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   » » Wiki: 18-bit
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18-bit
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18 binary digits have (1000000 , 40000 ) 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 produced by Digital Equipment Corporation from 1960 to 1975.

The 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 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.Digital Equipment Corporation, PDP-9 Utility Programs--Advanced Software System--Programmer's Reference Manual, full text, Digital Equipment Corporation, 1968, Appendix 1.

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

The format packs three characters in each 18-bit word, "PDP-7 Symbolic Assembler Programming Manual". p. 6; p. 38-39. 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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