Computer Chip

There are two main kinds of computer chips: (1) microprocessors, which carry out the instructions that make up computer programs, and (2) memory chips, which hold computer programs and other data. Memory chips are used primarily in computers. Microprocessors are used in computers and hundreds of other products. A microprocessor serves as the "heart" of every personal computer. Larger computers have more than one such chip. Other products controlled by microprocessors include video games, digital watches, microwave ovens, and some telephones.

Structure. The body of most chips is made of silicon. This material is used because it is a semiconductor. In its pure form, silicon does not conduct electricity at room temperature. But if certain impurities are added to silicon, it can carry an electric current. Manufacturers "dope" silicon chips with such impurities as boron and phosphorus. The doped regions form the chip's electronic components, which control the electric signals carried on the chip. The kind and arrangement of the impurities determine how each component controls signals. Most components serve as switches called transistors. Others serve mainly as capacitors, which store an electric charge; diodes, which prevent current from flowing in one direction but not the other; and resistors, which control voltage.

Some chips contain millions of components. Certain parts of these components measure less than 1 micrometer (0.001 millimeter) across. Manufacturers create thin lines of metal--usually aluminum--on the chip to connect these tiny components.

External connections. Most chips connect with other devices by means of a container called a package. One common type of container is a dual in-line package (DIP), an oblong box about 2 inches (5 centimeters) in length. Two rows of metal pins extend downward from the box, one from each long side. Each pin is connected by a wire to an electric terminal on the chip.

In a computer, the packaged chip is mounted on a circuit board. Printed circuits on the board connect the package--and thus the chip--with other devices.

Characteristics of microprocessors. Microprocessors perform essential computer operations. A microprocessor obtains instructions and data from an external memory device; performs arithmetic and logic operations with these data and data contained in its own memory circuits; and, after obtaining a result, moves the calculated data back to the external memory device.

Word length. Instructions and other data handled by computers are in the form of "words." A word is a group of bits. A bit is a binary digit--a 0 or a 1. Computers operate on the basis of bits. For example, the presence of an electric charge in a capacitor can represent a "1" and the absence of a charge can represent a "0."

The maximum word length that a microprocessor can handle helps determine how rapidly it can operate. The earliest microprocessors used 4-bit words. As microprocessors advanced, they were able to handle longer words--usually made up of 8, 16, 32, or 64 bits.

Clock speed is another important characteristic of a microprocessor. Bits travel through a computer in pulses of electric current that occur at regular intervals called clock cycles. Today's microprocessors run at speeds of more than one million cycles per second, or 1 megahertz (MHz).

Instruction sets. There are two basic types of microprocessors: (1) complex instruction set computer (CISC), and (2) reduced instruction set computer (RISC). The microprocessors used in the first computers were the kind now known as CISC. Since about 1986, RISC chips have appeared in certain computers called workstations.

A CISC chip has a very large instruction set--that is, it has many ways to carry out each instruction. For example, it may be able to add two numbers by following any one of 10 procedures. These procedures take various numbers of clock cycles. The number of cycles depends on such factors as the size of the numbers to be added and the location of the numbers in the computer system. A RISC chip uses instructions that are always the same length and can be executed in one clock cycle. By using its special circuits, a RISC chip can execute many times more instructions per second than can a comparable CISC chip.

Memory chips. Most computers use two types of memory chips: (1) read-only memory (ROM), and (2) random-access memory (RAM). A ROM chip retains its stored memory even when the computer is turned off, but the computer user cannot change the stored memory. In contrast, the user can change the memory stored in a RAM chip, but the chip holds its memory only as long as power is on.

There are two main kinds of RAM: static RAM (SRAM) and dynamic RAM (DRAM). SRAM holds its memory until the microprocessor changes it, but DRAM can only hold its memory for a few thousandths of a second. Therefore, a DRAM chip must be refreshed at least 100 times per second, or it will lose its data. To refresh a DRAM, the computer removes the information from each group of cells (memory storage units) in the chip, then puts the same information back. This may seem wasteful. But the time and expense involved are more than made up for by the amount of memory that can be fit into a small space on a DRAM and the cost of these devices. Most personal computers use DRAM's because they are inexpensive to make and can store much memory.

Researchers have developed memory chips with useful features of both ROM and RAM chips. These erasable ROM chips can be erased and reprogrammed, yet they do not lose their memory when the computer is turned off.

One successful design is the electrically, selectively erasable, programmable ROM (EEPROM) chip. A pulse of electric current can erase all the memory in a selected area of the chip. The user can then reprogram this area as if it were part of a RAM chip.

Chip manufacture. The manufacture of a computer chip begins with a wafer of doped silicon. The wafer measures from 1 to 8 inches (2.5 to 20 centimeters) in diameter. A photographic process reduces a large master design for the integrated circuit to microscopic size. Technicians use these microscopic designs, called masks, as stencils to make hundreds of chips on one wafer. After the wafer has been processed, it is divided into individual chips.

History. The first chips were patented in 1959 by two Americans--Jack Kilby, an engineer, and Robert Noyce, a physicist--who worked independently. During the 1960's, scientists developed chips for guided missiles and satellites. Engineers soon began to build smaller and faster computers by using chips in place of conventional circuits. The first microprocessors were produced in 1971 for use in desktop calculators.

Contributor: Charles Melear, M.S.E.E., Manager, Advanced Microcontroller Applications, Motorola Inc.

See also Capacitor; Electronics; Transistor.

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