IBM PC compatible

IBM PC compatible computers are those similar to the original IBM PC, XT, and AT, able to run the same software and support the same expansion cards as those. Such computers used to be referred to as PC clones, or IBM clones. They duplicate almost exactly all the significant features of the PC architecture, facilitated by IBM's choice of commodity hardware components and various manufacturers' ability to reverse engineer the BIOS firmware using a "clean room design" technique. Columbia Data Products built the first clone of the IBM personal computer by a clean room implementation of its BIOS.

Early IBM PC compatibles used the same computer bus as the original PC and AT models. The IBM AT compatible bus was later named the Industry Standard Architecture bus by manufacturers of compatible computers. The term "IBM PC compatible" is now a historical description only, since IBM has ended its personal computer sales.

Descendants of the IBM PC compatibles comprise the majority of personal computers on the market presently, although interoperability with the bus structure and peripherals of the original PC architecture may be limited or non-existent.

IBM Personal Computer

The IBM Personal Computer, commonly known as the IBM PC, is the original version and progenitor of the IBM PC compatible hardware platform. It is IBM model number 5150, and was introduced on August 12, 1981. It was created by a team of engineers and designers under the direction of Don Estridge of the IBM Entry Systems Division in Boca Raton, Florida.

The generic term "personal computer" was in use before 1981, applied as early as 1972 to the Xerox PARC's Alto, but because of the success of the IBM Personal Computer, the term "PC" came to mean more specifically a desktop microcomputer compatible with IBM's PC products. Within a short time of the introduction, third-party suppliers of peripheral devices, expansion cards, and software proliferated; the influence of the IBM PC on the personal computer market was substantial in standardizing a platform for personal computers. "IBM compatible" became an important criterion for sales growth; only the Apple Macintosh family kept significant market share without compatibility with the IBM personal computer.

IBM 3270 PC

The IBM 3270 PC (IBM System Unit 5271), released in October 1983, was an IBM PC XT containing additional hardware which could emulate the behaviour of an IBM 3270 terminal. It could therefore be used both as a standalone computer, and as a terminal to a mainframe.

IBM later released the 3270 AT (IBM System Unit 5273), which was a similar design based on the IBM PC AT. They also released high-end graphics versions of the 3270 PC in both XT and AT variants. The XT-based versions were called 3270 PC/G and 3270 PC/GX and they used a different System Unit 5371, while their AT counterparts (PC AT/G and PC AT/GX) had System Unit 5373.

Technology

The additional hardware occupied nearly all the free expansion slots in the computer. It included a video card which occupied 1-3 ISA slots (depending on what level of graphics support was required), and supported CGA and MDA video modes. The display resolution was 720×350, either on the matching 14-inch color monitor (model 5272) or in monochrome on an MDA monitor.