Machine Overview

The IBM System/360 Model 20 is the smallest member of the IBM System/360 family announced in November 1964. The Model 20 supports only a subset of the System/360 instruction set, with binary numbers limited to 16 bits and no floating point. In later years it would have been classified as a 16-bit minicomputer rather than a mainframe, but the term “minicomputer” was not current, and in any case IBM wanted to emphasise the compatibility of the Model 20 rather than its differences from the rest of the System/360 line. It does, however, have the full System/360 decimal instruction set, that allows for addition, subtraction, product, and dividend of up to 31 decimal digits.

The Model 20 was mainly intended for business applications. Its storage was limited, just 4K to 32K bytes of core storage, and it was extremely slow even by 1960s standards, performing around 5700 additions per second. This slow CPU was enough to generate business reports from punch cards since the card reader only read 8 cards per second. To reduce its price, the Model 20 implemented a subset of the S/360 instructions and used half-sized registers – 8, making it incompatible with the rest of the S/360 line. Despite its limitations, the Model 20 was the most popular S/360 model due to its low price, with more than 7,400 Model 20s in operation by the end of 1970. The monthly rental price of the Model 20 in the USA started at $1280 with the purchase price starting at $62,710.

Our machine specifications are thus-far unchecked as the machines location doesn’t currently really allow for in-depth investigations. Below is the list of items we have acquired:

2203System 360 Printer1
360 2020360/20 Processor2
2311Disk Drive2
2152System Console1
2560Punched Card Reader/Punch/Sorter1
2501Punched Card Reader1
2415 IITape Drives2
3125370/125 Processor1
1403Printer (for the blue system)1
29Manual Card Punch1
3504Possibly a 370 card reader, but half appears to be missing1
5471System Console1