The IBM 1620

An IBM 1620's Console


Just The Right Machine for Chula's Statistics Department

An extensive tour of the U.S.'s governmental institutions in 1955 convinced Prof. Bundhit that his Thai Statistics Department at Chula needed a computer.  Real statistical work involved manipulating large amounts of data and thus could hardly be done without a computer.  Prof. Bundhit therefore concluded that Thailand's future statisticians simply had to be trained to use one.

By the time the sixties came around, large computers were available.  However, their price tags were extremely high. IBM 7094, IBM's most powerful computer in 1963 , for example, cost more than $3 million a piece.  Fortunately, since late 1959 IBM came out with the 1620, a small, relatively affordable computer just powerful enough for training students and for small-scale scientific applications.  The price tag was under $100,000 for a minimal system to hundreds of thousands of dollars fully equipped.  The 1620 became very popular in U.S. universities, and was also used to train U.S. Navy personnel. Dr. Bundhit thought, quite correctly in retrospect, that the IBM 1620 was just the right machine for his department at Chula.  He not only found funding for the purchase of an IBM1620 and software, but also for Thailand's first computing center (now named after Dr. Bundhit).  Additionally the computing center also housed a departmental library, funded by one of Dr. Bundhit's grants.

Since the 1620 was popular in colleges and universities, IBM made some important pieces of software (such as compilers and even chess-playing software) available at a nominal (trifling) cost..

Now for some technical details... The 1620 was primarily a decimal machine.  In other words, it operated in base-10 numbers, unlike most of today's computers which use binary numbers.  Electrically speaking, the 1620 held and manipulated bits (binary digits 0 and 1), but the bits are used to encode decimal digits.  Note also that you could change the base of numbers  every time you booted the machine.

The 1620's arithmetic-logic unit (ALU), which is the circuit for doing calculations, used a lookup table to add, subtract, and multiply.  Division required additional hardware (and of course expense), but can also be simulated (using a lot of extra computation time) in software.

How much main memory (ferrite core) did the 1620 have?  Well, the basic model had 20,000 decimal digits!  This was arranged as a 100x100 array of 12-bit locations, each holding 2 decimal digits.  (Each decimal digit was encoded in binary using 6 bits.)

What about the clock speed?  What's clock speed, anyway?  Well, a computer's CPU usually has a "master clock" which ticks away (oscillates between a low and a high voltage level) at a fixed frequency.  This frequency is one of the things that determine how fast your computer system operates.  A 500 megahertz CPU (like a typical desktop computer), for example, has a clock that oscillates between a high and a low voltages 500 million times a second!  Well, the original (Model 1) 1620 had a clock that oscillated 50,000 times a second, ten thousand times slower than a typical desktop today!  Model 2, introduced in 1962, was twice as fast as Model 1.

So -- would you like to know more about the IBM 1620?  Did you know that there are even people restoring one? Please check out the links on the right-hand side!

Photos of Statistical Computing Center and IBM1620 from the Very First Issue of Sthitisarn
A Photo of Dr. Bundhit's IBM 1620
The IBM 1620 History Site
An IBM 1620 Simulator
The Computer Museum History Center's web site on the IBM 1620
Data Processing System
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