Editing 36-bit computing

{{Short description|Computer architecture bit width}}
{{refimprove|date=October 2009}}
{{N-bit|36|(six six-bit characters)}}
36-bit computers were popular in the early [[mainframe computer]] era from the 1950s through the early 1970s.

[[File:Friden calculator - Ridai Museum of Modern Science, Tokyo - DSC07579.JPG|thumb|300px|Friden mechanical calculator. The electronic computer word length of 36-bits was chosen, in part, to match its precision.]]

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 computing|8-bit]] bytes, with word sizes that were multiples of 8, notably the [[32-bit computing|32-bit]] [[IBM System/360]] [[mainframe computer|mainframe]] and [[VAX|Digital Equipment VAX]] and [[Data General Eclipse MV/8000|Data General MV series]] [[superminicomputer]]s. By the mid-1970s the conversion was largely complete, and [[microprocessor]]s quickly moved from 8-bit to 16-bit to 32-bit over a period of a decade. The number of 36-bit machines rapidly fell during this period, offered largely for [[backward compatibility]] purposes running [[legacy system|legacy programs]].

==History==
Prior to the introduction of computers, the state of the art in precision scientific and engineering calculation was the ten-digit, electrically powered, [[mechanical calculator]], such as those manufactured by [[Friden, Inc.|Friden]], [[Marchant Calculator|Marchant]] and [[Monroe Calculator Company|Monroe]]. These calculators had a column of keys for each digit, and operators were trained to use all their fingers when entering numbers, so while some specialized calculators had more columns, ten was a practical limit.{{cn|date=January 2019}} Computers, as the new competitor, had to match that accuracy. Decimal computers sold in that era, such as the [[IBM 650]] and the [[IBM 7070]], had a word length of ten digits, as did [[ENIAC]], one of the earliest computers.

Early binary computers aimed at the same market therefore often used a 36-[[bit]] [[Word (computer architecture)|word length]]. This was long enough to represent positive and negative integers to an accuracy of ten decimal digits (35 bits would have been the minimum). It also allowed the storage of six alphanumeric characters encoded in a [[six-bit character code]]. Computers with 36-bit words included the [[MIT Lincoln Laboratory]] [[TX-2]], the [[IBM 700/7000 series|IBM 701/704/709/7090/7094]], the [[UNIVAC 1103]]/[[UNIVAC 1103A|1103A]]/[[UNIVAC 1105|1105]] and [[UNIVAC 1100/2200 series|1100/2200 series]], the [[GE-600 series|General Electric GE-600]]/[[Honeywell 6000 series|Honeywell 6000]], the Digital Equipment Corporation [[PDP-6]]/[[PDP-10]] (as used in the [[DECsystem-10]]/[[DECSYSTEM-20]]), and the [[Symbolics#The 3600 series|Symbolics 3600 series]].

Smaller machines like the [[PDP-1]]/[[PDP-9]]/[[PDP-15]] used [[18-bit computing|18-bit]] words, so a double word was 36 bits.

These computers had addresses 12 to 18 bits in length. The addresses referred to 36-bit words, so the computers were limited to addressing between 4,096 and 262,144 words (24,576 to 1,572,864 six-bit characters).  The older 36-bit computers were limited to a similar amount of physical memory as well. Architectures that survived evolved over time to support larger virtual address spaces using [[memory segmentation]] or other mechanisms.

The common character packings included:
* six 6-bit IBM [[BCD (character encoding)|BCD]] or [[Fieldata]] characters (ubiquitous in early usage)
* six 6-bit ASCII characters, supporting the upper-case unaccented letters, digits, space, and most ASCII punctuation characters.  It was used on the PDP-6 and PDP-10 under the name [[sixbit]].
* six [[DEC Radix-50]] characters packed into 32 bits, plus four spare bits
* five 7-bit characters and 1 unused bit (the usual PDP-6/10 convention, called ''five-seven ASCII'')<ref name="cline" /><ref name="rfc114" />
* four 8-bit characters (7-bit [[ASCII]] plus 1 spare bit, or 8-bit [[EBCDIC]]), plus four spare bits
* four 9-bit characters<ref name="cline">
Marshall Cline.
[https://isocpp.org/wiki/faq/intrinsic-types#bytes-review "Would you please go over the rules about bytes, chars, and characters one more time?"]
</ref><ref name="rfc114">
{{IETF RFC|114}}: "A file transfer protocol"
</ref> (the [[Multics]] convention).

Characters were extracted from words either using [[machine code]] shift and mask operations or with special-purpose hardware supporting 6-bit, 9-bit, or variable-length characters. The Univac 1100/2200 used the ''partial word designator'' of the instruction, the "J" field, to access characters. The GE-600 used special indirect words to access 6- and 9-bit characters. the PDP-6/10 had [[addressing mode#Indirect_to_bit_field_within_word|special instructions to access arbitrary-length byte fields]].

The standard [[C (programming language)|C programming language]] requires that the size of the [[character_(computing)#char | <code>char</code>]] data type be at least 8 bits,<ref>{{cite book
 | title=ISO/IEC 9899:1999 specification
 | at=p. 20, § 5.2.4.2.1
 | url=https://c0x.shape-of-code.com/5.2.4.2.1.html
 | access-date=2023-07-24
}}</ref> and that all data types other than bitfields have a size that is a multiple of the character size,<ref>{{cite book
 | title=ISO/IEC 9899:1999 specification
 | at=p. 37, § 6.2.6.1 (4)
 | url=https://c0x.shape-of-code.com/6.2.6.1.html
 | access-date=2023-07-24
}}</ref> so standard C implementations on 36-bit machines would typically use 9-bit <code>char</code>s, although 12-bit, 18-bit, or 36-bit would also satisfy the requirements of the standard.<ref>
Marshall Cline.
[http://www.parashift.com/c++-faq-lite/bytes-review.html "C++ FAQ: the rules about bytes, chars, and characters"].
</ref>

By the time IBM introduced [[System/360]] with [[32-bit computing|32-bit]] full words, scientific calculations had largely shifted to [[floating point]], where double-precision formats offered more than 10-digit accuracy. The 360s also included instructions for variable-length decimal arithmetic for commercial applications, so the practice of using word lengths that were a power of two quickly became commonplace, though at least one line of 36-bit computer systems are still sold {{As of|2019|lc=y}}, the [[Unisys]] ClearPath Dorado series, which is the continuation of the [[UNIVAC 1100/2200 series]] of [[mainframe computer]]s.

[[CompuServe]] was launched using 36-bit [[PDP-10]] computers in the late 1960s.  It continued using [[PDP-10]] and DECSYSTEM-10-compatible hardware and retired the service in the late 2000s.

== Other uses in electronics ==
The LatticeECP3 [[FPGA]]s from [[Lattice Semiconductor]] include [[Binary multiplier|multiplier]] slices that can be configured to support the multiplication of two 36-bit numbers.<ref>{{cite web|url=https://www.latticesemi.com/view_document?document_id=32322|title=LatticeECP3 sysDSP Usage Guide|publisher=[[Lattice Semiconductor]]|access-date=April 29, 2019}}</ref>  The DSP block in [[Altera]] Stratix FPGAs can do 36-bit additions and multiplications.<ref>{{cite web|url=http://www.altera.com/devices/fpga/stratix-fpgas/stratix/stratix/features/stx-dsp.html|title=Digital Signal Processing (DSP) Blocks in Stratix Devices|publisher=[[Altera]]+accessdate=December 27, 2013}}</ref>

== See also ==
* [[Physical Address Extension]] (PAE)
* [[PSE-36]] (36-bit Page Size Extension)
* [[UTF-9 and UTF-18]]

==References==
{{Reflist}}

{{CPU technologies}}

[[Category:Data unit]]