IBM 1620

    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".

   IBM 1620 console (img:/pub/misc/IBM1620-console.jpg)

   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>.

   IBM 1620 at Tuck School of Business, Dartmouth College, Hanover,
   NH, USA (/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].

   (2018-09-11)