bobguo wrote:... I heard there are big differences in normal assembler programming. ...
You "heard?" From whom?
What nature of "usermods?"
JES2 exits? In these exits, you must use a variety of JES2 macros, and you often cannot use "normal" z/OS macros for what often seems to be the same purpose as JES2 macros.
The last time I did anything with JES2 exits was for z/OS 1.7 in 2006 or 2007, and my memory of that has faded.
In other system usermods, just like JES2 exits, the environment is often somewhat different than you are used to in regular programming, and that may require deviations from what you usually do for regular problem state programming.
However, the instructions are the same, and work the same way as they do in problem state, problem key, task mode programming. In that sense, there are no differences at all.