IBM System/360 Model 20

An IBM System/360 Model 20 (with front panels removed), with IBM 2560 MFCM (Multi-Function Card Machine) at right

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.[1] 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.

Developed by IBM in Böblingen, Germany,[2]:pp.217–218,352 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 MFCM and line printer, compared to around $3000 for a comparable 1401.[2]:p.446

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."[2]:p.639 Other models, however, brought in greater revenue.

Another IBM System/360 Model 20 CPU.

Architecture

The Model 20 was available with six memory sizes: 4, 8, 12, 16, 24 and 32 KiB. As in other models of System/360 memory is byte-addressable.[1]:p.1 It has eight 16 bit general purpose registers numbered R8 thru 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.[1]:p.4 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,[1]:pp.7–25[3]:p.384 with some incompatible instructions, such as a BASR (Branch And Store Register) rather than BALR (Branch And Link Register).

Peripherals

As of 1967 the following peripherals were supported:[1]:pp.36–110

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.[1]:pp.58–76 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.[4]

Multi-Function Card Machine

A peripheral first offered on the Model 20 is the IBM 2560 Multi-Function Card Machine (MFCM). Because of its reliability problems, often involving card jams, the acronym was frequently interpreted as referring to many less favorable names. Nevertheless the 2560 combines the functions of a card reader, card punch, interpreter, and card sorter, allowing the Model 20 to replace several separate pieces of punched-card equipment.

Because 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 was 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:[5]

Rollin/rollout allows the computer operator to interrupt a running batch program, execute an inquiry program, and then restart the batch job where it was interrupted.

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.

Languages

Other IBM programs

Clones

The UNIVAC 9200 and 9300 processors were clones of the Model 20.

See also

References

  1. 1 2 3 4 5 6 IBM Corporation (1964). IBM System/360 Model 20 Functional Characteristics (PDF).
  2. 1 2 3 Pugh, Emerson W.; Johnson, Lyle R.; Palmer, John H. (1991). IBM's 360 and Early 370 Systems. MIT Press. ISBN 0-262-16123-0.
  3. ↑ Pugh, Emerson W. (1995). Building IBM: Shaping an Industry and Its Technology. MIT Press. ISBN 0-262-16147-8.
  4. ↑ “IBM 2311 Disk Storage Drive Models 1, 11 and 12 Field Engineering Theory of Operation” describes the IBM 2311, including models 11 and 12, used on System/360 model 20
  5. ↑ IBM Corporation (1969). IBM System/360 Model 20 Disk Programming System Control and Service Programs (PDF).
  6. ↑ IBM Corporation (1968). IBM System/360 Model 20 Disk Programming System PL/I (PDF).

Further reading

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