How to find the OS Version of your computer#

For ATARI 400 and ATARI 800 Computers#

In Atari BASIC type the following command if you have an 400/800
PRINT PEEK(65528) 
and press the RETURN key

ValueOS Version
221400/800 OS Rev. A NTSC
214400/800 OS Rev. A PAL
243400/800 OS Rev. B NTSC
34400/800 OS Rev. B PAL (DOES THIS VERSION ACTUALLY EXIST???)

For ATARI XL and XE Computers#

If you have an XL or XE Computer, type
PRINT PEEK(65527)
and press the RETURN key

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

see also How to find the revision number of Atari Basic