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| IBM 1620 Model I Level G, running. |
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 2 decimal digits (Even-Odd digit pair for numeric data or 1 Alphameric character for text data) at the same time. Each decimal digit was 6-bits, composed of an odd parity Check bit, a Flag bit, and 4 BCD bits for the value of the digit in the following format:
C F 8 4 2 1The Flag bit had several uses:
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", and a 5-digit "Q Address". Fixed-point data "words" could be any size from 2 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 digits for the exponent and 2 to 100 digits for the mantissa).The machine had no programmer-accessible registers: all operations were memory to memory (including the index registers of the 1620 II).
Most of the logic circuitry of the 1620 was a type of resistor-transistor logicResistor-transistor logic RTL is a class of digital circuits built from bipolar junction transistors (BJT), and resistors; it is the earliest transistorized digital logic used. Its main limitation was its limited fan-in, usually 3 inputs being the limit b (RTL) using "drift" transistors (a type of transistor invented by Herbert KroemerHerbert Kroemer (born August 25, 1928) is a Professor of Electrical and Computer Engineering at University of California, Santa Barbara, received a Ph. in theoretical physics in 1952 from the University of Gottingen, Germany, with a dissertation on hot-el in 1957Events January January 2 San Francisco and Los Angeles stock exchanges merge. January 3 Hamilton Watch Company introduces the first electric watch January 4 After 69 years the last issue of Colliers magazine is published January 5 Russell Endean becomes t) for their speed, that IBM referred to as SDTRL. Other IBM circuit types used were referred to as: Alloy (some logic, but mostly various non-logic functions, named for the kind of transistors used), CTRL (another type of RTL, but slower than SDTRL), CTDL (a type of diode-transistor logicDiode-transistor logic DTL is a class of digital circuits built from bipolar junction transistors (BJT), diodes and resistors; it is the direct ancestor of transistor-transistor logic. See also: resistor-transistor logic (RTL), transistor-transistor logic (DTL)), and DL (another type of RTL, named for the kind of transistor used, "drift" transistors).
These circuits were constructed of individual discrete components mounted on single sided paper-phenolic printed circuit boards about 3 inches wide by 5 inches tall with a 16 pin goldFor alternative meanings, see gold (disambiguation Gold is a chemical element in the periodic table that has the symbol Au ( L. aurum and atomic number 79. A soft, shiny, yellow, heavy, malleable, ductile (trivalent and univalent) transition metal, gold d plated edge connector, that IBM referred to as SMS cards (Standard Modular System). The amount of logic on one card was similar to that in one 7400 seriesThe 7400 series of TTL integrated circuit SSI devices were historically important as the first widespread family of IC devices. Modern variants of the family are still used today for "glue logic". Early 7400 series parts were constructed using bipolar tra SSI or simpler MSI package (e.g., 3 to 5 logic gates or a couple of flip-flops).
These boards were inserted in sockets on racks, that 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:
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