[3] The IBM 1620 Model I was a variable "word" length decimal (BCD) computer using core memory.

[13] Since the Model I used in-memory lookup tables for addition/subtraction,[14] limited bases (5 to 9) unsigned number arithmetic could be performed by changing the contents of these tables, but noting that the hardware included a ten's complementer for subtraction (and addition of oppositely signed numbers).

However an optional special feature in hardware for octal input/output, logical operations, and base conversion to/from decimal was available.

Although bases other than 8 and 10 were not supported, this made the Model II very practical for applications that needed to manipulate data formatted in octal by other computers (e.g., the IBM 7090).

The general RN (read numeric) and WN (write numeric) instructions had assembly language mnemonics that supplied the "device" code in the second address field, and the control code in the low-order digit of the second address field.

To simplify input and output, there were two instructions: The Model II used a modified Selectric typewriter, which could type at 15.5 cps – a 55% improvement.

Available peripherals were: The standard "output" mechanism for a program was to punch cards, which was faster than using the typewriter.

The "operating system" for the computer constituted the human operator, who would use controls on the computer console, which consisted of a front panel and typewriter, to load programs from the available bulk storage media such as decks of punched cards or rolls of paper tape that were kept in cabinets nearby.

Later, the model 1311 disc storage device attached to the computer enabled a reduction in the fetch and carry of card decks or paper tape rolls, and a simple "Monitor" operating system could be loaded to help in selecting what to load from disc.

Instead there was a button on the console called Modify which when pressed together with the Check Reset button, when the computer was in Manual mode, would set the computer in a mode that would clear all of memory in a tenth of one second regardless of how much memory you had; when you pressed Start.

Loading from either tape or disk required first typing a "bootstrap" routine on the console typewriter.

Programs were prepared ahead of time, offline, on paper tape or punched cards.

But usually the programmers were allowed to run the programs personally, hands-on, instead of submitting them to operators as was the case with mainframe computers at that time.

Aside from debugging, scientific programming is typically exploratory, by contrast to commercial data processing where the same work is repeated on a regular schedule.

The next most important items on the console were the buttons labeled Start, Stop-SIE, and Instant Stop-SCE.

All of main memory could be cleared from the console by entering and executing a transfer instruction from address to address +1, this would overwrite any word mark, that would normally stop a transfer instruction, and wrap around at the end of memory.

The FORTRAN II compiler and SPS assembler were somewhat cumbersome to use[26][27] by modern standards, however, with repetition, the procedure soon became automatic and you no longer thought about the details involved.

One of these was developed by Bob Richardson,[28][29] a programmer at Rice University, the FLAG (FORTRAN Load-and-Go) compiler.

For instance, at Auckland University a batch job processor for student assignments (typically, many small programs not requiring much memory) chugged through a class lot rather faster than the later IBM 1130 did with its disk-based system.

The various decks of cards constituting the compiler and loader no longer need be fetched from their cabinets but could be stored on disk and loaded under the control of a simple disk-based operating system: a lot of activity becomes less visible, but still goes on.

These boards were inserted into sockets mounted in door-like racks which IBM referred to as gates.

The machine had the following "gates" in its basic configuration: There were two different types of core memory used in the 1620: The address decoding logic of the Main memory also used two planes of 100 pulse transformer cores per module to generate the X-Y Line half-current pulses.

There were two models of the 1620, each having totally different hardware implementations: In 1958 IBM assembled a team at the Poughkeepsie, New York development laboratory to study the "small scientific market".

Initially the team consisted of Wayne Winger (Manager), Robert C. Jackson, and William H. Rhodes.

To compete effectively would require use of technologies that IBM had developed for larger computers, yet the machine would have to be produced at the least possible cost.

Following transfer to San Jose, someone there jokingly suggested that the code name CADET actually stood for "Can't Add, Doesn't Even Try", referring to the use of addition tables in memory rather than dedicated addition circuitry (and SDTRL actually stood for "Sold Down The River Logic" became a common joke among the CEs).

[33] In 1963 an IBM 1620 was installed at IIT Kanpur providing the kicker for India's software prowess.

[34] In 1964 at the Australian National University, Martin Ward used an IBM 1620 model I to calculate the order of the Janko group J1.

[35] In 1966 the ITU produced an explanatory film on a 1963 system for typesetting by computer at the Washington Evening Star, using an IBM 1620 and a Linofilm phototypesetter.