Editing ASCII (section)

==Design considerations==

===Bit width===
The X3.2 subcommittee designed ASCII based on the earlier teleprinter encoding systems. Like other [[character encoding]]s, ASCII specifies a correspondence between digital bit patterns and [[character (computing)|character]] symbols (i.e. [[grapheme]]s and [[control character]]s). This allows [[Digital data|digital]] devices to communicate with each other and to process, store, and communicate character-oriented information such as written language. Before ASCII was developed, the encodings in use included 26 [[English alphabet|alphabetic]] characters, 10 [[numerical digit]]s, and from 11 to 25 special graphic symbols. To include all these, and control characters compatible with the [[CCITT|Comité Consultatif International Téléphonique et Télégraphique]] (CCITT) [[International Telegraph Alphabet No. 2]] (ITA2) standard of 1932,<ref>{{cite web |url=http://handle.itu.int/11.1004/020.1000/4.5.43.en.101 |title=Telegraph Regulations and Final Protocol (Madrid, 1932) |access-date=9 Jun 2024 |archive-url=https://web.archive.org/web/20230821020920/https://search.itu.int/history/HistoryDigitalCollectionDocLibrary/4.5.43.en.101.pdf |archive-date=21 August 2023}}</ref><ref name="bdcode">{{cite web |author-last=Smith |author-first=Gil |title=Teletype Communication Codes |publisher=Baudot.net |date=2001 |url=http://www.baudot.net/docs/smith--teletype-codes.pdf |access-date=2008-07-11 |archive-url=https://web.archive.org/web/20080820043949/http://www.baudot.net/docs/smith--teletype-codes.pdf |archive-date=August 20, 2008 |url-status=live }}</ref> [[FIELDATA]] (1956{{citation needed|date=June 2016|reason=My sources state 1957 rather than 1956, but Wikipedia states 1956 in various places. This needs to be sorted out with better sources.}}), and early [[EBCDIC]] (1963), more than 64 codes were required for ASCII.

ITA2 was in turn based on [[Baudot code]], the 5-bit telegraph code Émile Baudot invented in 1870 and patented in 1874.<ref name="bdcode" />

The committee debated the possibility of a [[Shift code|shift]] function (like in [[ITA2]]), which would allow more than 64 codes to be represented by a [[six-bit character code|six-bit code]]. In a shifted code, some character codes determine choices between options for the following character codes. It allows compact encoding, but is less reliable for [[data transmission]], as an error in transmitting the shift code typically makes a long part of the transmission unreadable. The standards committee decided against shifting, and so ASCII required at least a seven-bit code.<ref name="Mackenzie_1980"/>{{rp|pages=215 §13.6, 236 §4}}

The committee considered an eight-bit code, since eight bits ([[octet (computing)|octet]]s) would allow two four-bit patterns to efficiently encode two digits with [[binary-coded decimal]]. However, it would require all data transmission to send eight bits when seven could suffice. The committee voted to use a seven-bit code to minimize costs associated with data transmission. Since perforated tape at the time could record eight bits in one position, it also allowed for a [[parity bit]] for [[error checking]] if desired.<ref name="Mackenzie_1980"/>{{rp|pages=217 §c, 236 §5}} [[8-bit computing|Eight-bit]] machines (with octets as the native data type) that did not use parity checking typically set the eighth bit to 0.<ref name="Sawyer_1995">{{cite book |author-first1=Stanley A. |author-last1=Sawyer |author-first2=Steven George |author-last2=Krantz |title=A TeX Primer for Scientists |url=https://books.google.com/books?id=bXLDwmIJNkUC&pg=PA13 |date=1995 |publisher=[[CRC Press]] |isbn=978-0-8493-7159-2 |page=13 |bibcode=1995tps..book.....S |access-date=October 29, 2016 |archive-url=https://web.archive.org/web/20161222151907/https://books.google.com/books?id=bXLDwmIJNkUC&pg=PA13 |archive-date=December 22, 2016 |url-status=live }}</ref>

===Internal organization===
The code itself was patterned so that most control codes were together and all graphic codes were together, for ease of identification. The first two so-called ''ASCII sticks''{{Efn|name="NB_Stick"}}<ref name="Bemer_1980_Inside"/> (32 positions) were reserved for control characters.<ref name="Mackenzie_1980"/>{{rp|220, 236 8,9)}} The [[Space (punctuation)|"space" character]] had to come before graphics to make [[sorting algorithm|sorting]] easier, so it became position 20<sub>[[hexadecimal|hex]]</sub>;<ref name="Mackenzie_1980"/>{{rp|237 §10}} for the same reason, many special signs commonly used as separators were placed before digits. The committee decided it was important to support uppercase [[sixbit code pages|64-character alphabets]], and chose to pattern ASCII so it could be reduced easily to a usable 64-character set of graphic codes,<ref name="Mackenzie_1980"/>{{rp|228, 237 §14}} as was done in the [[DEC SIXBIT]] code (1963). [[Lower case|Lowercase]] letters were therefore not interleaved with [[uppercase]]. To keep options available for lowercase letters and other graphics, the special and numeric codes were arranged before the letters, and the letter ''A'' was placed in position 41<sub>[[hexadecimal|hex]]</sub> to match the draft of the corresponding British standard.<ref name="Mackenzie_1980"/>{{rp|238 §18}} The digits 0–9 are prefixed with 011, but the remaining [[Nibble|4 bits]] correspond to their respective values in binary, making conversion with [[binary-coded decimal]] straightforward (for example, 5 in encoded to 011''0101'', where 5 is ''0101'' in binary).

Many of the non-alphanumeric characters were positioned to correspond to their shifted position on typewriters; an important subtlety is that these were based on ''mechanical'' typewriters, not ''electric'' typewriters.<ref name="Savard">{{cite web |title=Computer Keyboards |url=http://www.quadibloc.com/comp/kybint.htm |author-first=John J. G. |author-last=Savard |access-date=2014-08-24 |archive-url=https://web.archive.org/web/20140924183236/http://www.quadibloc.com/comp/kybint.htm |archive-date=September 24, 2014 |url-status=live }}</ref> Mechanical typewriters followed the [[de facto standard|''de facto'' standard]] set by the [[Remington No. 2]] (1878), the first typewriter with a shift key, and the shifted values of <code>23456789-</code> were <code>"#$%_&'()</code>{{snd}} early typewriters omitted ''0'' and ''1'', using ''O'' (capital letter ''o'') and ''l'' (lowercase letter ''L'') instead, but <code>1!</code> and <code>0)</code> pairs became standard once 0 and 1 became common. Thus, in ASCII <code>!"#$%</code> were placed in the second stick,{{Efn|name="NB_Stick"}}<ref name="Bemer_1980_Inside"/> positions 1–5, corresponding to the digits 1–5 in the adjacent stick.{{Efn|name="NB_Stick"}}<ref name="Bemer_1980_Inside"/> The parentheses could not correspond to ''9'' and ''0'', however, because the place corresponding to ''0'' was taken by the space character. This was accommodated by removing <code>_</code> (underscore) from ''6'' and shifting the remaining characters, which corresponded to many European typewriters that placed the parentheses with ''8'' and ''9''. This discrepancy from typewriters led to [[bit-paired keyboard]]s, notably the [[Teletype Model 33]], which used the left-shifted layout corresponding to ASCII, differently from traditional mechanical typewriters.

Electric typewriters, notably the [[IBM Selectric]] (1961), used a somewhat different layout that has become ''de facto'' standard on computers{{snd}} following the [[IBM PC]] (1981), especially [[Model M]] (1984){{snd}} and thus shift values for symbols on modern keyboards do not correspond as closely to the ASCII table as earlier keyboards did. The <code>/?</code> pair also dates to the No. 2, and the <code>,&lt; .&gt;</code> pairs were used on some keyboards (others, including the No. 2, did not shift <code>,</code> (comma) or <code>.</code> (full stop) so they could be used in uppercase without unshifting). However, ASCII split the <code>;:</code> pair (dating to No. 2), and rearranged mathematical symbols (varied conventions, commonly <code>-* =+</code>) to <code>:* ;+ -=</code>.

Some then-common typewriter characters were not included, notably <code>½ ¼ ¢</code>, while <code>^ ` ~ </code> were included as diacritics for international use, and <code>&lt; &gt;</code> for mathematical use, together with the simple line characters <code>\ |</code> (in addition to common <code>/</code>). The ''@'' symbol was not used in continental Europe and the committee expected it would be replaced by an accented ''À'' in the French variation, so the ''@'' was placed in position 40<sub>[[hexadecimal|hex]]</sub>, right before the letter A.<ref name="Mackenzie_1980"/>{{rp|243}}

The control codes felt essential for data transmission were the start of message (SOM), end of address (EOA), [[end of message]] (EOM), end of transmission (EOT), "who are you?" (WRU), "are you?" (RU), a reserved device control (DC0), synchronous idle (SYNC), and acknowledge (ACK). These were positioned to maximize the [[Hamming distance]] between their bit patterns.<ref name="Mackenzie_1980"/>{{rp|243–245}}

===<span class="anchor" id="Order"></span>Character order===
ASCII-code order is also called ''ASCIIbetical'' order.<ref>{{cite magazine |url=https://www.pcmag.com/encyclopedia_term/0,2542,t=ASCIIbetical&i=38025,00.asp |title=ASCIIbetical definition |magazine=[[PC Magazine]] |access-date=2008-04-14 |archive-url=https://web.archive.org/web/20130309183509/http://www.pcmag.com/encyclopedia_term/0%2C2542%2Ct%3DASCIIbetical%26i%3D38025%2C00.asp |archive-date=March 9, 2013 |url-status=live }}</ref> [[Collation]] of data is sometimes done in this order rather than "standard" alphabetical order ([[collating sequence]]). The main deviations in ASCII order are:
* All uppercase come before lowercase letters; for example, "Z" precedes "a"
* Digits and many punctuation marks come before letters

An intermediate order converts uppercase letters to lowercase before comparing ASCII values.