IBM 1620

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 an especially attractive first computer to learn on; hundreds of thousands of students had their first computer experience on the IBM 1620. Core memory cycle times were 20 microseconds for the (earlier) Model I, 10 microseconds for the Model II (about a thousand times slower than typical computer main memory in 2006). The Model II was introduced in 1962. The IBM 1620 Model I was a variable word length, binary-coded decimal (BCD) computer using magnetic-core memory. The Model I core could hold 20,000 decimal digits with each digit stored in six bits. More memory could be added with the IBM 1623 Storage Unit, Model 1 which held 40,000 digits, or the 1623 Model 2 which held 60,000.