Modern computer systems (since 1950s) evolved from being bulky, unreliable and expensive, to being tiny, reliable, cheaper and faster. Improvements in size, expense and performance was due to the evolution in computer hardware; from vacuum tubes, magnetic tapes, integrated circuits, to very large-scale integration (VLSI) of thousands of transistors and other circuit elements onto a single chip. The drop in cost and size led to the introduction of personal computers (PCs) for use in office, schools, and homes. Together with developments such as network technologies (LAN, WAN, WLAN, the Internet, etc), and application softwares, computer systems are no longer just calculators, but multipurpose devices. We use these devices for watching movies, making phone calls, sending emails, controlling remote devices, driving, etc