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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 a design
error. 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 from struggling Pacific Microsystems(?) 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.