AVR-Chat

I have looked in the part data sheets and the "AVR Instruction Set"
manual and I can't find this info.

If it isn't documented, you should assume that the behavior is 
undefined.  That implies that the behavior may even change over time 
with die revisions for a given model and may differ between models.

Don Kinzer
ZBasic Microcontrollers
http://www.zbasic.net