
IBM System/360 Model 20

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 emphasize 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. 
Developed by IBM in Böblingen, Germany, the system was intended for data processing and as a replacement for tab equipment. An incompatible small computer, the IBM 1130 introduced the following year, was designed for scientific and engineering computing.
The design of the Model 20 was the result of a tug of war within IBM. John Haanstra had wanted a low-end machine that could execute IBM 1401 instructions. Fred Brooks wanted the machine to have System/360 architecture. The final result was a compromise where cost concerns predominated: the system leased for under $2000 per month including "Multi-Function Card Machine" and line printer, compared to around $3000 for a comparable 1401.
Based on number of systems sold the Model 20 was the most successful model of System/360. According to Pugh "The number of Model 20 processors installed by the end of 1970 in the United States exceeded 7,400." Other models, however, brought in greater revenue. Despite their popularity there are relatively few Model 20s in existence in working condition in 2020.
Architecture.
The Model 20 was available with six memory sizes: 4, 8, 12, 16, 24 and 32 KB. As in other models of System/360 memory is byte-addressable. It has eight 16-bit general purpose registers numbered R8 through R15 which can be used in computations as base registers. All of memory is also directly addressable through a feature, called direct addressing, that combines the twelve bit displacement and the low-order bits of what would normally be the base register field of the instruction (R0-R7) to form a combined fifteen bit address. No storage protection is provided, except for the low 144 bytes of "protected area".
The instruction set is a subset of System/360 consisting of 37 instructions instead of 143, with some incompatible instructions, such as a BASR (Branch And Store Register) rather than BALR (Branch And Link Register). 
Like most other models of System/360 the Model 20 is microprogrammed; it uses transformer read-only storage (TROS).
Peripherals.
As of 1967 the following peripherals were supported:
The system can also have an integrated communications adapter. Two adapter versions were available, one for Synchronous transmit-receive (STR) and one for binary synchronous communications (Bisync).
2311 Disk Storage.
Model 20 disk storage uses IBM 2311 disk drives, model 11 or 12, attached to an integrated "storage control feature" on the CPU. If multiple drives are attached they have to be the same model. Both models use the 1316 disk pack which provides a maximum of 203 cylinders with 10 tracks per cylinder. The model 11 uses all cylinders, the model 12 uses only the outermost 103. Each track is divided into ten sectors of 270 bytes each. The model 11 has a formatted capacity of 5.4 million bytes, and the model 12 of 2.5 million.
IBM 2560 Multi-Function Card Machine.
The IBM 2560 Multi-Function Card Machine (MFCM) is a peripheral first offered on the Model 20. Due to its reliability problems, often involving card jams, the acronym was frequently interpreted as referring to many less favorable names.
The 2560 has two hoppers into which punch cards can be placed. The cards in the second hopper can contain punched cards to be read or blank cards to be punched. Two models were offered:
Card flow is: a read station, a punch station, and on the A2 a print station allowing text to be printed on cards. Cards then flowed into one of the stackers after being processed. This allows it to function as a card reader and a card punch. As a program can issue a Write Card instruction to print on the card after reading card data with a Read Primary Card or Read Secondary Card instruction, it can also, under program control, function as an interpreter and, as a program can issue a Primary Card Stacker Select or Secondary Card Stacker Select instruction to select the stacker into which a card should be put after reading card data, it can function as a card sorter. This allows the Model 20 to replace several separate pieces of punched-card equipment.
IBM's announcement proclaimed the 2560 as providing "a card-handling capability never before possible on a single pass through the system." Since all I/O is performed by the processor, the system is particularly suited to controlling timing-dependent devices such as the 2560, where a card can be read, punched into, printed on, and sent to a specified output stacker with tight timing requirements if full rated speed is to be maintained.
Software.
Disk Programming System.
The Disk Programming System (DPS) is a control program for a Model 20 with at least 12 KiB of memory and one disk drive. The components of DPS are:
"Rollout/Rollin" allows the computer operator to interrupt a running batch program, execute an inquiry program, and then restart the batch job where it was interrupted.
Tape Programming System.
Tape Programming Support (TPS) supports systems with magnetic tape drives but no disk drives.
The minimum configuration for TPS is 4096 bytes of main storage, a card reader, a card punch, a printer and two magnetic tape drives.
On such systems TPS provides an assembler and tape manipulation utility programs.
Additional capabilities are provided on systems with 8192 bytes of main storage,
and still more with four or more magnetic tape drives.
Card Programming Support.
Card Programming System (CPS) is a set of control programs for a Model 20 computer system with only card input/output devices.
Clones.
The UNIVAC 9200 and 9300 processors were clones of the Model 20.
Remaining machines.
Despite having been sold or leased in very large numbers for a mainframe system of its era, only a few of System/360 Model 20 computers remain. These are primarily the property of museums or collectors. Examples of existing systems include:
A running list of remaining System/360s can be found at World Inventory of remaining System/360 CPUs.