CPU and Main Memory

The CPU, for central processing unit, follows the instructions of the soft­ware to manipulate data into information. The CPU consists of two parts: (1) the control unit and 21 the arithmetic/logic unit. The two components are connected by a kind of electronic "roadway" called a bus.

The control unit tells the rest of the computer system how to carry out a program's instructions. It directs the movement of elec­tronic signals between main memory and the arithmetic/logic unit. It also directs these electronic signals between main memory and the input and output devices.

The arithmetic/logic unit, or ALU, performs arithmetic operations and logical operations and controls the speed of those operations.

As you might guess, arithmetic operations are the fundamental math operations: addition, subtraction, multiplication, and division.

Logical operations arc comparisons. That is, the ALU compares two pieces of data to see whether one is equal to (=), greater than (>), or less I than (<) the other. (The comparisons can also be combined, as in "greater than or equal to" and "less than or equal to.")

Main Memory

Main memory—variously known as memory, primary storage, internal memory, or RAM (for random access memory)—is working storage. It has three tasks. (1) It holds data for processing. (2) It holds instructions (the pro­grams) for processing the data. (3) It holds processed data (that is, informa­tion) waiting to be sent to an output or secondary-storage device. Main memory is contained on special microchips called RAM chips, as we describe in a few pages. This memory is in effect the computer's short-term capacity. It determines the total size of the programs and data files it can work on at; any given moment.

CPU Chip

Most personal computers today use CPU chips (microprocessors) of two kinds—those made by Intel and those by Motorola—although that situation may be changing. Workstations generally use RISC chips.

Intel makes chips for IBM and IBM-com­patible computers such as Compaq, Dell, Gateway, Tandy, Toshiba, and Zenith. Variations of Intel chips are made by other companies—-for exam­ple, Advanced Micro Devices (AMD), Cyrix, and Chips and Technologies.

Intel has identified its chips by numbers—8086, 8088, 80286, 80386, 80486—and is now marketing its newest chips under the names Pentium and Pentium Pro. The higher the number, the newer and more powerful the chip and the faster the processing speed, which means that software runs more efficiently. The chips are commonly referred to by their last three digits, such as '386 and '486.

Some chips have different versions—for example, "386SX" or "486DX." SX chips are usually less expensive than DX chips and run more slowly. Thus, they are more appropriate for home use, whereas DX chips are more appropriate for business use. SL chips are designed to reduce power con­sumption and so are used in portable computers. DX2 and DX4 chips are usually used for heavy-duty information processing.

Macintosh computers. These chip numbers include the 68000, 68020, 68030, and 68040. In 1994, Motorola joined forces with IBM and Apple and produced the PowerPC chip.

Sun Microsystems, Hewlett-Packard, and Digital Equipment use RISC chips in their desktop workstations, although the technology is also showing up in some portables.

RISC stands for reduced instruction set computing. With RISC chips a great many needless instructions are eliminated. Thus, a RISC computer system operates with fewer instructions than those required in conven­tional computer systems. RISC-equipped workstations have been found to work 10 times faster than conventional computers. A problem, however, is that software has to be modified to work with them.

Дата добавления: 2015-11-14