Development of the PC - Upgrading and Repairing PCs - page 7

become universally adopted.
The first generation of modern programmed electronic computers to take advantage of these
improvements appeared in 1947. This group of machines included EDVAC and UNIVAC, the first
commercially available computers. These computers included, for the first time, the use of true
random access memory (RAM) for storing parts of the program and the data that is needed quickly.
Typically, they were programmed directly in machine language, although by the mid-1950s progress
had been made in several aspects of advanced programming. The standout of the era is the UNIVAC
(Universal Automatic Computer), which was the first true general-purpose computer designed for
both alphabetical and numerical uses. This made the UNIVAC a standard for business, not just
science and the military.
Modern Computers
From UNIVAC to the latest desktop PCs, computer evolution has moved rapidly. The first-generation
computers were known for using vacuum tubes in their construction. The generation to follow would
use the much smaller and more efficient transistor.
From Tubes to Transistors
Any modern digital computer is largely a collection of electronic switches. These switches are used
to represent and control the routing of data elements called binary digits (or bits ). Because of the on-
or-off nature of the binary information and signal routing the computer uses, an efficient electronic
switch was required. The first electronic computers used vacuum tubes as switches, and although the
tubes worked, they had many problems.
The type of tube used in early computers was called a triode and was invented by Lee De Forest in
1906 (see Figure 1.1 ) . It consists of a cathode and a plate, separated by a control grid, suspended in a
glass vacuum tube. The cathode is heated by a red-hot electric filament, which causes it to emit
electrons that are attracted to the plate. The control grid in the middle can control this flow of
electrons. By making it negative, you cause the electrons to be repelled back to the cathode; by
making it positive, you cause them to be attracted toward the plate. Thus, by controlling the grid
current, you can control the on/off output of the plate.
Figure 1.1. The three main components of a basic triode vacuum tube.
Unfortunately, the tube was inefficient as a switch. It consumed a great deal of electrical power and
gave off enormous heat—a significant problem in the earlier systems. Primarily because of the heat