Joined: 23 Nov 2006 Posts: 19270 Location: Inside the Matrix
Either by looking in source libraries at your site or by doing a web search for sample assembler code you can evaluate which are "most" common.
There will be a difference between business application code and system support code. From my experience, there are many, many common instructions. There are some seldom used instructions and macros, but even those are too numerous to list here.
If you want to learn assembler from ground zero, i'd recomend going to the Murach publishing web site and buy a copy of the assembler book written by Kevin McQuillen. It will be a good investment.