How a Microprocessor Works

Microprocessor History

A microprocessor - also known as a CPU or Central Processing Unit - is a complete computation engine that is fabricated on a single chip. The first microprocessor was the Intel 4004, introduced in 1971. The 4004 was not very powerful - all it could do was add and subtract, and it could only do that four bits at a time. But it was amazing that everything was on one chip. Prior to the 4004, engineers built computers either from collections of chips or from discrete components (transistors wired one at a time). The 4004 powered one of the first portable electronic calculators.

The first microprocessor to make it into a home computer was the Intel 8080, a complete 8-bit computer on one chip introduced in 1974. The first microprocessor to make a real splash in the market was the Intel 8088, introduced in 1979 and incorporated into the IBM PC (which first appeared in 1982 or so). If you are familiar with the PC market and its history, you know that the PC market moved from the 8088 to the 80286 to the 80386 to the 80486 to the Pentium to the Pentium-II to the new Pentium-III. All of these microprocessors are made by Intel and all of them are improvements on the basic design of the 8088. The new Pentiums-IIIs can execute any piece of code that ran on the original 8088, but the Pentium-III runs about 3,000 times faster!

The following table helps you to understand the differences between the different processors that Intel has introduced over the years.

Name	Date	Transistors	Microns	Clock speed	Data width	MIPS
8080	1974	6,000	6	2 MHz	8	0.64 MIPS	First home computers
8088	1979	29,000	3	5 MHz	16 bits, 8 bit bus	0.33 MIPS	First IBM PC
80286	1982	134,000	1.5	6 MHz	16 bits	1 MIPS	IBM ATs. Up to 2.66 MIPS at 12 MHz
80386	1985	275,000	1.5	16 MHz	32 bits	5 MIPS	Eventually 33 MHz, 11.4 MIPS
80486	1989	1,200,000	1	25 MHz	32 bits	20 MIPS	Eventually 50 MHz, 41 MIPS
Pentium	1993	3,100,000	0.8	60 MHz	32 bits, 64 bit bus	100 MIPS	Eventually 200 MHz
Pentium II	1997	7,500,000	0.35	233 MHz	32 bits, 64 bit bus	400 MIPS?	Eventually 450 MHz, 800 MIPS?
Pentium III	1999	9,500,000	0.25	450 MHz	32 bits, 64 bit bus	1,000 MIPS?

What is a Chip?

A chip is also called an integrated circuit. Generally it is a small, thin piece of silicon onto which the transistors making up the microprocessor have been etched. A chip might be as large as an inch on a side and can contain as many as 10 million transistors. Simpler processors might consist of a few thousand transistors etched onto a chip just a few millimeters square. See How Silicon Chips Are Made for details on how transistors are fabricated on silicon.

Information about this table:

The date is the year that the processor was first introduced. Many processors are re-introduced at higher clock speeds for many years after the original release date.
Transistors is the number of transistors on the chip. You can see that the number of transistors on a single chip has risen steadily over the years.
Microns is the width, in microns, of the smallest wire on the chip. For comparison, a human hair is 100 microns thick. As the feature size on the chip goes down, the number of transistors rises.
Clock speed is the maximum rate that the chip can be clocked. Clock speed will make more sense in the next section.
Data Width is the width of the ALU. An 8-bit ALU can add/subtract/multiply/etc. two 8-bit numbers, while a 32-bit ALU can manipulate 32-bit numbers. An 8-bit ALU would have to execute 4 instructions to add two 32-bit numbers, while a 32-bit ALU can do it in one instruction. In many cases the external data bus is the same width as the ALU, but not always. The 8088 had a 16-bit ALU and an 8-bit bus, while the modern Pentiums fetch data 64 bits at a time for their 32-bit ALUs.
MIPS stands for Millions of Instructions Per Second, and is a rough measure of the performance of a CPU. Modern CPUs can do so many different things that MIPS ratings lose a lot of their meaning, but you can get a general sense of the relative power of the CPUs from this column.

From this table you can see that, in general, there is a relationship between clock speed and MIPS. The maximum clock speed is a function of the manufacturing process and delays within the chip. There is also a relationship between the number of transistors and MIPS. For example, the 8088 clocked at 5 MHz but only executed at 0.33 MIPS (about 1 instruction per 15 clock cycles). Modern processors can often execute at a rate of 2 instructions per clock cycle. That improvement is directly related to the number of transistors on the chip and will make more sense in the next section.