Bajt

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Bajt
Sistemjedinica izvedena iz bita
Jedinicadigitalne informacije, veličina podataka
SimbolB ili o (kada se radi o 8 bita) 
Umnošci bajta
Odomaćena upotreba i
(značenje po SI sistemu) 		Standard za binarne
prefikse po IEC 60027-2
Naziv Oznaka Količina Naziv Oznaka Količina
kilobajt kB 210 (103) kibibajt KiB 210
megabajt MB 220 (106) mebibajt MiB 220
gigabajt GB 230 (109) gibibajt GiB 230
terabajt TB 240 (1012) tebibajt TiB 240
petabajt PB 250 (1015) pebibajt PiB 250
eksabajt EB 260 (1018) eksbibajt EiB 260
zetabajt ZB 270 (1021) zebibajt ZiB 270
jotabajt YB 280 (1024) jobibajt YiB 280
Vidi još: bit, nibl, neodređenost umnožaka

Bajt (engl. byte, kao drugačije napisan oblik reči bite sa značenjem „odgrizak") je jedinica mere podataka u računarstvu. Sastoji se od odgovarajućeg broja bitova, ali kao mera nije apsolutno standardizovana - bajtovi različitih računara mogu imati različit broj bitova, npr. između 4 i 32. Činjenični standard je, međutim, 8 bitova ("oktet") i danas je teško naći računare sa različitom definicijom bajta. Reč bajt je skraćenica od engleskih reči by eight i znači „za osam”[1], ali je takođe skraćenica i od binary term.

Razlog za postojanje ove jedinice informacije je praktičan. Da bi se definisala osnovna veličina podataka dovoljna za smeštanje jednog slova odnosno znaka interpunkcije, ali takođe i za prenos komunikacionim kanalima kao i radi definisanja osnovne adresibilne jedinice podataka. U skladu sa složenošću procesorske arhitekture definisana je i osnovna veličina reči kojom barata procesor.[2][3] Tako se vremenom od 8-bitnog procesora prešlo na 16-bitne, 32-bitne i trenutno na 64-bitne. Tako je i veličina procesorske reči, širina adresne linije, registara i adresibilna jedinica za pristup postala značajno veća, međutim još uvek svi procesori čuvaju, kompatibilnosti radi, mogućnost da barataju pojedinačnim bajtovima. Zato i danas (uslovno) važi stara definicija bajta: Bajt je najmanja adresibilna jedinica količine podataka.

Da bi se razlikovali bajtovi proizvoljne veličine od uobičajene 8-bitne definicije, dokumenti mrežnog protokola kao što je Internet Protokol (RFC 791) označavaju 8-bitni bajt kao oktet.[4] Ti bitovi u oktetu se obično broje sa ciframa od 0 do 7 ili 7 do 0 u zavisnosti od bitne orijentacije. Prvi bit je broj 0, što čini osmi bit brojem 7.

Veličina bajta je istorijski zavisila od hardvera i nisu postojali definitivni standardi koji bi nalagali veličinu. Korišćene su veličine od 1 do 48 bita.[5][6][7][8] Šestobitni kod karaktera bio je često korišćena implementacija u ranim sistemima kodiranja, a računari koji koriste šest-bitne i devet-bitne bajtove bili su uobičajeni tokom 1960-ih. Ovi sistemi su često imali memorijske reči od 12, 18, 24, 30, 36, 48 ili 60 bita, što odgovara 2, 3, 4, 5, 6, 8 ili 10 šesto-bitnih bajtova. U ovoj eri, grupisanje bitova u toku instrukcija često se nazivalo slogovima[a] ili slabovima, pre nego što je termin bajt postao uobičajen.

Savremeni de fakto standard od osam bita, kako je dokumentovano u ISO/IEC 2382-1:1993, je podesni stepen dvojke koji dozvoljava binarno kodirane vrednosti od 0 do 255 za jedan bajt—2 na stepen od 8 je 256.[9] Međunarodni standard IEC 80000-13 je kodifikovao ovo zajedničko značenje. Mnogi tipovi aplikacija koriste informacije koje se mogu predstaviti u osam ili manje bitova i dizajneri procesora obično optimizuju za ovu upotrebu. Popularnost velikih komercijalnih računarskih arhitektura je pomogla u sveprisutnom prihvatanju 8-bitnog bajta.[10] Moderne arhitekture obično koriste 32- ili 64-bitne reči, koje se sastoje od četiri ili osam bajtova, respektivno.

Simbol jedinice za bajt je označen kao veliko slovo B od strane Međunarodne elektrotehničke komisije (IEC) i Instituta inženjera elektrotehnike i elektronike (IEEE).[11] Na međunarodnom nivou, jedinični oktet, simbol o, eksplicitno definiše sekvencu od osam bitova, eliminišući potencijalnu dvosmislenost termina „bajt”.[12][13]

Etimologija i istorija[uredi | uredi izvor]

Termin bajt je skovao Verner Buholc u junu 1956. godine,[5][14][15][b] tokom rane faze dizajna za IBM Streč[16][17][2][14][15][18][19] računara, koji je imao adresiranje na instrukcije bitova i promenljive dužine polja (VFL) sa veličinom bajta kodiranom u instrukciji.[14] Pravopis reči je namerno izabran kako bi se izbeglo slučajno poistovećivanje sa bitom.[2][14][20][v]

Napomene[uredi | uredi izvor]

  1. ^ Termin slog je korišćen za bajtove koji sadrže instrukcije ili sastojke instrukcija, a ne za bajtove podataka.
  2. ^ Mnogi izvori pogrešno ukazuju na rođendan termina bajt u julu 1956. godine, međutim Verner Buholc je tvrdio da je taj termin skovan u junu 1956. Zapravo, najraniji dokument koji to podržava datira iz 1956-06-11. Buholc je izjavio da je prelazak na 8-bitne bajtove zamišljen u avgustu 1956. godine, ali najraniji dokument koji je pronađen da koristi ovaj pojam datira iz septembra 1956. godine.
  3. ^ Neke kasnije mašine, na primer, Burroughs B1700, CDC 3600, DEC PDP-6, DEC PDP-10, imale su mogućnost da rade na proizvoljnim bajtovima koji nisu veći od veličine reči.

Reference[uredi | uredi izvor]

  1. ^ „Politikin zabavnik, broj 2919”. Arhivirano iz originala 14. 03. 2012. g. Pristupljeno 10. 04. 2010. 
  2. ^ a b v Blaauw, Gerrit Anne; Brooks, Jr., Frederick Phillips; Buchholz, Werner (1962), „4: Natural Data Units” (PDF), Ur.: Buchholz, Werner, Planning a Computer System – Project Stretch, McGraw-Hill Book Company, Inc. / The Maple Press Company, York, PA., str. 39—40, LCCN 61-10466, Arhivirano iz originala (PDF) 2017-04-03. g., Pristupljeno 2017-04-03, „Terms used here to describe the structure imposed by the machine design, in addition to bit, are listed below.
    Byte denotes a group of bits used to encode a character, or the number of bits transmitted in parallel to and from input-output units. A term other than character is used here because a given character may be represented in different applications by more than one code, and different codes may use different numbers of bits (i.e., different byte sizes). In input-output transmission the grouping of bits may be completely arbitrary and have no relation to actual characters. (The term is coined from bite, but respelled to avoid accidental mutation to bit.)
    A     word consists of the number of data bits transmitted in parallel from or to memory in one memory cycle. Word size is thus defined as a structural property of the memory. (The term catena was coined for this purpose by the designers of the Bull fr computer.)
    Block refers to the number of words transmitted to or from an input-output unit in response to a single input-output instruction. Block size is a structural property of an input-output unit; it may have been fixed by the design or left to be varied by the program.”     
  3. ^ Bemer, Robert William (1959), „A proposal for a generalized card code of 256 characters”, Communications of the ACM, 2 (9): 19—23, S2CID 36115735, doi:10.1145/368424.368435 
  4. ^ Postel, J. (septembar 1981). Internet Protocol DARPA INTERNET PROGRAM PROTOCOL SPECIFICATION (na jeziku: engleski). p. 43. doi:10.17487/RFC0791 Slobodan pristup. RFC 791. Pristupljeno 28. 8. 2020. „octet An eight bit byte.” 
  5. ^ a b Buchholz, Werner (1956-06-11). „7. The Shift Matrix” (PDF). The Link System. IBM. str. 5—6. Stretch Memo No. 39G. Arhivirano iz originala (PDF) 2017-04-04. g. Pristupljeno 2016-04-04. „[…] Most important, from the point of view of editing, will be the ability to handle any characters or digits, from 1 to 6 bits long.
    Figure 2 shows the Shift Matrix to be used to convert a 60-bit word, coming from Memory in parallel, into characters, or 'bytes' as we have called them, to be sent to the Adder serially. The 60 bits are dumped into magnetic cores on six different levels. Thus, if a 1 comes out of position 9, it appears in all six cores underneath. Pulsing any diagonal line will send the six bits stored along that line to the Adder. The Adder may accept all or only some of the bits.
    Assume that it is desired to operate on 4 bit decimal digits, starting at the right. The 0-diagonal is pulsed first, sending out the six bits 0 to 5, of which the Adder accepts only the first four (0–3). Bits 4 and 5 are ignored. Next, the 4 diagonal is pulsed. This sends out bits 4 to 9, of which the last two are again ignored, and so on.
    It is just as easy to use all six bits in alphanumeric work, or to handle bytes of only one bit for logical analysis, or to offset the bytes by any number of bits. All this can be done by pulling the appropriate shift diagonals. An analogous matrix arrangement is used to change from serial to parallel operation at the output of the adder. […]    ”     
  6. ^ 3600 Computer System – Reference Manual (PDF). K. St. Paul, Minnesota, USA: Control Data Corporation (CDC). 1966-10-11 [1965]. 60021300. Arhivirano iz originala (PDF) 2017-04-05. g. Pristupljeno 2017-04-05. „Byte – A partition of a computer word.”  (NB. Discusses 12-bit, 24-bit and 48-bit bytes.)
  7. ^ Rao, Thammavaram R. N.; Fujiwara, Eiji (1989). McCluskey, Edward J., ur. Error-Control Coding for Computer Systems. Prentice Hall Series in Computer Engineering (1 izd.). Englewood Cliffs, NJ, USA: Prentice Hall. ISBN 0-13-283953-9. LCCN 88-17892.  (NB. Example of the usage of a code for "4-bit bytes".)
  8. ^ Tafel, Hans Jörg (1971). Einführung in die digitale Datenverarbeitung [Introduction to digital information processing] (na jeziku: nemački). Munich: Carl Hanser Verlag. str. 300. ISBN 3-446-10569-7. „Byte = zusammengehörige Folge von i.a. neun Bits; davon sind acht Datenbits, das neunte ein Prüfbit”  (NB. Defines a byte as a group of typically 9 bits; 8 data bits plus 1 parity bit.)
  9. ^ ISO/IEC 2382-1: 1993, Information technology – Vocabulary – Part 1: Fundamental terms. 1993. „byte
    A string that consists of a number of bits, treated as a unit, and usually representing a character or a part of a character.
    NOTES
    1 The number of bits in a byte is fixed for a given data processing system.
    2 The number of bits in a byte is usually 8.    ”     
  10. ^ „Computer History Museum – Exhibits – Internet History – 1964: Internet History 1962 to 1992”. Computer History Museum. 2017 [2015]. Arhivirano iz originala 2017-04-03. g. Pristupljeno 2017-04-03. 
  11. ^ Jaffer, Aubrey (2011) [2008]. „Metric-Interchange-Format”. Arhivirano iz originala 2017-04-03. g. Pristupljeno 2017-04-03. 
  12. ^ Kozierok, Charles M. (2005-09-20) [2001]. „The TCP/IP Guide – Binary Information and Representation: Bits, Bytes, Nibbles, Octets and Characters – Byte versus Octet”. 3.0. Arhivirano iz originala 2017-04-03. g. Pristupljeno 2017-04-03. 
  13. ^ ISO 2382-4, Organization of data (2 izd.). „byte, octet, 8-bit byte: A string that consists of eight bits.” 
  14. ^ a b v g Buchholz, Werner (februar 1977). „The Word 'Byte' Comes of Age...”. Byte Magazine. 2 (2): 144. „[…] The first reference found in the files was contained in an internal memo written in June 1956 during the early days of developing Stretch. A byte was described as consisting of any number of parallel bits from one to six. Thus a byte was assumed to have a length appropriate for the occasion. Its first use was in the context of the input-output equipment of the 1950s, which handled six bits at a time. The possibility of going to 8 bit bytes was considered in August 1956 and incorporated in the design of Stretch shortly thereafter. The first published reference to the term occurred in 1959 in a paper 'Processing Data in Bits and Pieces' by G A Blaauw, F P Brooks Jr and W Buchholz in the IRE Transactions on Electronic Computers, June 1959, page 121. The notions of that paper were elaborated in Chapter 4 of Planning a Computer System (Project Stretch), edited by W Buchholz, McGraw-Hill Book Company (1962). The rationale for coining the term was explained there on page 40 as follows:
    Byte     denotes a group of bits used to encode a character, or the number of bits transmitted in parallel to and from input-output units. A term other than character is used here because a given character may be represented in different applications by more than one code, and different codes may use different numbers of bits (ie, different byte sizes). In input-output transmission the grouping of bits may be completely arbitrary and have no relation to actual characters. (The term is coined from bite, but respelled to avoid accidental mutation to bit.)
    System/360 took over many of the Stretch concepts, including the basic byte and word sizes, which are powers of 2. For economy, however, the byte size was fixed at the 8 bit maximum, and addressing at the bit level was replaced by byte addressing. […]    ”     
  15. ^ a b „Timeline of the IBM Stretch/Harvest era (1956–1961)”. Computer History Museum. jun 1956. Arhivirano iz originala 2016-04-29. g. Pristupljeno 2017-04-03. „1956 Summer: Gerrit Blaauw, Fred Brooks, Werner Buchholz, John Cocke and Jim Pomerene join the Stretch team. Lloyd Hunter provides transistor leadership.
    1956 July [    sic]: In a report Werner Buchholz lists the advantages of a 64-bit word length for Stretch. It also supports NSA's requirement for 8-bit bytes. Werner's term "Byte" first popularized in this memo.”      (NB. This timeline erroneously specifies the birth date of the term "byte" as July 1956, while Buchholz actually used the term as early as June 1956.)
  16. ^ Buchholz, Werner (1956-07-31). „5. Input-Output” (PDF). Memory Word Length. IBM. str. 2. Stretch Memo No. 40. Arhivirano iz originala (PDF) 2017-04-04. g. Pristupljeno 2016-04-04. „[…] 60 is a multiple of 1, 2, 3, 4, 5, and 6. Hence bytes of length from 1 to 6 bits can be packed efficiently into a 60-bit word without having to split a byte between one word and the next. If longer bytes were needed, 60 bits would, of course, no longer be ideal. With present applications, 1, 4, and 6 bits are the really important cases.
    With 64-bit words, it would often be necessary to make some compromises, such as leaving 4 bits unused in a word when dealing with 6-bit bytes at the input and output. However, the LINK Computer can be equipped to edit out these gaps and to permit handling of bytes which are split between words. […]    ”     
  17. ^ Buchholz, Werner (1956-09-19). „2. Input-Output Byte Size” (PDF). Memory Word Length and Indexing. IBM. str. 1. Stretch Memo No. 45. Arhivirano iz originala (PDF) 2017-04-04. g. Pristupljeno 2016-04-04. „[…] The maximum input-output byte size for serial operation will now be 8 bits, not counting any error detection and correction bits. Thus, the Exchange will operate on an 8-bit byte basis, and any input-output units with less than 8 bits per byte will leave the remaining bits blank. The resultant gaps can be edited out later by programming […]” 
  18. ^ Raymond, Eric Steven (2017) [2003]. „byte definition”. Arhivirano iz originala 2017-04-03. g. Pristupljeno 2017-04-03. 
  19. ^ Bemer, Robert William (2000-08-08). „Why is a byte 8 bits? Or is it?”. Computer History Vignettes. Arhivirano iz originala 2017-04-03. g. Pristupljeno 2017-04-03. „[…] I came to work for IBM, and saw all the confusion caused by the 64-character limitation. Especially when we started to think about word processing, which would require both upper and lower case. […] I even made a proposal (in view of STRETCH, the very first computer I know of with an 8-bit byte) that would extend the number of punch card character codes to 256 […].” 
  20. ^ Blaauw, Gerrit Anne; Brooks, Jr., Frederick Phillips; Buchholz, Werner (jun 1959). „Processing Data in Bits and Pieces”. IRE Transactions on Electronic Computers: 121. 

Literatura[uredi | uredi izvor]

Spoljašnje veze[uredi | uredi izvor]

Bajt na Vikimedijinoj ostavi.
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