Thank you, Justin, for your lessons. Clean and simple, I like it very much.
Please don't take me wrong, but this Iron Man riff is unfortunately not 100% correct. I apologies for posting google-translated version of translated article, but I failed find the original source. In interview to GuitarWorld Tony Iommi said that he plays this riff on "lower E and A strings". Links contains explanations ("when the riff is played on the lower E and A strings, instead of the A and D, it sounds much darker and gloomier"), tabs and sample audio fragments.
Google-powered translation - _ttp://translate.google.com/translate?hl=en&sl=ru&tl=en&prev=_dd&u=http%3A%2F%2Fwww.guitars.ru%2F02%2Finfo.php%3Fz995
"Original" article (Russian) - _ttp://www.guitars.ru/02/info.php?z995
I apologies once again for not being allowed to post external links...