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, ...
, 40000
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, hexa ...
) distinct combinations.
18 bits was a common
word
A word is a basic element of language that carries an semantics, objective or pragmatics, practical semantics, meaning, can be used on its own, and is uninterruptible. Despite the fact that language speakers often have an intuitive grasp of w ...
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 Massachusetts ...
,
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 th ...
,
PDP-7
The PDP-7 was a minicomputer produced by Digital Equipment Corporation as part of the PDP series. Introduced in 1964, shipped since 1965, it was the first to use their Flip-Chip technology. With a cost of , it was cheap but powerful by the st ...
,
PDP-9
The PDP-9, the fourth of the five 18-bit minicomputers produced by Digital Equipment Corporation, was introduced in 1966. A total of 445 PDP-9 systems were produced, of which 40 were the compact, low-cost PDP-9/L units..
History
The 18-bit PDP ...
and
PDP-15
The PDP-15 was the fifth and last of the 18-bit minicomputers produced by Digital Equipment Corporation. The PDP-1 was first delivered in December 1959 and the first PDP-15 was delivered in February 1970. More than 400 of these successors to th ...
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 unt ...
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
36-bit computers were popular in the early mainframe computer era from the 1950s through the early 1970s.
Starting in the 1960s, but especially the 1970s, the introduction of 7-bit ASCII and 8-bit EBCDIC led to the move to machines using 8-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 an ...
produced several 18-bit computers, including the
UNIVAC 418
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 ...
and several military systems.
The
IBM 7700 Data Acquisition System The IBM 7700 Data Acquisition System was announced by IBM on December 2, 1963.
It is capable of collecting data from as many as 32 sources simultaneously, process the data and transmit results to up to 16 remote printers, display units or plot boa ...
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, succeeding t ...
Standard Spacecraft Computer
NSSC-1
The NASA Standard Spacecraft Computer-1 (NSSC-1) is a computer developed as a standard component for the MultiMission Modular Spacecraft at the Goddard Space Flight Center (GSFC) in 1974. The basic spacecraft was built of standardized components a ...
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 empl ...
(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 ...
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
RADIX 50 or RAD50 (also referred to as RADIX50, RADIX-50 or RAD-50), is an uppercase-only character encoding created by Digital Equipment Corporation (DEC) for use on their DECsystem, PDP, and VAX computers.
RADIX 50's 40-character re ...
, called Radix 50
8 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
The Baudot code is an early character encoding for telegraphy invented by Émile Baudot in the 1870s. It was the predecessor to the International Telegraph Alphabet No. 2 (ITA2), the most common teleprinter code in use until the advent of ASCII. ...
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 punc ...
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 of ...
code, which folds lowercase to uppercase letters.
References
DIGITAL Computing Timelime: 18-bit architecture''Architectural Evolution in DEC’s 18b Computers'' Bob Supnik, 2006.
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