Processor memory holds the image and is used for running processes, while io memory is used by the interfaces to hold packets in memory prior to switching to appropriate interface , A sh mem will show you how much memory is being used
>sh mem
Head Total(b) Used(b) Free(b) Lowest(b) Largest(b)
Processor 61C57420 70945760 5155608 65790152 65614448 65737592
I/O 6000000 33554432 1864124 31690308 31652000 31653180
When you see the largest & free column fall to a small amount you should be concerned . in the above output there is 65MB free in the largest column ,
On a 1600 you may be able to use the memory-size iomem xx command to change the amount of io mem on a % basis that is allocated , you must reload the box for this to take affect
