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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 CPU and ALU architectures are those that are based on registers, address buses, or data buses of that size.

1 2 4 8 12 16 18 24 26 31 32 36 48 60 64 128 256 512
8 16 32 64
16 32 40 64 80 128 256
×½ ×1 ×2 ×4 ×8
32 64 128

18 binary digits have 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 architecturesEdit

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 encodingEdit

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.[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.

ReferencesEdit

  1. ^ Digital Equipment Corporation, PDP-9 Utility Programs--Advanced Software System--Programmer's Reference Manual, full text, Digital Equipment Corporation, 1968, Appendix 1.
  2. ^ a b "PDP-7 Symbolic Assembler Programming Manual". p. 6; p. 38-39.