how can I check the OS memory.

I'd check the memory of our system by STAD

I could check the memory who use , how much they possess , which T-code etc..

but i'd like to check the OS memory detailed specific time.

I think STAD doesnt show the list of the momory which used by OS.

for Example) when i do back up by DB13 , it also usees the memory.

but i cant check only by STAD.

where can i check it?