Byte addressing

Byte Addressing  Refers to hardware architectures supporting qui Accessing individual bytes of data Rather than only larger units called Expired words , Which would be  word-addressable  . Such computers are sometimes called  byte machines  [1]  (in contrast to  word machines  ).  [2]

The basic unit of digital storage is called a bit , storing a single 0 or 1.

Many common architectures can address more than 8 bits of data at a time. For example, the Intel 386SX processor can handle 16-bit (two-byte) data, since it is transferred over a 16-bit bus . However, data in memory may be of various lengths.

A 64-bit architecture machine might still need to access 8-bit data over its 64-bit line address, and have the data returned in the 8-bit bottom of its longer data line.

Byte addressable memory refers to architectures where data can be accessed and addressed in units that are narrower than the bus. An eight bit processor like the Intel 8008 addresses eight bits, but this is the full width of the bus, this is considered as word addressable. The 386SX, which addresses memory in 8 bit units but can be fetched and store it 16 bit at a time, is termed byte addressable.

Bytes have not always meant 8 bits, in fact, depending on the platform, byte sizes of 1  [3]  to 48 bits  [4]  have been used in the past. Therefore, the term ” byte ” is used where the context makes byte-length ambiguous.

For example in the 1980s, Honeywell mainframes had 36 bit words, and were byte addressable in 9 bit bytes, or “nonets”, they used 7- or 8-bit character codes, which were stored one to each 9-bit byte , making characters individually addressable.

To illustrate why byte addressing is useful, consider the IBM 7094 which is word addressable and has no concept of a byte. It has 36 bit words and six-bit character codes to a word.

To change the 15th character in a string, the program has the value of the third character of the third word in the string, “or” in the new one, and then store back the amended word. At least six machine instructions. Usually these are relegated to a subroutine, so every single one of the following features is taken into account.

With byte addressing, which can be achieved in one instruction: Text programs are easier to write, smaller, and run faster.

The advantage of wording is that it can be addressed in the same number of bits. The IBM 7094 has 15-bit addresses, so could address 32768 words of 36-bit. The machines were often built with a full complement of addressable memory. Addressing 32768 bytes of 6 bits would have been much less useful for scientific and engineering users.

Or consider the 32 bit Pentium processor. Its 32-bit address can address 4 billion different items. Using word addressing, a 32-bit address bus could address 4 Gigawords; 16 gigabytes gold using the modern 8 bit byte. If the 386SX and its successors had used word addressing, scientists, engineers, and gamers could have enjoyed programs that were 4x larger on 32 bit machines.

Then again, word processing, HTML rendering, and all other text applications would have run more slowly.

When computers were so expensive that they were only used for science and engineering, word addressing was the obvious mode. As it became cost-effective to use computers for handling text, hardware designers moved to byte addressing.

Some systems, such as the PDP-10 , can be used to address the problem by using a  pointer  , which contains both a word address and the index of a specific byte within the word.

See also

  • endianness
  • Data structure alignment

References

  1. Jump up^   Hansen, HR (1986).  Wirtschaftsinformatik  (in German).  I  (5 ed.). Stuttgart, Germany: Gustav Fischer. p. 125.
  2. Jump up^  “Wortmaschine” (in German). Archived from the original on 2017-04-09 . Retrieved 2017-04-09 .
  3. Jump up^   Buchholz, Werner (February 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 bytewas described as one of a number of parallel bits from one to six. Thus a byte was assumed to have an appropriate length for the occasion. It was used 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 into the design of Stretching a little afterwards. The first published in 1959 in a paper “Processing Data in Bits and Parts” by G A Blaauw, F 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 there Explained 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 Given a 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 can be completely arbitrary and have no relation to actual characters. (The term is coined from  cock  , aim respelled to Avoid accidental mutation to  bit  .) 
    System / 360For example, the economics of economics and economics of economics, which are powers of 2. For economy, however, the size of the variable was fixed at the 8-bit maximum, and it was replaced by byte addressing. […]
  4. Jump up^  3600 Computer System – Reference Manual  (PDF) . K. St. Paul, MN, USA: Control Data Corporation (CDC). 1966-10-11 [1965]. 60021300. Archived (PDF) from the original on 2017-04-05 . Retrieved 2017-04-05 .  Byte – A partition of a computer word.  (NB Discusses 12-bit, 24-bit and 48-bit bytes.)

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