Writing s-functions to be used in real time environmnets

A model is available with me which is built such as to generate code using RTW to run on specific target. Iam tracing out now how various functinalities have been implemented in it. While goig through, i came across several s-fucnctions. I have a theoritical idea of s-functions.

Can any one help me how to decide that a particular place deserves an s-function? I dont have any programs already written in other langauges. What does mdlRTW method do inside an s-function? is it for inlining the s-function?

[Purushottama Rao]

s-functions are written for a) finding out processor load b) loading xml files which contains data for the model c)interfacing the code with the application written in windows..etc.