How do you calculate the linux process's Acutal Memory Usage and Not Virtual Memory Usage through the information supplied by /proc//smaps or maps or status or stat.

To be more precise I need the heap usage only. I need to do this on an PowerPc based embedded system and hence I do not have utilities like exmap, valgrind etc. I understand the concepts of shared/non shared memory.

I have read through the other topics given in this forum about the same but they talk more using the tools mentioned. What I need is the native way of calculation done by the same tools so that I may write a shell script for the same.