IBM 1620

IBM 1620

The IBM 1620 was announced by IBM on October 21, 1959 and marketed as an inexpensive "scientific computer". After a total production of about two thousand machines, it was withdrawn on November 19, 1970. Modified versions of the 1620 were used as the CPU of the IBM 1710 and IBM 1720 Industrial Process Control Systems (making it the first digital computer considered reliable enough for real-time process control of factory equipment).

Being variable word length decimal, as opposed to fixed-word-length pure binary, made it as especially attractive first computer to learn on — and hundreds of thousands of students had their first experiences with a computer on the IBM 1620.

Core memory cycle times were 20 microseconds for the Model I, 10 microseconds for the Model II (about a thousand times slower than typical computer main memory in 2006).

Many in the user community recall the 1620 being referred to as "CADET", jokingly meaning "Can't Add, Doesn't Even Try", referring to the use of addition tables in memory rather than dedicated addition circuitry. For an explanation of all three known interpretations of the machine's code name see the section on the machine's development history.

The 1620's architecture

It was a variable "word" length decimal (BCD) computer with a memory that could hold anything from 20,000 to 60,000 decimal digits increasing in 20,000 decimal digit increments. (While the 5-digit addresses could have addressed 100,000 decimal digits, no machine larger than 60,000 decimal digits was ever built.)

Memory was accessed two decimal digits at the same time (even-odd digit pair for numeric data or one "alphameric" character for text data). Each decimal digit was 6 bits, composed of an odd parity Check bit, a Flag bit, and four BCD bits for the value of the digit in the following format: C F 8 4 2 1The Flag bit had several uses:
*In the least significant digit it was set to indicate a negative number (signed magnitude).
*It was set to mark the most significant digit of a number ("wordmark").
*In the least significant digit of 5-digit addresses it was set for indirect addressing (an option on the 1620 I, standard on the 1620 II). Multi-level indirection could be used (you could even put the machine in an infinite indirect addressing loop).
*In the middle 3 digits of 5-digit addresses (on the 1620 II) they were set to select one of 7 index registers.In addition to the valid BCD digit values there were three "special" digit values (these could NOT be used in calculations): C F 8 4 2 1 1 0 1 0 - Record Mark (right most end of record) 1 1 0 0 - Numeric Blank (blank for punched card output formatting) 1 1 1 1 - Group Mark (right most end of a group of records for disk I/O)

Instructions were fixed length (12 decimal digits), consisting of a 2-digit "op code", a 5-digit "P Address" (usually the "destination" address, and a 5-digit "Q Address" (usually the "source" address or the "source" immediate value). Some instructions, such as the B (branch) instruction, only used the P Address, and later smart assemblers included a "B7" instruction that generated an 7-digit branch instruction (op code, P address, and one extra digit because the next instruction had to start on an even-numbered digit).

Fixed-point data "words" could be any size from two decimal digits up to all of memory not used for other purposes.

Floating-point data "words" (using the hardware floating point option) could be any size from 4 decimal digits up to 102 decimal digits (2 to 100 digits for the mantissa and 2 digits for the exponent).

The machine had no programmer-accessible registers: all operations were memory to memory (including the index registers of the 1620 II).

Character and Op codes

The table below lists Alphameric mode Characters (and Op codes).

Paper Tape reader/punch

The 1621 Tape reader and 1624 Tape punch controls.
*Power switch – With this switch on the reader is powered anytime the 1620 is powered.
*Reel-Strip switch – This switch selects whether Reels or Strips of paper tape are used.
*Reel power key – Applies power to the supply and takeup Reels to position the tape for reading and places the reader in ready state.
*Non-process runout key – Feeds tape until the reader is empty and takes the reader out of ready state.

Management was not entirely convinced that core memory could be made to work in small machines, so Gerry Ottaway was loaned to the team to design a drum memory as a backup. During acceptance testing by the Product Test Lab, repeated core memory failures were encountered and it looked likely that management's predictions would come true. However at the last minute it was found that the muffin fan used to blow hot air through the core stack was malfunctioning, causing the core to pick up noise pulses and fail to read correctly. After the fan problem was fixed, there were no further problems with the core memory and the drum memory design effort was discontinued as unnecessary.

Transferred to San Jose for production

Following announcement of the IBM 1620 on October 21, 1959, due to an internal reorganization of IBM, it was decided to transfer the computer from the Data Processing Division at Poughkeepsie (large scale mainframe computers only) to the General Products Division at San Jose (small computers and support products only) for manufacturing.

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. This stuck and became very well known among the user community.

Implementation "levels"

*Model I
**Level A; prototype.
***All flip-flops in the design were transistorized versions of the original "Eccles-Jordan trigger circuit". While this machine was fully functional, it was found that the capacitor coupling used in these proved troublesome in the noisy signal environment of relays and timing cam driven switches used to drive the console typewriter. This necessitated a "complete redesign" of the machine to use "S-R" flip-flops instead (except for two triggers used to generate clocks for the "S-R" flip-flops). However usage of the term "Trigger" was retained in all the documentation when referring to a flip-flop, as it was IBM's conventional term (as "alphamerics" was their term for alphanumerics).
***This is the only level using a one piece vertical control panel, when the design was transferred from Poughkeepsie to San Jose it was redesigned to the two piece angled control panel used on all production models.
**Level B; first production.
***This is the only level using a burnished aluminum lower control panel, later levels finished this panel with white.
**Level C; introduction of 1622 card reader/punch.
**Level D; introduction of 1311 disk drives and addition of optional "Gate J" containing disk control logic.
**Level E; introduction of Floating Point option.
**Level F
**Level G; introduction of Interrupt option (needed for IBM 1710).
***Did not support BT & BB subroutines in interrupt code!
***Disk control logic on "Gate J" logic merged into "Gate A" & "Gate B".
****Made possible because much of logic was compacted using cards designed for the Model II.
**Level H; improved Interrupt option that supported BT & BB subroutines in interrupt code.
***Final version of the Model I.
*Model II (no information on "Levels" available at this time)::The 1620 Model II introduced basic ALU hardware for addition and subtraction (making "Can't Add, Doesn't Even Try" no longer applicable) and index registers.
*Model III
**Work was begun on a 1620 Model III but the project was quickly canceled as IBM wanted to promote sales of their new System/360 and discontinue the old lines.


Related peripheral units

Available peripherals were:
*IBM 1621 - Paper tape reader
*IBM 1622 - Punch card reader/punch
*IBM 1624 - Paper tape punch (sat inside the 1621 on a shelf)
*IBM 1626 - Plotter controller
*IBM 1627 - Plotter
*IBM 1311 - Disk drive: Model 3 master drive controlling up to 3–Model 2 slave drives.
*IBM 1443 - Printer, flying type bar

Use in film and television

The fictional computer Colossus of ' used about a dozen scrapped 1620 front panels purchased on the surplus market, in various orientations. A similar arrangement was used in a late episodeFact|date=April 2008 of "The Man from U.N.C.L.E." to portray a "THRUSH""' supercomputer.


External links

* [ IBM 1620 restoration project]
* [ 1620 Data Processing System]
* [ IBM 1620 documents] from
* [ IBM 1620 Simulator Applet] (part of the IBM 1620 restoration project)

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