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
The basic machine had 20,000 decimal digits of
ferrite corememory 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
indirectaddressing if you had that option installed. A few "illegal"
bit combinations were used to store things like record marks
and "numeric blanks".
five digits just before the entry point to the routine, so you
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
Because the 1620 was popular with colleges, IBM ran a clearing
house of software for a nominal cost such as
Snobol,
The model II, released about three years later, could add and
to 10 microseconds, instruction fetch sped up by a few cycles
model I's options were standard on the model II, like
from a model C to a
Selectric. Later still, IBM marketed
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
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.
Victor E. McGee, pictured).
["Basic Programming Concepts and the IBM 1620 Computer",
Leeson and Dimitry, Holt, Rinehart and Winston, 1962].
(1997-08-05)
