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

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In 1957, Ken Olson and Harlan Anderson founded Digital Equipment
Corporation (DEC), capitalized at $100,000, and 70% owned by American
Research and Development Corporation.  Olson and Anderson had designed
major parts of the AN/FSQ-7, the TX-0 and the TX-2 computers at
Lincoln Labs.  They wanted to call their company Digital Computer
Corporation, but the venture capitalists insisted that they avoid the
term Computer and hold off on building computers.

With facilities in an old woolen mill in Maynard Massachusetts, DEC's
first product was a line of transistorized digital "systems modules"
based on the modules used in building TX-2 at Lincoln Labs; these
were plug-in circuit boards with a few logic gates per board.  Starting
in 1960, DEC finally began to sell computers (the formal acceptance of
the first PDP-1 by BBN is reported in Computers and Automation, April
1961, page 8B).  Soon after this, there were enough users that DECUS,
the Digital Equipment Computer User's Society was founded.

DEC's first computer, the PDP-1, sold for only $120,000 at a time when
other computers sold for over $1,000,000.  (A good photo of a PDP-1 is
printed in Computers and Automation, Dec. 1961, page 27).  DEC quoted
prices as low as $85,000 for minimal models.  The venture capitalist's
insistance on avoiding the term computer was based on the stereotype
that computers were big and expensive, needing a computer center and a
large staff; by using the term Programmable Data Processor, or PDP, DEC
avoided this stereotype.  For over a decade, all digital computers sold
by DEC were called PDPs.  (In early DEC documentation, the plural form
"PDPs" is used as a generic term for all DEC computers.)

In the early 1960's, DEC was the only manufacturer of large computers
without a leasing plan.  IBM, Burroughs, CDC and other computer
manufacturers leased most of their machines, and many machines were
never offered for outright sale.  DEC's cash sales approach led to the
growth of third party computer leasing companies such as DELOS, a
spinoff of BB&N.

DEC built a number of different computers under the PDP label, with a
huge range of price and performance.  The largest of these are fully
worthy of large computer centers with big support staffs.  Some early
DEC computers were not really built by DEC.  With the PDP-3 and LINC,
for example, customers built the machines using DEC parts and
facilities.  Here is the list of PDP computers:

    MODEL  DATE  PRICE    BITS NUMBER COMMENTS
    =====  ====  ======== ==== ====== ========
    PDP-1  1960  $120,000 18       50 DEC's first computer
    PDP-2            NA   24        - Never built?  Prototype only?
    PDP-3            NA   36          One built by a customer*, not by DEC.
    PDP-4  1962   $60,000 18       45 Predecessor of the PDP-7.
    PDP-5  1963   $27,000 12    1,000 The ancestor of the PDP-8.
    PDP-6  1964  $300,000 36       23 A big computer; 23 built, most for MIT.
    PDP-7  1965   $72,000 18      120 Widely used for real-time control.
    PDP-8  1965   $18,500 12  ~50,000 The smallest and least expensive PDP.
    PDP-9  1966   $35,000 18      445 An upgrade of the PDP-7.
    PDP-10 1967  $110,000 36   **~700 A PDP-6 followup, great for timesharing.
    PDP-11 1970   $10,800 16 >600,000 DEC's first and only 16 bit computer.
    PDP-12 1969   $27,900 12      725 A PDP-8 relative.
    PDP-13           NA             - Bad luck, there was no such machine.
    PDP-14                        *** A ROM-based programmable controller.
    PDP-15 1970   $16,500 18      790 A TTL upgrade of the PDP-9.
    PDP-16 1972      NA   8/16      ? A register-transfer module system.

*  Scientific Engineering Institute of Waltham MA.  SEI was aledgedly
     founded in 1956 by the CIA to study the effects of microwaves (radar)
     on the human brain.  If so, the PDP-3 may have been used as an
     instrumentation computer.  More info on the CIA connection and the
     use of the PDP-3 would be nice!
** Includes DECsystem 20.

Corrections and additions to this list are welcome!  The prices given
are for minimal systems in the year the machine was first introduced.
Most of the production run numbers come from "Computer Engineering" by
Bell, Mudge and McNamara, 1978, or from Computers and Automation's
computer census figures published regularly throughout the 1960's.
The bits column in the table indicates the word size.  Note that the
DEC PDP-10 became the DECSYSTEM-20 as a result of marketing
considerations, and DEC's VAX series of machines began as the Virtual
Address eXtension of the never-produced PDP-11/78.

It is worth mentioning that it is widely (but somewhat incorrectly)
accepted that the Data General Nova (see photo, Computers and
Automation, Nov. 1968, page 48) grew out of the PDP-X, a 16-bit
multi-register version of the PDP-8 designed by Edson DeCastro, Henry
Burkhardt and Dick Soggee.  (DeCastro was one of DEC's key design
engineers; his name appears on many of the blueprints for machines
from the PDP-5 up through the PDP-8/L).

A prototype PDP-X was built at DEC; this and a competing 16-bit design
were apparently submitted to Harold McFarland at Carnegie-Mellon
University for evaluation; McFarland (and perhaps Gordon Bell, who was
at C-MU at the time) evaluated the competing designs and rejected both
in favor of what we now know as the PDP-11.  (I was at Carnegie-Mellon
at the time, and McFarland gave a guest lecture in a class I attended
telling part of this story.)  Some speculate, incorrectly, that Bell
rejected the Nova design because the competing proposal used the
register-transfer notation he had introduced in "Bell and Newell,
Computer Structures -- Readings and Examples".  An alternate and equally
unfounded story is that the reason DEC never produced a PDP-13 was
because the number 13 had been assigned to what became the Nova.

In any case, when DeCastro, Burkhardt and Soggee founded Data General,
Ken Olson at DEC was very angry, claiming for a long time that the
Nova design was stolen.  Gordon Bell and others concluded that the
Nova design was sufficiently original that a lawsuit was unwarranted,
but the feud between DeCastro and Olson lasted until after Ken Olson
left DEC.  It is more correct to say that the Nova is a reaction to the
PDP-X than to say that it is based on the PDP-X.  I am indebted to
Jim Campbell, retired VP at Data General, for some of the details of
this story.

Today, all of the PDP machines are in DEC's corporate past, except the
PDP-11 family, which survives as a line of microcomputers; DEC has
promised to discontinue PDP-11 sales on Sept. 30, 1996.  Occasionally,
some lab has built a machine out of DEC hardware and called it a PDP
with a new number.  For example, the Australian Atomic Energy Commission
once upgraded a PDP-7 by adding a PDP-15 on the side; they called the
result a PDP-22.  There is also a story about the PDP-2 1/2, built by
Ed Rawson of the American Science Institute out of surplus modules that
were originally used in the prototype PDP-2.

In 1998, Compaq purchased DEC, and it is unclear how long DEC will retain
any semblance of its original identity as a division of a larger company.

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Last Update March 27 2014 @ 02:11 PM