why computer takes more execution time when a function is called ?

i want theoritical explanation.............that is what happens internally..........
this is diadvantages of Function
Why it takes more execution time ?

This is a very brief answer to your question. For a more detailed answer try googling for it or wait for some one else to post it

When a function is called the context of the current function (the contents of the stacks, the values in the registers etc, the line number of the function call) is saved.
Then the control switches over to the piece of code which responds to the function you have called. If you call any other function from this function then the same process that I have described above happens. When the code reaches the return statement, then all the data stored previously is reloaded. If any value is to be returned from the function that value is put in the appropriate variable