Monitoring and reacting to 4 input lines every 1us or 10us is a serious task for a standard 8051. It is likely that most of your CPU time will be spent doing this task alone, IF YOUR MICRO CAN EVEN DO IT AT ALL.

I think I have said this before: you are in over your head. You are doing this as an academic excercise. CHOOSE AN EASIER PROJECT!! Maybe put it down for a while and then come back to it. Learn "multitasking" on a different quicky project.

And another thing, I think the way that you ask for help rubs people the wrong way. Your posts are tedious to read. You present long and overly intricate problem definitions and you expect the solution to be delivered on a silver platter. That doesn't go over very well when it happens time and time again.

- Lee