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PDP-8 Frequently Asked Questions (posted every other month)
Section - What is a PDP-8?

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The PDP-8 family of minicomputers were built by Digital Equipment
Corporation between 1965 and 1990, although it is worth noting that the
term minicomputer first came into prominence after the machine was
introduced.  The first use of the term appears to have been made by
the head of DEC's operations in England, John Leng.  He sent back a
sales report that started: "Here is the latest minicomputer activity
in the land of miniskirts as I drive around in my [Austin] Mini Minor."
The term quickly became part of DEC's internal jargon and spread from
there; the first computer explicitly sold as a minicomputer, though,
was made by by Interdata (See the Interdata ad in Computers and
Automation, May 1968, page 10).

The PDP-8 was largely upward compatible with the PDP-5, a machine that
was unveiled on August 11, 1963 at WESCON, and the inspiration for that
machine came from two earlier machines, the LINC and the CDC 160.  All
of these machines were characterized by a 12 bit word with little or no
hardware byte structure, typically 4K words of memory, and simple but
powerful instruction sets.

Although some people consider the CDC 160 the first minicomputer, the
PDP-8 was the definitive minicomputer.  By late 1973, the PDP-8 family
was the best selling computer in the world, and it is likely that it was
only displaced from this honor by the Apple II (which was displaced by
the IBM PC).  Most models of the PDP-8 set new records as the least
expensive computer on the market at the time of their introduction.
The PDP-8 has been described as the model-T of the computer industry
because it was the first computer to be mass produced at a cost that
just about anyone could afford.

C. Gordon Bell has said that the basic idea of the PDP-8 was not really
original with him.  He gives credit to Seymour Cray (of CDC and later
Cray) for the idea of a single-accumulator 12 bit minicomputer.  Cray's
CDC 160 family (see CACM, march 1961, photo on page 244, text on page
246) was such a machine, and in addition to the hundreds of CDC 160
systems sold as stand-alone machines, a derivative 12 bit architecture
was used for the I/O processors on Cray's first great supercomputer,
the CDC 6600.

Note that Cray's 12 bit machines had 6 basic addressing modes with
variable length instruction words and other features that were far from
the simple elegance of the PDP-8.  Despite its many modes, the CDC 160
architecture lacked the notion of current page addressing, it had no
unconditional jump instruction, and the I/O instructions all blocked
the CPU until I/O complete.  As a result, the PDP-8 is both far more
flexible and it supports much tighter programming styles.

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Top Document: PDP-8 Frequently Asked Questions (posted every other month)
Previous Document: What is a PDP?
Next Document: What is the PDP-8 instruction set?

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Last Update March 27 2014 @ 02:11 PM