I have to confess I have not jotted down my entire problem here. I am working on a data acquisition system which reads input from a few Octal Latches and sends the output to the parallel port of a computer. The latches are in turn connected to 8 different single line inputs. When ever any inputs go from LOW to HIGH, I have to report this to the parallel port. I figured out that if you have the last input (say LS = 0000100) in a register then inverting all the bits (i.e. getting 11110111) and ANDing it with the new input (say NS= 01010001) can give you the bits that went from LOW to HIGH, i.e. (01010001). This way the bit No that are high will identify the origin of the HIGH signal (in this case, 2, 4, 7). I will finally have to add these numbers with another number to indicate which Octal Latch the signal comes from. That way the parallel port will receive the absolute identification of the origin of the HIGH signals.