18-bit computing

{{Short description|Computer architectures using an 18-bit word}}

Eighteen binary digits have 262,144 (1000000 octal, 40000 hexadecimal) distinct combinations.

Eighteen bits was a common word size for smaller computers in the 1960s, when large computers often using 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 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.{{Cite manual|title=PDP-9 Utility Programs--Advanced Software System--Programmer's Reference Manual|publisher=Digital Equipment Corporation|year=1968|location=Maynard, Massachusetts|url=http://www.bitsavers.org/pdf/dec/pdp9/DEC-9A-GUAB-D_UTILITIES.pdf|archive-date=January 25, 2019|url-status=live|archive-url=https://web.archive.org/web/20190125062212/http://www.bitsavers.org/pdf/dec/pdp9/DEC-9A-GUAB-D_UTILITIES.pdf|page=A1-1|chapter=Linking Loader}}

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 format packs three characters in each 18-bit word,{{Cite manual|title=PDP-7 Symbolic Assembler Programming Manual|publisher=Digital Equipment Corporation|year=1965|location=Maynard, Massachusetts|pages=6, 38–39|url=http://bitsavers.trailing-edge.com/pdf/dec/pdp7/PDP-7_AsmMan.pdf|archive-url=https://web.archive.org/web/20170523224514/http://bitsavers.trailing-edge.com/pdf/dec/pdp7/PDP-7_AsmMan.pdf|archive-date=May 23, 2017|url-status=live|access-date=June 18, 2015}} each 6-bit character obtained by stripping the high bits from the 7-bit ASCII code, which folds lowercase to uppercase letters.

18-bit computing

Example computer architectures

Character encoding

References

Further reading