View source for 18-bit computing

{{Short description|Computer architectures using an 18-bit word}}
{{more footnotes|date=October 2009}}
{{N-bit|18|(2.25 [[Octet (computing)|octets]])}}

Eighteen binary digits have 262,144 (1000000 [[octal]], 40000 [[hexadecimal]]) distinct combinations.

Eighteen bits was a common [[Word (computer architecture)|word]] size for smaller computers in the 1960s, when large computers often using [[36-bit|36 bit words]] and [[Six-bit character code|6-bit character sets]], sometimes implemented as [[BCD (character encoding)|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]] [[minicomputer]]s produced by [[Digital Equipment Corporation]] from 1960 to 1975. [[Digital Equipment Corporation|Digital's]] [[PDP-10]] used [[36-bit]] words but had 18-bit addresses.

The [[UNIVAC]] division of [[Remington Rand]] 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|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 system]]s 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==
Eighteen-bit machines use a variety of character encodings.

The [[DEC Radix-50]], called Radix 50<sub>8</sub> format, packs three characters plus two bits in each 18-bit word.<ref>{{Cite manual|title=PDP-9 Utility Programs--Advanced Software System--Programmer's Reference Manual|publisher=[[Digital Equipment Corporation]]|year=1968|location=Maynard, Massachusetts|url=http://www.bitsavers.org/pdf/dec/pdp9/DEC-9A-GUAB-D_UTILITIES.pdf|archive-date=January 25, 2019|url-status=live|archive-url=https://web.archive.org/web/20190125062212/http://www.bitsavers.org/pdf/dec/pdp9/DEC-9A-GUAB-D_UTILITIES.pdf|page=A1-1|chapter=Linking Loader}}</ref>

The Teletype packs three characters in each 18-bit word; each character a 5-bit [[Baudot code]] and an upper-case bit.<ref name="pdp7" />

The [[DEC SIXBIT]] format packs three characters in each 18-bit word,<ref name="pdp7">{{Cite manual|title=PDP-7 Symbolic Assembler Programming Manual|publisher=[[Digital Equipment Corporation]]|year=1965|location=Maynard, Massachusetts|pages=6, 38–39|url=http://bitsavers.trailing-edge.com/pdf/dec/pdp7/PDP-7_AsmMan.pdf|archive-url=https://web.archive.org/web/20170523224514/http://bitsavers.trailing-edge.com/pdf/dec/pdp7/PDP-7_AsmMan.pdf|archive-date=May 23, 2017|url-status=live|access-date=June 18, 2015}}</ref> each 6-bit character obtained by stripping the high bits from the 7-bit [[ASCII]] code, which folds lowercase to uppercase letters.

==References==
{{reflist}}

== Further reading ==
* [http://research.microsoft.com/Users/gbell/Digital/timeline/18-bit.htm DIGITAL Computing Timelime: 18-bit architecture]
* [http://simh.trailing-edge.com/docs/architecture18b.pdf ''Architectural Evolution in DEC’s 18b Computers''], Bob Supnik, 2006.

{{CPU technologies}}

[[Category:Computer data]]