In computer architecture, 18-bit integers, memory addresses, and other data units are 18 bits (2.25 octets) wide. Architectures with 18-bit CPUs and ALUs use registers, address buses, or data buses of this size. Eighteen bits represent 262,144 distinct values (1000000 octal or 40000 hexadecimal). This was a common word size in 1960s smaller computers, contrasting with larger systems using 36-bit words and 6-bit character sets, often based on BCD extensions. Additionally, 18-bit teletypes were 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 UNIVAC division of Remington Rand 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
Eighteen-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,3 each 6-bit character obtained by stripping the high bits from the 7-bit ASCII code, which folds lowercase to uppercase letters.
Further reading
- DIGITAL Computing Timelime: 18-bit architecture
- Architectural Evolution in DEC’s 18b Computers, Bob Supnik, 2006.
References
"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. http://www.bitsavers.org/pdf/dec/pdp9/DEC-9A-GUAB-D_UTILITIES.pdf ↩
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. http://bitsavers.trailing-edge.com/pdf/dec/pdp7/PDP-7_AsmMan.pdf ↩
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. http://bitsavers.trailing-edge.com/pdf/dec/pdp7/PDP-7_AsmMan.pdf ↩