Display memory usage.

Syntax
      MEM 
      MEM /C
      MEM /D
      MEM /P

Key
   /P   List programs in memory
        with the memory address and size of each 

   /D   List Programs(as /P) and also Devices

   /C   List programs in conventional memory and
        list programs in upper memory

MEM will only display details about the current CMD shell environment, programs running in a separate shell (or WIN32 programs) will not be listed - so it won't tell you anything about total memory usage.