11. From Vacuum Tubes to Transistors to Microchips

Old-time radios used vacuum tubes—light bulb-size electronic tubes with glowing filaments. The last computer to use these tubes, the ENIAC, which was turned on in 1946, employed 18,000 of them. Unfortunately, a tube fail­ure occurred every 7 minutes, and it took more than 15 minutes to find and 1 replace the faulty tube. Thus, it was difficult to get any useful computing work done. Moreover, the ENIAC was enormous, occupying 1500 square feet and weighing 30 tons.

The transistor changed all that. A transistor is essentially a tiny electri­cally operated switch that can alternate between "on" and "off" many mil­lions of times per second. The first transistors were one-hundredth the size of a vacuum tube, needed no warm up time, consumed less energy, and were taster and more reliable. Moreover, they marked the begin­ning of a process of miniaturization that has not ended yet. In 1960 one tran­sistor fit into an area about a half-centimeter square. This was sufficient to permit Zenith, for instance, to market a transistor radio weighing about 1 pound (convenient, they advertised, for "pocket or purse"). Today more than 3 million transistors can be squeezed into a half centimeter, and a Sony head­set radio, for example, weighs only 6.2 ounces.

In the old days, transistors were made individually and then formed into an electronic circuit with the use of wires and solder. Today transistors are part of an integrated circuit; that is, an entire electronic circuit, including wires, is all formed together on a single chip of special material, silicon, as part of a single manufacturing process. An integrated circuit embodies what is called solid-state technology. Solid state means that the electrons are trav­eling through solid material—in this case silicon. They do not travel through a vacuum, as was the case with the old radio vacuum tubes.

12. 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.