A processor or central processing unit (CPU) is the hardware that executes instructions in a computer, and it is composed of circuitry such as registers, the arithmetic logic unit, and the control unit. All these components can each be obtained as integrated circuits (ICs), also known as microchips,and placed together in a circuit board as shown in Figure 1.1. In fact, that is how computers were built in the past. However, for other than educational purposes, there is absolutely no reason to do this any more, as processors themselves are now built as ICs. To be more precise, they are built into a very specific part of an IC, a single piece of semiconducting material known as die¹.
Microprocessors (MPUs) are processors built in adie; the die integrates all the transistors that form all the components of a CPU. Figure 1.2 shows the Eniac-on-a-chip, a die replica of the first general-purpose computer, which weighted over thirty metric tons
A microprocessor by itself is not a computer.It would require at least some form of memory and I/O to make one. In the case of a desktop computer or laptop, all these components are soldered as part of the motherboard. Is it possible to make computer boards even smaller? The short answer is yes, and in the next section we discuss how micro-controllers achieve this.