This unit (commonly called "1620" until the Model II was introduced) was produced as inexpensively as IBM could make it, in order to keep the price low. One industry magazine (Datamation) mentioned that the 1620 was the first IBM computer for which the basic system could be leased for a monthly rate less than its number :-)
It did not even have conventional ALU hardware: all arithmetic was done by table lookup in core memory. Addition and Subtraction used a 100 digit table (@ address 00300..00399). Multiplication used a 200 digit table (@ address 00100..00299). In the basic machine division used software subroutines, but optional divide hardware could be installed using a repeated subtraction algorithm. Floating point arithmetic instructions were an available option (if the divide option was installed).
| IBM 1620 Model I Level G, running. |
The first 20,000 decimal digits of Core memory were internal to the CPU itself (which reduced the floor space requirements of the basic system). Expansion to either 40,000 or 60,000 decimal digits required the addition of an IBM 1623 Memory unit. The Memory cycle time was 20μs (i.e., the memory speed was 50kHz = 1/20th of a MHz).
The central processor clock speed was 1MHz, which was divided by 20 by a 10 position ring counter to provide the system timing/control signals.
Instructions took 8 Memory cycles (160μs) to fetch and a variable number of Memory cycles to execute. Indirect addressing added 4 Memory cycles (80μs) for each level of indirection.
:Image:1620 Model I Layout.png shows the internal layout of the "gates" of the Model I.