18 binary digits have (1000000

octal The octal numeral system, or oct for short, is the radix, base-8 number system, and uses the Numerical digit, digits 0 to 7. This is to say that 10octal represents eight and 100octal represents sixty-four. However, English, like most languages, ...

hexadecimal In mathematics and computing, the hexadecimal (also base-16 or simply hex) numeral system is a positional numeral system that represents numbers using a radix (base) of 16. Unlike the decimal system representing numbers using 10 symbols, h ...

) distinct combinations. 18 bits was a common

word A word is a basic element of language that carries an objective or practical meaning, can be used on its own, and is uninterruptible. Despite the fact that language speakers often have an intuitive grasp of what a word is, there is no consen ...

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 The PDP-1 (''Programmed Data Processor-1'') is the first computer in Digital Equipment Corporation's PDP series and was first produced in 1959. It is famous for being the computer most important in the creation of hacker culture at Massachusett ...

PDP-4 The PDP-4 was the successor to the Digital Equipment Corporation's PDP-1. History This 18-bit machine, first shipped in 1962, was a compromise: "with slower memory and different packaging" than the PDP-1, but priced at $65,000 - less than half t ...

, PDP-7, PDP-9 and PDP-15

minicomputer A minicomputer, or colloquially mini, is a class of smaller general purpose computers that developed in the mid-1960s and sold at a much lower price than mainframe and mid-size computers from IBM and its direct competitors. In a 1970 survey, ...

s produced by

Digital Equipment Corporation Digital Equipment Corporation (DEC ), using the trademark Digital, was a major American company in the computer industry from the 1960s to the 1990s. The company was co-founded by Ken Olsen and Harlan Anderson in 1957. Olsen was president unti ...

from 1960 to 1975. Digital's

PDP-10 Digital Equipment Corporation (DEC)'s PDP-10, later marketed as the DECsystem-10, is a mainframe computer family manufactured beginning in 1966 and discontinued in 1983. 1970s models and beyond were marketed under the DECsystem-10 name, especi ...

used 36-bit words but had 18-bit addresses. The

UNIVAC UNIVAC (Universal Automatic Computer) was a line of electronic digital stored-program computers starting with the products of the Eckert–Mauchly Computer Corporation. Later the name was applied to a division of the Remington Rand company and ...

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 The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research. NASA was established in 1958, succeedi ...

Standard Spacecraft Computer NSSC-1 was developed as a standard component for the

MultiMission Modular Spacecraft Multi-mission Modular Spacecraft, also known as the MMS, was originally designed by NASA to serve the largest array of functions for the space program possible to decrease the cost of space missions. It was designed to operate in four distinct are ...

at the

Goddard Space Flight Center The Goddard Space Flight Center (GSFC) is a major NASA space research laboratory located approximately northeast of Washington, D.C. in Greenbelt, Maryland, United States. Established on May 1, 1959 as NASA's first space flight center, GSFC emp ...

(GSFC) in 1974. The

flying-spot store The flying-spot store was an optical digital memory used in early stored program control components of electronic switching systems. The flying-spot store used a photographic plate as the store of binary data. Each ''spot'' on the plate was an op ...

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 A six-bit character code is a character encoding designed for use on computers with word lengths a multiple of 6. Six bits can only encode 64 distinct characters, so these codes generally include only the upper-case letters, the numerals, some pun ...

format packs three characters in each 18-bit word, each 6-bit character obtained by stripping the high bits from the 7-bit

ASCII ASCII ( ), abbreviated from American Standard Code for Information Interchange, is a character encoding standard for electronic communication. ASCII codes represent text in computers, telecommunications equipment, and other devices. Because ...

code, which folds lowercase to uppercase letters.

References

DIGITAL Computing Timelime: 18-bit architecture

''Architectural Evolution in DEC’s 18b Computers''
Bob Supnik, 2006. {{CPU technologies Computer data