Just a FYI, the author of this tutorial got ousted out of AVRFreaks for bad behavior and plagiarism (especially tutorial plagiarism!). [0] Another tutorial of his had several errors. [1] I would personally recommend either [2] or [3] as a beginner's tutorial for AVR assembly and [4] for all your other questions.
[0] http://www.avrfreaks.net/index.php?name=PNphpBB2&file=vi...
[1] http://www.reddit.com/r/programming/comments/aqkt2/dirty_mat...
[2] http://www.avr-asm-tutorial.net/avr_en/beginner/index.html
[3] http://www.avrbeginners.net/
[4] http://www.avrfreaks.net/index.php?name=PNphpBB2&file=in...