18-bit computing
Computer architectures using an 18-bit word From Wikipedia, the free encyclopedia
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.
 | This article includes a list of general references, but it lacks sufficient corresponding inline citations. (October 2009) |
Eighteen binary digits have 262,144 (1000000 octal, 40000 hexadecimal) distinct combinations.
Eighteen bits was a common word size for smaller computers in the 1960s, when large computers often using 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.
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 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 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.
Eighteen-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.
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