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Incomputer architecture,18-bitintegers,memory addresses, or otherdata units are those that are 18bits (2.25octets) wide. Also, 18-bitcentral processing unit (CPU) andarithmetic logic unit (ALU) architectures are those that are based onregisters,address buses, ordata buses of that size.
Eighteen binary digits have 262,144 (1000000octal, 40000hexadecimal) distinct combinations.
Eighteen bits was a commonword size for smaller computers in the 1960s, when large computers often using36 bit words and6-bit character sets, sometimes implemented asextensions 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 thePDP-1,PDP-4,PDP-7,PDP-9 andPDP-15minicomputers produced byDigital Equipment Corporation from 1960 to 1975.Digital'sPDP-10 used36-bit words but had 18-bit addresses.
TheUNIVAC division ofRemington Rand produced several 18-bit computers, including the 1963UNIVAC 418 and several military systems.
TheIBM 7700 Data Acquisition System was announced byIBM on December 2, 1963.
The 1964CDC 6000 series and successorsCDC 7600 andCDC Cyber 70 and 170 series used60-bit words but had 18-bit addresses.
TheBCL Molecular 18 was a group of systems designed and manufactured in the UK in the 1970s and 1980s.
TheNASA Standard Spacecraft ComputerNSSC-1 was developed as a standard component for theMultiMission Modular Spacecraft at theGoddard Space Flight Center (GSFC) in 1974.
Theflying-spot store digital memory in the first experimentalelectronic 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.
TheDEC 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-bitBaudot code and an upper-case bit.[2]
TheDEC SIXBIT format packs three characters in each 18-bit word,[2] each 6-bit character obtained by stripping the high bits from the 7-bitASCII code, which folds lowercase to uppercase letters.