Determining physical memory usage

[Rouben Rostamian]

Is there a utility to determine what fraction of the physical memory 
is being used on a machine?

The closest I can come to is to execute the ps command (with
optional flags) to get the percentage of memory used by individual
processes, then add up all the percentages.  [I am not even sure
if the memory referred by ps is virtual or physical memory.]

I have a DECstation 3100 with 8MB of memory.  The reason I want to check
the memory usage is to determine whether the memory gets saturated at all,
and whether investing in additional memory modules is worthwhile. 

I tried 'man -k mem' but did not find anything useful there.
I would appreciate any pointers and suggestions.

--
Rouben Rostamian                               Phone: 301 455-2458
Department of Mathematics                      e-mail:
University of Maryland Baltimore County        rostamian at umbc.bitnet
Baltimore, MD 21228                            rostamian at umbc3.umbc.edu