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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.
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, sometimes implemented as extensions of BCD, were the norm. There were also 18-bit teletypes experimented with in the 1940s.
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. Digital's PDP-10 used 36-bit words but had 18-bit addresses.
The BCL Molecular 18 was a group of systems designed and manufactured in the UK in the 1970s and 1980s.
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.
18-bit machines use a variety of character encodings.
- "Linking Loader". PDP-9 Utility Programs--Advanced Software System--Programmer's Reference Manual (PDF). Maynard, Massachusetts: Digital Equipment Corporation. 1968. p. A1-1. Archived (PDF) from the original on January 25, 2019.
- PDP-7 Symbolic Assembler Programming Manual (PDF). Maynard, Massachusetts: Digital Equipment Corporation. 1965. pp. 6, 38–39. Archived (PDF) from the original on May 23, 2017. Retrieved June 18, 2015.