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18-bit computing

March 06, 2023

In computer architecture, 18-bit integers, memory addresses, or other data units are those that are 18 bits (2.25 octets) wide. Also, 18-bit central processing unit (CPU) and arithmetic logic unit (ALU) architectures are those that are based on registers, address buses, or data buses of that size.

18 binary digits have 262144 (1000000 octal, 40000 hexadecimal) distinct combinations.

18 bits was a common word size for smaller computers in the 1960s, when large computers often used 36 bit words and 6-bit character sets, sometimes implemented as extensions of BCD, were the norm. There were also 18-bit teletypes experimented with in the 1940s.

Example computer architectures

Possibly the most well-known 18-bit computer architectures are the PDP-1, PDP-4, PDP-7, PDP-9 and PDP-15 minicomputers produced by Digital Equipment Corporation from 1960 to 1975. Digital's PDP-10 used 36-bit words but had 18-bit addresses.

The UNIVAC produced several 18-bit computers, including the UNIVAC 418 and several military systems.

The IBM 7700 Data Acquisition System was announced by IBM on December 2, 1963.

The BCL Molecular 18 was a group of systems designed and manufactured in the UK in the 1970s and 1980s.

The NASA Standard Spacecraft Computer NSSC-1 was developed as a standard component for the MultiMission Modular Spacecraft at the Goddard Space Flight Center (GSFC) in 1974.

The flying-spot store digital memory in the first experimental electronic switching systems used nine plates of optical memory that were read and written two bits at a time, producing a word size of 18 bits.

Character encoding

18-bit machines use a variety of character encodings.

The DEC Radix-50, called Radix 508 format, packs three characters plus two bits in each 18-bit word.[1]

The Teletype packs three characters in each 18-bit word; each character a 5-bit Baudot code and an upper-case bit.[2]

The DEC SIXBIT format packs three characters in each 18-bit word,[2] each 6-bit character obtained by stripping the high bits from the 7-bit ASCII code, which folds lowercase to uppercase letters.

References

  1. ^ "Linking Loader". PDP-9 Utility Programs--Advanced Software System--Programmer's Reference Manual (PDF). Maynard, Massachusetts: Digital Equipment Corporation. 1968. p. A1-1. (PDF) from the original on January 25, 2019.
  2. ^ a b PDP-7 Symbolic Assembler Programming Manual (PDF). Maynard, Massachusetts: Digital Equipment Corporation. 1965. pp. 6, 38–39. (PDF) from the original on May 23, 2017. Retrieved June 18, 2015.
  • DIGITAL Computing Timelime: 18-bit architecture
  • Architectural Evolution in DEC’s 18b Computers, Bob Supnik, 2006.

March 06, 2023
computing, this, article, includes, list, general, references, lacks, sufficient, corresponding, inline, citations, please, help, improve, this, article, introducing, more, precise, citations, october, 2009, learn, when, remove, this, template, message, comput. This article includes a list of general references but it lacks sufficient corresponding inline citations Please help to improve this article by introducing more precise citations October 2009 Learn how and when to remove this template message In computer architecture 18 bit integers memory addresses or other data units are those that are 18 bits 2 25 octets wide Also 18 bit central processing unit CPU and arithmetic logic unit ALU architectures are those that are based on registers address buses or data buses of that size 18 binary digits have 262144 1000000 octal 40000 hexadecimal distinct combinations 18 bits was a common word size for smaller computers in the 1960s when large computers often used 36 bit words and 6 bit character sets sometimes implemented as extensions of BCD were the norm There were also 18 bit teletypes experimented with in the 1940s Example computer architectures EditPossibly the most well known 18 bit computer architectures are the PDP 1 PDP 4 PDP 7 PDP 9 and PDP 15 minicomputers produced by Digital Equipment Corporation from 1960 to 1975 Digital s PDP 10 used 36 bit words but had 18 bit addresses The UNIVAC produced several 18 bit computers including the UNIVAC 418 and several military systems The IBM 7700 Data Acquisition System was announced by IBM on December 2 1963 The BCL Molecular 18 was a group of systems designed and manufactured in the UK in the 1970s and 1980s The NASA Standard Spacecraft Computer NSSC 1 was developed as a standard component for the MultiMission Modular Spacecraft at the Goddard Space Flight Center GSFC in 1974 The flying spot store digital memory in the first experimental electronic switching systems used nine plates of optical memory that were read and written two bits at a time producing a word size of 18 bits Character encoding Edit18 bit machines use a variety of character encodings The DEC Radix 50 called Radix 508 format packs three characters plus two bits in each 18 bit word 1 The Teletype packs three characters in each 18 bit word each character a 5 bit Baudot code and an upper case bit 2 The DEC SIXBIT format packs three characters in each 18 bit word 2 each 6 bit character obtained by stripping the high bits from the 7 bit ASCII code which folds lowercase to uppercase letters References Edit Linking Loader PDP 9 Utility Programs Advanced Software System Programmer s Reference Manual PDF Maynard Massachusetts Digital Equipment Corporation 1968 p A1 1 Archived PDF from the original on January 25 2019 a b PDP 7 Symbolic Assembler Programming Manual PDF Maynard Massachusetts Digital Equipment Corporation 1965 pp 6 38 39 Archived PDF from the original on May 23 2017 Retrieved June 18 2015 DIGITAL Computing Timelime 18 bit architecture Architectural Evolution in DEC s 18b Computers Bob Supnik 2006 Retrieved from https en wikipedia org w index php title 18 bit computing amp oldid 1059010096, wikipedia, wiki, book, books, library,

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