Here is the simple function
void SomeFunction()
{
Some100MillisecondVariable = Some100MillisecondVariable + 1
}
and i want this function to be called after every 100 milliseconds.

as Andy says, this is basic manual stuff, read about it.

Shazad,

There is (at least) two ways to come up to speed on the '51

1) spend a day or two familiarizing yourself with "the bible" and the Keil manuals, then ask a question or two here on points you do not understand and you will be up to a reasonable speed in a week or so.

2) haphazardly write some code if it is small and ask "what is wrong". If it is large ask for the code. And you will be up to a reasonable speed in a year or so.

You choose the method

Erik