The evolution of computer has passed through a number of stages before it reached the present state of development. The earliest and simplest device that was used for calculations was the Abacus. Abacus was first developed by Chinese in 3000 B.C. It is still in use in some parts of Japan and China. It was simply a rectangular wooden frame, with rods carrying round beads. The frame is divided into two parts. One part consists of five beads whereas other consists of only two beads. In the part which consists of five beads, each bead represents digit one and in the second part each bead represents digit five. It is used for addition and subtraction.