CPU usage: PS versus TOP - Different output

Hi

When monitoring a Linux environment with PS command, reported CPU value for a certain process differs, sometimes greatly, from the value seen in TOP.

I understood this is not a bug, they report different data.
I can't understand the nature of this difference.

My Question: Can someone explain it in simple manner, or alternativly provide a link to such documentation.

Thanks :-)
Liav

PS versus TOP at same instance or different instance !!!

If it is in different time, then it will differ because at different instance a process will occupy different percentage of it..

That also interests me, only what i know is that ps reports the "actual" state of proces at the moment, top instead does some kind of summary and reports values during time

1) I'm talking about the same instance at the same time. (meaning the same process id number).
2) The way I understand it NOW (from the man help) is that:
PS: reports on the accumulate CPU usage (from the time process started).
It gives some sort of an average.

TOP: reports on the CPU usage from the last time it was sampled. (E.G. if the sample
interval is 60 minutes, than it will report the CPU usage in the last 60 minutes).

My question is still open:
a) can someone point out on a good documentation about this?
b) Is there a simple command to calculate the CPU usage the
way top does it (without using top)? (top is too heavy, and if I have to filter out the
lines I need, it will be havier).

I need sort of Real Time CPU usage and not the average.

Thanks :-)

Also I vote for this.... someone please clearly explain from practice view (or give good documentation) about how linux treat system resources: processes, memory, CPU usage, swap, load.... I think all of those questions should be replied together cause it is linked. I have read all what I have found, but still unclear many questions. Thanks a lot gurus