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In computer architecture, 18-bit integers, memory addresses, or other data units are those that are at most 18 bits (2.25 octets) wide. Also, 18-bit CPU and ALU architectures are those that are based on registers, address buses, or data buses of that size.
Contents
18 binary digits have 7005262144000000000♠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 were the norm.
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.
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.
The Teletype packs three characters in each 18-bit word; each character a 5-bit Baudot code and an upper-case bit.
The DEC SIXBIT format packs three characters in each 18-bit word, each 6-bit character obtained by stripping the high bits from the 7-bit ASCII code, which folds lowercase to uppercase letters.