|400/800 OS Rev. A PAL|72b3fed4|eb1f32f5d9f382db1bbfb8d7f9cb343a|CO15199, CO15299, CO12399B
|243|400/800 OS Rev. B NTSC|0e86d61d|4177f386a3bac989a981d3fe3388cb6c|CO12499B, CO14599B, 12399B
|34|400/800 OS Rev. B PAL; please further see [Rarity 10]|0c913dfc|89d5e5f4713267667ab713449944f8a9 Kr0tki-Version|__we are still searching for this. Please help us ;-)__

!!For ATARI XL and XE Computers
If you have an XL or XE Computer, type 

%%prettify 
{{{
PRINT PEEK(65527)
}}}
/%
and press the RETURN key

||Value||Version
|10|XL/XE OS Rev 10
|11|XL/XE OS Rev 11
|1|XL/XE OS Rev 1
|2|XL/XE OS Rev 2
|3|XL/XE OS Rev 3
|59|XL/XE OS Rev 3B (Arabic)
|4|XL/XE OS Rev 4

see also [How to find the revision number of Atari Basic]