IBM 1620

IBM 1620

   <computer> A computer built by {IBM} and released in late
   1959.  The 1620 cost from around $85,000(?) up to hundreds of
   thousands of dollars(?) according to the configuration.  It
   was billed as a "small scientific computer" to distinguish it
   from the business-oriented {IBM 1401}.  It was regarded as
   inexpensive, and many schools started out with one.

   It was either developed for the US Navy to teach computing, or
   as a replacement for the very successful {IBM 650} which did
   quite well in the low end scientific market.  Rumour has it
   that the Navy called this computer the CADET - Can't Add,
   Doesn't Even Try.

   The {ALU} used lookup tables to add, subtract and multiply but
   it could do address increments and the like without the
   tables.  You could change the number base by adjusting the
   tables, which were input during the boot sequence from
   {Hollerith} cards.  The divide instruction required additional
   hardware, as did {floating point} operations.

   The basic machine had 20,000 decimal digits of {ferrite core
   memory} arranged as a 100 by 100 array of 12-bit locations,
   each holding two digits.  Each digit was stored as four
   numeric bits, one flag bit and one parity bit.  The numeric
   bits stored a decimal digit (values above nine were illegal).

   Memory was logically divided into fields.  On the high-order
   digit of a field the flag bit indicated the end of the field.
   On the low-order digit it indicated a negative number.  A flag
   bit on the low order of the address indicated {indirect
   addressing} if you had that option installed.  A few "illegal"
   bit combinations were used to store things like record marks
   and "numeric blanks".

   On a {subroutine} call it stored the {return address} in the
   five digits just before the entry point to the routine, so you
   had to build your own {stack} to do {recursion}.

   The enclosure was grey, and the core was about four or five
   inches across.  The core memory was kept cool inside a
   temperature-controlled box.  The machine took a few minutes to
   warm up after power on before you could use it.  If it got too
   hot there was a thermal cut-out switch that would shut it
   down.

   Memory could be expanded up to 100,000 digits in a second
   cabinet.  The cheapest package used {paper tape} for I/O.  You
   could also get {punched cards} and later models could be
   hooked up to a 1311 {disk drive} (a two-{megabyte} {washing
   machine}), a 1627 {plotter}, and a 1443 {line printer}.

   Because the 1620 was popular with colleges, IBM ran a clearing
   house of software for a nominal cost such as {Snobol},
   {COBOL}, chess games, etc.

   The model II, released about three years later, could add and
   subtract without tables.  The {clock period} decreased from 20
   to 10 microseconds, instruction fetch sped up by a few cycles
   and it added {index registers} of some sort.  Some of the
   model I's options were standard on the model II, like
   {indirect addressing} and the {console} {teletype} changed
   from a model C to a {Selectric}.  Later still, IBM marketed
   the {IBM 1710}.

   A favorite use was to tune a FM radio to pick up the
   "interference" from the lights on the console.  With the right
   delay loops you could generate musical notes.  Hackers wrote
   {interpreters} that played music from notation like "C44".

   1620 consoles were used as props to represent {Colossus} in
   the film "The Forbin Project", though most of the machines had
   been scrapped by the time the film was made.

   A fully configured 1620
   (http://uranus.ee.auth.gr/TMTh/exhibit.htm).

   IBM 1620 console picture
   (http://foldoc.org/pub/misc/IBM1620-console.jpg).

   IBM 1620 at Tuck School of Business, Dartmouth College,
   Hanover, NH, USA
   (http://foldoc.org/pub/misc/IBM1620-Tuck1960s.jpg) (Thanks
   Victor E. McGee, pictured).

   ["Basic Programming Concepts and the IBM 1620 Computer",
   Leeson and Dimitry, Holt, Rinehart and Winston, 1962].

   (1997-08-05)