On a command line: STRSST
Select 1 Start a Service Tool
Select 7 Hardware Service Manager
Select 2 Logical Hardware Resources
Select 3 Main Storage Resources
Add everything listed under ‘size’ to determine the total amount of main storage.
Simplest command is probably WRKSHRPOOL. The amount of main memory available to the system is shown as ‘Main storage size’ at the top of the display.
The easiest way I have found if to use rhe WRKHDWRSC *PRC command on the command line. It will tell you the processor number and the number of memory chips and the size of each. then you can consult the system builder book to see if there is room for any more chips and the placement order. Some of the systems have to have memory put in pairs or quads and loaded in a particular order.
Hope this helps