{"id":96211,"date":"2020-10-09T07:42:43","date_gmt":"2020-10-09T05:42:43","guid":{"rendered":"https:\/\/prohoster.info\/blog\/administrirovanie\/pochemu-v-ebcdic-bukvy-idut-ne-podryad"},"modified":"2020-10-09T07:42:43","modified_gmt":"2020-10-09T05:42:43","slug":"pochemu-v-ebcdic-bukvy-idut-ne-podryad","status":"publish","type":"post","link":"https:\/\/prohoster.info\/en\/blog\/administrirovanie\/pochemu-v-ebcdic-bukvy-idut-ne-podryad","title":{"rendered":"Why don't the letters in EBCDIC appear consecutively?","gt_translate_keys":[{"key":"rendered","format":"text"}]},"content":{"rendered":"

The ASCII standard was adopted in 1963, and it's unlikely that anyone now uses a character set where the first 128 characters differ from ASCII. However, until the end of the last century, EBCDIC was widely used \u2014 the standard encoding for IBM mainframes and their Soviet clones, ES EVM. EBCDIC remains the primary encoding in z\/OS \u2014 the standard OS for modern IBM Z mainframes.<\/p>\n

What immediately stands out when looking at EBCDIC is that the letters do not appear consecutively: between I<\/code> and J<\/code> and between R<\/code> and S<\/code> there are unused positions (on ES EVM, these gaps were filled with<\/a><\/noindex> Cyrillic symbols). Who could think of encoding letters with unequal gaps between adjacent letters?<\/p>\n

\"Why<\/p>\n

The name EBCDIC (\"Extended BCDIC\") hints that this encoding, unlike ASCII, was not created from scratch but was based on the six-bit BCDIC code that had been used since IBM 704<\/a><\/noindex> (1954):<\/p>\n

\"Why<\/p>\n

There is no direct backward compatibility: a convenient feature of BCDIC, lost in the transition to EBCDIC, was that digits 0<\/code>\u20149<\/code> correspond to codes 0-9. Nevertheless, there were already gaps of seven codes between I<\/code> and J<\/code> and eight codes between R<\/code> and S<\/code> in BCDIC. So where did they come from?<\/p>\n

The history of (E)BCDIC begins simultaneously with the history of IBM \u2014 long before electronic computers. IBM was formed from the merger of four companies, the most technologically advanced among them being the \"Tabulating Machine Company,\" founded in 1896 by Herman Hollerith \u2014 the inventor tabulator.<\/a><\/noindex>The first tabulators simply counted the number of punch cards processed in certain places; but in 1905, Hollerith began producing decimal<\/em> tabulators. Each card for a decimal tabulator consisted of fields of arbitrary lengths, and the numbers recorded in these fields in the familiar decimal form were summed across the entire deck. The division of the card into fields was determined by connecting wires on the tabulator's switching panel. For example, on this punch card belonging to Hollerith, held<\/a><\/noindex> at the Library of Congress, the number 23456789012345678 is clearly punched, although it's unclear how it was divided into fields:<\/p>\n

\"Why<\/p>\n

The most observant might notice that Hollerith's card has 12 rows for holes, even though ten are sufficient for digits; and in BCDIC, only 12 out of 16 possible codes are used for each value of the two highest bits.<\/p>\n

Of course, this is not a coincidence. Hollerith originally intended the additional rows for \"special marks,\" which were not summed but simply counted \u2014 as in the very first tabulators. (Today we would call them \"bit fields.\") Additionally, among the \"special marks,\" one could set group indicators: if not only final sums but also intermediate ones were required during tabulation, the tabulator would stop when it detected a change in any of the group indicators, and the operator had to write down the intermediate sums from the digital tabulator onto paper, reset the tabulator, and resume tabulation. For example, when calculating accounting balances, a group of cards could correspond to a single date or a single counterparty.<\/p>\n

By 1920, when Hollerith had already retired, \"printing tabulators\" had come into use, which connected to the teleprinter and could print intermediate sums themselves without requiring operator intervention. The complexity now lay in determining what each printed number referred to. In 1931, IBM decided to designate letters using \"special marks\": a mark in row 12 denoted a letter from A<\/code> up to I<\/code>, in row 11 \u2014 from J<\/code> up to R<\/code>, in row 0 \u2014 from S<\/code> up to Z<\/code>. The new \"alphabetical tabulator\" could print the name of each group of cards along with intermediate sums; meanwhile, an unpunched column became a space between characters. Note that S<\/code> is denoted by the combination of holes 0+2, while the combination 0+1 was initially not used for fear that two holes next to each other in one column could cause mechanical issues in the reader.<\/p>\n

\"Why<\/p>\n

Now the BCDIC table can be viewed from a slightly different angle:<\/p>\n

\"Why<\/p>\n

Except for the fact that 0 and space have been switched, the higher two bits determine the \"special mark\" that has been punched in the punch card for the corresponding symbol since 1931; the lower four bits determine the digit punched in the main part of the card. Support for characters & - \/<\/code> It was added to IBM tabulators in the 1930s, and the encoding of these characters in BCDIC corresponds to the hole combinations they create. When support for an even larger number of characters was needed, a series of 8 began to be punched as an additional \"special mark\" \u2014 thus, a single column could have up to three holes. This format of punch cards remained virtually unchanged until the end of the century. In the USSR, IBM's encodings for Latin letters and punctuation were retained, while for Cyrillic letters, multiple \"special marks\" were punched in rows 12, 11, and 0 \u2014 not limited to three holes in a single column.<\/p>\n

When the IBM 704 computer was created, there was not much thought given to the character encoding for it: they simply used the encoding already in use at that time for punch cards, and only moved the 0 to its place. In 1964, during the transition from BCDIC to EBCDIC, the lower four bits of each character were left unchanged, although the upper bits were slightly shuffled. Thus, the punch card format selected by Hollerith at the beginning of the last century influenced the architecture of all IBM computers, up to and including IBM Z.<\/p>\n

Source: habr.com<\/a> <\/p>","protected":false,"gt_translate_keys":[{"key":"rendered","format":"html"}]},"excerpt":{"rendered":"

\u0421\u0442\u0430\u043d\u0434\u0430\u0440\u0442 ASCII \u0431\u044b\u043b \u043f\u0440\u0438\u043d\u044f\u0442 \u0432 1963, \u0438 \u0441\u0435\u0439\u0447\u0430\u0441 \u0432\u0440\u044f\u0434 \u043b\u0438 \u043a\u0442\u043e-\u043d\u0438\u0431\u0443\u0434\u044c \u0438\u0441\u043f\u043e\u043b\u044c\u0437\u0443\u0435\u0442 \u043a\u043e\u0434\u0438\u0440\u043e\u0432\u043a\u0443, \u043f\u0435\u0440\u0432\u044b\u0435 128 \u0441\u0438\u043c\u0432\u043e\u043b\u043e\u0432 \u043a\u043e\u0442\u043e\u0440\u043e\u0439 \u043e\u0442\u043b\u0438\u0447\u0430\u043b\u0438\u0441\u044c \u0431\u044b \u043e\u0442 ASCII. \u0422\u0435\u043c \u043d\u0435 \u043c\u0435\u043d\u0435\u0435, \u0434\u043e \u043a\u043e\u043d\u0446\u0430 \u043f\u0440\u043e\u0448\u043b\u043e\u0433\u043e \u0432\u0435\u043a\u0430 \u0430\u043a\u0442\u0438\u0432\u043d\u043e \u0438\u0441\u043f\u043e\u043b\u044c\u0437\u043e\u0432\u0430\u043b\u0430\u0441\u044c EBCDIC \u2014 \u0441\u0442\u0430\u043d\u0434\u0430\u0440\u0442\u043d\u0430\u044f \u043a\u043e\u0434\u0438\u0440\u043e\u0432\u043a\u0430 \u0434\u043b\u044f \u043c\u0435\u0439\u043d\u0444\u0440\u0435\u0439\u043c\u043e\u0432 IBM \u0438 \u0438\u0445 \u0441\u043e\u0432\u0435\u0442\u0441\u043a\u0438\u0445 \u043a\u043b\u043e\u043d\u043e\u0432 \u0415\u0421 \u042d\u0412\u041c. 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