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{{Short description|Low-end IBM computer model from 1960s}}
{{Infobox computing device
|
| title = IBM System/360 Model 20
| aka =
| logo = File:IBM
|
| image = LCM - IBM System 360 - 01.jpg
| caption = An IBM System/360 Model 20 (with front panels removed), with IBM 2560 MFCM (Multi-Function Card Machine) at right▼
| image_size = 270px
| caption = An IBM System/360 Model 20, with [[IBM 1442]] at right and [[IBM 2203]] line printer on left
| developer =
| manufacturer = International Business Machines Corporation ([[IBM]])
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| os =
| power =
| cpu =
| memory = 4–32 KB Core
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| display =
| graphics =
| input =
| controllers =
| connectivity =
| platform =
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| dimensions =
| weight =
| compatibility=
| predecessor =
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| website =
}}
The '''IBM System/360 Model 20''' is the smallest member of the [[IBM System/360]] family announced in November 1964.
Developed by IBM in [[Böblingen]], Germany,<ref name=Pugh>{{cite book|last1=Pugh|first1=Emerson W.|last2=Johnson|first2=Lyle R.|last3=Palmer|first3=John H.|title=IBM's 360 and Early 370 Systems|year=1991|publisher=MIT Press|isbn=0-262-16123-0|url=https://archive.org/details/ibms360early370s0000pugh|url-access=registration}}</ref>{{rp|pp.217–218,352}} the system was intended for [[Computer data processing|data processing]] and as a replacement for [[Unit record equipment|
The design of the Model 20 was the result of a tug of war within IBM.
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."<ref name=Pugh />{{rp|p.639}} Other models, however, brought in greater revenue. Despite their popularity there are relatively few Model 20s in existence in working condition in 2020.<ref>{{cite web |last1=Bradley |first1=Adam |last2=Blackburn |first2=Christopher |last3=Vaughan |first3=Peter |title=IBM 360 Model 20 Rescue and Restoration |url=https://ibms360.co.uk/ |accessdate=Mar 2, 2020}}</ref>
[[File:IBM system 360.JPG |thumb|Another IBM System/360 Model 20 CPU.]]
▲
==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.<ref name=FuncChar />{{rp|p.1}} It has eight 16-bit [[Processor register|general purpose registers]] numbered R8 through R15 which can be used in computations as base [[Addressing mode#Base plus offset, and variations|register]]s. All of memory is also directly addressable through a feature, called direct addressing, that combines the twelve
The instruction set is a subset of System/360 consisting of 37 instructions instead of 143,<ref name=FuncChar />{{rp|pp.7–25}}<ref>{{cite book|last=Pugh|first=Emerson W.|title=Building IBM: Shaping an Industry and Its Technology|year=1995|publisher=MIT Press|isbn=0-262-16147-8|url=http://mitpress.mit.edu/catalog/item/default.asp?ttype=2&tid=7339|access-date=2012-10-15|archive-date=2012-05-05|archive-url=https://web.archive.org/web/20120505165349/http://mitpress.mit.edu/catalog/item/default.asp?ttype=2&tid=7339|url-status=dead}}</ref>{{rp|p.384}} with some incompatible instructions, such as a BASR (Branch And Store Register) rather than BALR (Branch And Link Register).
*'''Binary arithmetic''' uses the S/360 "halfword" instructions to operate on 16-bit quantities. Load Halfword, Add Halfword, Subtract Halfword, Compare Halfword, and Store Halfword were available, along with Add Register and Subtract register,
*'''Decimal arithmetic''' includes the complete S/360 decimal instruction set for operations on [[Packed decimal#Packed BCD|packed decimal]] operands of up to 31 digits plus sign in storage.
*'''Logical instructions''' are Move Characters, Move with offset, Move Immediate, Move Numerics, Move Zones, Compare Logical Immediate, Compare Logical Characters, Edit, And immediate, Or immediate, Test under Mask, and Translate.
*'''Branch instructions''' include standard S/360 Branch on Condition. Instead of "Branch and Link" the instruction set uses "Branch and Store" (BASR, X'0D', and BAS, X'4D') which functions similarly except that a 16-bit link address is stored. "Set PSW" (SPSW, X'B1'), which loads a 32 bit ''Program Status Word'' (PSW) replaces the standard "Load PSW". The Model 20 has no concept of ''privileged instructions'' or a distinction between ''supervisor state'' and ''user state'' so SPSW is available to user programs.
*'''Input/output instructions''' are unique to the Model 20. Lacking [[channel I/O|I/O channels]] all peripherals are directly controlled by the processor.
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As of 1967 the following [[peripheral]]s were supported:<ref name=FuncChar />{{rp|pp.36–110}}
*[[IBM 2415]] Magnetic Tape Unit with up to 6 [[Magnetic tape data storage|tape drives]].
*[[#IBM 2560 Multi-Function Card Machine|IBM 2560]] Multi-Function Card Machine.
*[[IBM 2520]] Card Read/Punch or Card Punch.
*[[IBM 1419]] Magnetic Ink Character Reader.
*[[IBM 2311]] Disk Storage (Models 11 and 12).
*[[IBM 1442]] Model 5 Card Punch.
*[[IBM 2203]] Line Printer.
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===2311 Disk Storage===
Model 20 disk storage uses [[History of IBM magnetic disk drives#IBM 2311|IBM 2311 disk drives]], model 11 or 12, attached to an integrated "storage control feature" on the CPU.<ref name=FuncChar />{{rp|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 [[Cylinder (disk drive)|cylinders]] with 10 [[Track (disk drive)|tracks]] per cylinder. The model 11 uses all cylinders, the model 12 uses only the outermost 103. Each track is divided into ten [[Disk sector|sector]]s of 270 bytes each. The model 11 has a formatted capacity of 5.4 million bytes, and the model 12 of 2.5 million.<ref>
===IBM 2560 Multi-Function Card Machine===
The '''IBM 2560 Multi-Function Card Machine''' ('''MFCM''')<ref>{{cite conference
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.▼
|url=https://dl.acm.org/doi/10.1145/1464182.1464221
|title=The IBM 2560 multi-function card machine |author=C. E. Spurrier
|book-title=Proceedings of the April 26–28, 1966, Spring joint computer conference - AFIPS '66 (Spring)
|year=1966|pages=315–321
|doi=10.1145/1464182.1464221
|isbn=9781450378925
|s2cid=24960497
|doi-access=free
▲
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:
The 2560 has two hoppers into which punch cards can be placed as input or as blank cards to be punched, a read station, a punch station, a print station allowing text to be printed on cards, and five stackers into which cards may be placed after they are processed.<ref name=FuncChar/>{{rp|41-43}} This allows it to function as a [[Punched card input/output|card reader]] and a [[Punched card input/output|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 (computing)#Punched card interpreter|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 [[IBM card sorter|card sorter]]. This allows the Model 20 to replace several separate pieces of punched-card equipment.▼
* Model A1: five card stackers, and an optional printing/interpreter, with two, four or six print heads.<ref name=SystemSummary.360_20>{{cite book
|title=IBM System/360 System Summary |at=Model 20 |id=A22-6810 |publisher=[[IBM]]}}</ref>
* Model A2: four card stackers.
▲
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.▼
IBM's announcement proclaimed the 2560 as providing "a card-handling capability never before possible on a single pass through the system."<ref>{{cite web
|url=http://ed-thelen.org/comp-hist/IBM-ProdAnn/360-20.pdf
|title=IBM 360 Model 20 customer engineering announcement}}</ref><ref name=SystemSummary.360_20 />{{rp|page=13}}<ref>{{cite book
|url=http://bitsavers.org/pdf/ibm/360/fe/2020/Z26-5908_360-20_FE_Manual_Of_Instruction_Aug65.pdf
▲
==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:<ref>{{cite book
* The ''[[Initial Program Load#IBM Initial Program Load|Initial Program Loader]]'' (IPL).
* A small ''resident monitor'' which handles disk input/output, error recovery, and job-to-job transition, and loads programs to be executed from the ''core-image library'' on disk.
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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.<ref>
===Card Programming Support===
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===Languages===
* [[IBM RPG|Report Program Generator (RPG)]].
* [[PL/I]] subset, DPS only.<ref>{{cite book
* [[IBM Basic assembly language|Basic Assembler]] versions were available for CPS, TPS, and DPS including [[Input/Output Control System]] (IOCS) macros and processing routines.
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==Clones==
The [[UNIVAC 9000 series|UNIVAC 9200 and 9300
==Remaining machines==
Despite having been sold or leased in very large numbers for a [[mainframe computer|mainframe]] system of its era, only a few
* Two Model 20 processors along with numerous peripherals (forming at least one complete system) located in Nürnberg, Germany were purchased on [[eBay]] in April/May 2019 for €3710 by two UK enthusiasts who, over the course of some months, moved the machine to [[Creslow Park]] in [[Buckinghamshire]], United Kingdom. The system was in a small, abandoned building left untouched for decades, and apparently had been used in that building since all peripherals were still fully wired and interconnected
*
▲* A Model 20 processor along with some peripherals is currently on public display at the [[Deutsches Museum]] in [[Munich]], [[Germany]].<ref name="ibm360deutsches">{{cite web|url=http://www.deutsches-museum.de/en/exhibitions/communication/computers/universal-computers/|title=Deutsches Museum - Computers with semiconductor components|access-date=2020-03-18|year=2020}}</ref>
A running list of remaining System/360s can be found at [http://www.ibmsystem3.nl/System360/ World Inventory of remaining System/360 CPUs].
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==Further reading==
*[http://bitsavers.trailing-edge.com/pdf/ibm/360/model20/E20-0098-0_basicApplications_360-20.pdf ''Basic Applications — System/360 Model 20 (E20-0098-0)''] "The purpose of this manual is to describe the IBM System/360, Model 20, and show how it may be used to process six basic applications: billing, accounts receivable, sales analysis, inventory control, accounts payable, and payroll."
* [http://bitsavers.informatik.uni-stuttgart.de/pdf/ibm/360/model20/C26-3810-3_360-20_diskUtils_Mar69.pdf IBM System/360 Model 20 / Disk Programming System / Disk Utility Programs]
*[http://bitsavers.trailing-edge.com/pdf/ibm/360/model20/C24-9006-4_260-20_CtlPgm_Mar69.pdf IBM System/360 Model 20 Disk Programming System Control and Service Programs] "This publication describes the purpose and functions of the Control and Service programs of the IBM System/360 Model 20 Disk Programming System (DPS)."
*[http://bitsavers.trailing-edge.com/pdf/ibm/360/model20/GC26-3602-5_360_20_Card_Programming_Support_Basic_Assembler_Language_Jan71.pdf IBM System/360 Model 20 / Card Programming Support / Basic Assembler Language]
▲*[http://bitsavers.trailing-edge.com/pdf/ibm/360/model20/GC26-3602-5_360_20_Card_Programming_Support_Basic_Assembler_Language_Jan71.pdf IBM System/360 Model 20 / Card Programming Support / Basic Assembler Language] “This reference publication provides programmers with the information required to write programs in the Basic Assembler language of the IBM System/360 Model 20.”
{{IBM System/360 line}}
{{DEFAULTSORT:IBM System 360 Model 20}}
[[Category:IBM System/360 mainframe line|System
[[Category:16-bit computers]]
[[Category:Remote job entry]]
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