IBM PC Clones
The original PC design was followed up in 1983 by the IBM XT, which was an incrementally improved design; it omitted support for the cassette, had more card slots, and was available with a 10MB hard drive. Although mandatory at first, the hard drive was later made an option and a two floppy disk XT was sold. While the architectural memory limit of 640K was the same, later versions were more readily expandable.
Although the PC and XT included a version of the BASIC language in read-only memory, most were purchased with disk drives and run with an operating system; three operating systems were initially announced with the PC. One was CP/M-86 from Digital Research, the second was PC-DOS from IBM, and the third was the UCSD p-System (from the University of California at San Diego). PC-DOS was the IBM branded version of an operating system from Microsoft, previously best known for supplying BASIC language systems to computer hardware companies. When sold by Microsoft, PC-DOS was called MS-DOS. The UCSD p-System OS was built around the Pascal programming language and was not marketed to the same niche as IBM's customers. Neither the p-System nor CPM-86 was a commercial success.
Because MS-DOS was available as a separate product, some companies attempted to make computers available which could run MS-DOS and programs. These early machines, including the ACT Apricot, the DEC rainbow 100, the Hewlett-Packard HP-150, the Seequa Chameleon and many others were not especially successful, as they required a customized version of MS-DOS, and could not run programs designed specifically for IBM's hardware. (See List of early non-IBM-PC-compatible PCs.) The first truly IBM PC compatible machines came from Compaq, although others soon followed.
Because the IBM PC was based on relatively standard integrated circuits, and the basic card-slot design was not patented, the key portion of that hardware was actually the BIOS software embedded in read-only memory. This critical element got reverse engineered, and that opened the floodgates to the market for IBM PC imitators, which were dubbed "PC clones". At the time that IBM had decided to enter the personal computer market in response to Apple's early success, IBM was the giant of the computer industry and was expected to crush Apple's market share. But because of these shortcuts that IBM took to enter the market quickly, they ended up releasing a product that was easily copied by other manufacturers using off the shelf, non-proprietary parts. So in the long run, IBM's biggest role in the evolution of the personal computer was to establish the de facto standard for hardware architecture amongst a wide range of manufacturers. IBM's pricing was undercut to the point where IBM was no longer the significant force in development, leaving only the PC standard they had established. Emerging as the dominant force from this battle amongst hardware manufacturers who were vying for market share was the software company Microsoft that provided the operating system and utilities to all PC's across the board, whether authentic IBM machines or the PC clones.
In 1984, IBM introduced the IBM Personal Computer/AT (more often called the PC/AT or AT) built around the Intel 80286 microprocessor. This chip was much faster, and could address up to 16MB of RAM but only in a mode that largely broke compatibility with the earlier 8086 and 8088. In particular, the MS-DOS operating system was not able to take advantage of this capability.
Read more about this topic: History Of Personal Computers, The IBM PC