Here is the User Guide for the assembler, with some examples:
http://www.atmel.com/dyn/resources/prod_documents/doc1022.pdf
You can download the assembler for free at
http://www.avrfreaks.net/index.php?module=Freaks%20Tools&func=viewItem&item _type=tool&item_id=103
Finally, maybe this site can offer you some useful literature:
http://www.avr-asm-tutorial.net/avr_en/index.html

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