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This item is from the PC Hardware FAQ, by Willie Lim and Ralph Valentino with numerous contributions by others. (v1.25).

1.09 What is the history of the IBM PC?

Around 1978 and '79, the market served by IBM's Data Entry Systems division began to change. Instead of terminals and minicomputers or mainframes, customers began demanding autonomous, low cost, single-user computers with minimal compute power or connectivity, but compliance to standards like the ASCII alphabet and the BASIC programming language. The closest product in IBM's line was the 5110, a closed, BASIC-in-ROM machine with a tiny built-in character display. The 5110 was uncompetitive, and IBM started losing bids from key customers, mostly government agencies.

Data Entry commissioned a consulting firm (Boca Associates?) to design a stop-gap machine to fill what was perceived within IBM as a short-lived, specialized niche. It was intended that the stop-gap machine would only be offered for a couple of years until it would be replaced in "The Product Line" by an internal IBM design. Some IBM executives believed the single-user desktop system was a fad which would die out when the shortcomings of such systems became appreciated.

The motherboard design was based very closely on a single-board computer described in a 1978 (?) Intel application note. (Anybody got an original copy of this collector's item? Among other things, Intel argues that 640KB is more memory than single-user applications will ever need, because of the efficiency of segmented memory "management"!) The expansion slot "bus" is based on an Intel bus called Multibus 1, which Intel introduced in its microprocessor software development equipment in the mid '70s. The Monochrome and Color Graphics Display Adapters are based on application notes for the Motorola 6845 video controller chip, except that the strangely interlaced pixel addresses in the CGA appears to have been extremely short sighted. The "event driven" keyboard is an original design, but the concept is from the Xerox Alto and Star graphics workstations. The keyboard noise and "feel" are intended to emulate those of the IBM Selectric typewriter. The Cassette Interface design is original, but similar in concept to the one on the Radio Shack TRS-80.

Data Entry Division approached Digital Research Inc. to offer its popular CP/M-86 operating system on the machine, but DRI rebuffed them. IBM's second choice was BASIC-in-ROM vendor Microsoft, which had no OS product at the time but quickly purchased a crude disk operating system called 86-DOS from Seattle Computer Products to offer it to IBM. Its command interpreter was an imitation of Unix' Bourne Shell, with the special characters changed to avoid infringing AT&T's rights.

Data Entry Division began bidding this system in various State procurements, without any plan to offer it to the public.

It became obvious that the Cassette Interface and optional 360KB Flexible Disk Drive were inadequate. The Cassette Interface was dropped, and an optional Fixed Disk Drive offered on a revised model known as the IBM Personal Computer XT. (A fixed, or "hard" disk had been offered on the PC by special order, with a Xebec controller, but few were sold.) The disk controller was designed around the Western Digital 1010 chip, and its design is taken directly from a WD application note.

The XT succeeded beyond all expectations. IBM offered the system to the public after it became clear that no other division was going to come up with anything timely. IBM published complete schematics and ROM listings, encouraging clones.

In 1984, IBM introduced an upwardly compatible model based on the Intel 80286. The expansion slot "bus" was extended to 16-bit data path width the same way Intel had extended Multibus: by adding data and address bits, a signal for boards to announce their capability to perform 16-bit transfers, and byte swapping on the motherboard to support the 8-bit boards.

 

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