popular lines of microcomputers manufactured by IBM. There are many variations of each; this article will mention only the most historically important.

The IBM Personal Computer (PC), introduced in 1981,was the first of a family of very popular microcomputers, including not only IBM products but also "clones" (imitations) made by other companies. The original IBM PC used very little proprietary technology. Thus, it was easy for competitors to build compatible machines without violating patents. See clone ; PC compatibility .

IBM maintained a high level of upward compatibility within the PC and PS/2 line. This means that later-model machines would run virtually all software written for earlier models. Nowadays, most PCcompatible computers are made by other companies; IBM continues to market PC-compatibles that are very similar to competitors' products.

IBM's two original machines, the PC and PC XT, are virtually identical, featuring 4.77-MHz 8088 microprocessors with an 8-bit bus. The only difference was that the XT had a 10-megabyte hard disk and had eight expansion slots instead of five. The PC AT, introduced in 1984, was the first PC to use the 80286 microprocessor, enabling programs to run much faster. The PC AT had what is now known as the ISA (Industry Standard Architecture) bus; it accepted both 8-bit (XT-style) and 16-bit plug-in cards.

The PS/2 machines were introduced in 1987 and discontinued in 1995. They were more compact than comparably configured PCs or ATs, and all but the lowest models used the Micro Channel bus, which made it possible in some situations to use more than one CPU in a single machine.

These computers use theASCII character set (see ASCII ). In addition, they define printed representations for all character codes from 0 to 255.

Figure 135 shows the printable part of the special character set. Under DOS or at a Windows command prompt , these characters are typed by holding down the Alt key and typing the appropriate number on the numeric keypad at the right side of the keyboard. For example, to type a shaded block, hold down Alt, type 178, and then release Alt.