Well the thing is that it's the OS that schedules what processes are running on which processors. Like I've said, I don't know much about AIX, but I doubt that an incorrect application installation could "trick" the OS into only scheduling on a processor subset.

In Solaris, there is a command called psrset (and a stripped-down companion called pbind) that allow the user to control what processors will be available for specified processes. bindprocessor seems to be the closest thing I can find in AIX, but it seems much more limited, so I was hoping it might lead you to the more advanced command in AIX, but maybe it doesn't exist.

Actually, now that I look, top gives me the CPU ID of running processes in Solaris. Maybe it'll give it to you in AIX, too.