IBM 1620
IBM 1620
(computer)It was either developed for the US Navy to teach computing, oras a replacement for the very successful IBM 650 which didquite well in the low end scientific market. Rumour has itthat the Navy called this computer the CADET - Can't Add,Doesn't Even Try.
The ALU used lookup tables to add, subtract and multiply butit could do address increments and the like without thetables. You could change the number base by adjusting thetables, which were input during the boot sequence fromHollerith cards. The divide instruction required additionalhardware, 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 fournumeric bits, one flag bit and one parity bit. The numericbits stored a decimal digit (values above nine were illegal).
Memory was logically divided into fields. On the high-orderdigit of a field the flag bit indicated the end of the field.On the low-order digit it indicated a negative number. A flagbit 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 marksand "numeric blanks".
On a subroutine call it stored the return address in thefive digits just before the entry point to the routine, so youhad to build your own stack to do recursion.
The enclosure was grey, and the core was about four or fiveinches across. The core memory was kept cool inside atemperature-controlled box. The machine took a few minutes towarm up after power on before you could use it. If it got toohot there was a thermal cut-out switch that would shut itdown.
Memory could be expanded up to 100,000 digits in a secondcabinet. The cheapest package used paper tape for I/O. Youcould also get punched cards and later models could behooked 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 clearinghouse of software for a nominal cost such as Snobol,COBOL, chess games, etc.
The model II, released about three years later, could add andsubtract without tables. The clock period decreased from 20to 10 microseconds, instruction fetch sped up by a few cyclesand it added index registers of some sort. Some of themodel I's options were standard on the model II, likeindirect addressing and the console teletype changedfrom a model C to a Selectric. Later still, IBM marketedthe IBM 1710.
A favorite use was to tune a FM radio to pick up the"interference" from the lights on the console. With the rightdelay loops you could generate musical notes. Hackers wroteinterpreters that played music from notation like "C44".
1620 consoles were used as props to represent Colossus inthe film "The Forbin Project", though most of the machines hadbeen scrapped by the time the film was made.
A fully configured 1620.
IBM 1620 console picture.
IBM 1620 at Tuck School of Business, Dartmouth College, Hanover, NH, USA (ThanksVictor E. McGee, pictured).
["Basic Programming Concepts and the IBM 1620 Computer",Leeson and Dimitry, Holt, Rinehart and Winston, 1962].