Information AboutEndianness |
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For hardware, the refers to a field indicating the order of the bytes in a number field as NUMSEX, and the Mac OS X operating system refers to "byte sex" in its compiler tools {Link without Title} .Generally speaking, endianness is a particular attribute of a representation format - which byte of a UCS-2 character would be stored at the lower address, etc. Byte order is an important consideration in network programming, since two computers with different byte orders may be communicating. Failure to account for varying endianness is notorious among Computer Programmer s as a source of Bugs . ENDIANNESS AND HARDWARE Most modern Computer Processor s agree on bit ordering "inside" individual bytes (this was not always the case). This means that any single-byte value will be read the same on almost any computer one may send it to. Integers are usually stored as sequences of bytes, so that the encoded value can be obtained by simple concatenation. The two most common of them are:
Again, big-endian does not mean "ending big", but "big end first". Intel's X86 processors use the little-endian format (sometimes called the ''Intel format''). Motorola processors have generally used big-endian. PowerPC (which includes Apple's Macintosh line prior to the Intel Switch ) and System/370 also adopt big-endian. SPARC historically used big-endian, though version 9 is bi-endian (see below). Bi-endian hardware Some architectures (including ARM , PowerPC (but not the PPC970/G5 ), DEC Alpha , SPARC V9 , MIPS , PA-RISC and IA64 ) feature switchable endianness. That can improve performance or simplify the logic of networking devices and software. The word ''bi-endian'', said of hardware, denotes the capability to compute or pass data in either of two different endian formats (usually big-endian and little-endian). Many of these architectures can be switched via software to default to a specific endian format (usually done when the computer starts up); however, on some systems the default endianness is selected by hardware on the motherboard and cannot be changed via software (e.g., the DEC Alpha, which runs only in big-endian mode on the Cray T3E ). Note that "bi-endian" refers primarily to how a processor treats ''data'' accesses. ''Instruction'' accesses (fetches of instruction words) on a given processor may still assume a fixed endianness, even if ''data'' accesses are fully bi-endian. Note, too, that some nominally bi-endian CPUs may actually employ internal "magic" (as opposed to really switching to a different endianness) in one of their operating modes. For instance, some PowerPC processors in little-endian mode act as little-endian from the point of view of the executing programs but they do not actually store data in memory in little-endian format (multi-byte values are swapped during memory load/store operations). This can cause problems when memory is transferred to an external device if some part of the software, e.g. a device driver, does not account for the situation. Floating-point and endianness On some machines, while integers were represented in little-endian form, floating-point numbers were represented in big-endian form. Because there are many floating formats, and a lack of a standard "network" representation, no standard for transferring floating point values has been made. This means that floating point data written on one machine may not be readable on another — even if both use IEEE 754 floating point arithmetic (as the endian-ness of the memory representation is not part of the IEEE specification). [http://www.cs.cmu.edu/People/rgs/pl-exp-conv.html DISCUSSION, BACKGROUND, ETYMOLOGY The choice of big-endian vs. little-endian has been the subject of on Wikisource The terms ''little-endian'' and ''endianness'' have a similar ironic intent. Endian FAQ – includes the paper Internet Engineering Note (IEN) 137: ''On Holy Wars and a Plea for Peace'' ftp mirror by Danny Cohen (1 April 1980), but adds much more context. An often cited argument in favour of big-endian is that it is consistent with the ordering used in natural languages. But that is far from being Universal , both in spoken and written form:
Little-endian has the property that, in the absence of Alignment restrictions, values can be read from memory at different widths without using different addresses. For example, a 32-bit memory location with content 4A 00 00 00 can be read at the same address as either 8-bit (value = 4A), 16-bit (004A), or 32-bit (0000004A). (This example works only if the value makes sense in all three sizes, which means the value fits in just 8 bits.) This little-endian property is rarely used, and doesn't imply that little-endian has any performance advantage in variable-width data access. A note on some non-idiomatic usages: some authors extend the usage of the word "endianness", and of related terms, to entities such as talks about "descending order year-month-day", not about "big-endian format"), do not have widespread usage, and are generally (other than for date formats) employed in a metaphorical sense. EXAMPLES OF STORING THE VALUE <TT>0X0A0B0C0D</TT> IN MEMORY Note: the prefix 0x indicates Hexadecimal notation. To further illustrate the above notions this section provides example layouts of a 32-bit number in the most common variants of endianness. There is no general guarantee that a platform will use one of these formats but in practice there are few if any exceptions. All the examples refer to the storage in memory of the value 0x0A0B0C0D. Big-endian
The Most Significant Byte (''MSB'') value, which is 0x0A in our example, is stored at the memory location with the lowest address, the next byte value in significance, 0x0B, is stored at the following memory location and so on. This is akin to Left-to-Right reading order in hexadecimal.
The most significant atomic element stores now the value 0x0A0B, followed by 0x0C0D. Little-endian
The Least Significant Byte (''LSB'') value, 0x0D, is at the lowest address. The other bytes follow in increasing order of significance.
The least significant 16-bit unit stores the value 0x0C0D, immediately followed by 0x0A0B.
The 16-bit atomic element byte ordering may look a little backwards as written above, but this is because little-endian is best written with addressing increasing towards the left. If we write the bytes this way then the ordering makes slightly more sense: The Least Significant Byte (''LSB'') value, 0x0D, is at the lowest address. The other bytes follow in increasing order of significance. The least significant 16-bit unit stores the value 0x0C0D, immediately followed by 0x0A0B. Middle-endian Still other architectures, generically called ''middle-endian'' or ''mixed-endian'', may have a more complicated ordering; PDP-11 , for instance, stored some 32-bit words, counting from the most significant, as: 2nd byte first, then 1st, then 4th, and finally 3rd.
Note that this can be interpreted as storing the most significant "half" (16-bits) followed by the less significant half (as if big-endian) but with each half stored in little-endian format. This ordering is known as ''PDP-endianness''. The ARM Architecture can also produce this format when writing a 32-bit word to an address 2 bytes from a 32-bit word Alignment . ENDIANNESS IN NETWORKING Networks generally use big-endian order, and thus it is called network order when sending information over a network in a common format. The historical reason is that this allowed Routing while a Telephone Number was being composed. In fact, the Internet Protocol defines a standard big-endian ''network byte order''. This byte order is used for all numeric values in the packet headers and by many higher level protocols and file formats that are designed for use over IP. The Berkeley Sockets API defines a set of functions to convert 16- and 32-bit integers to and from network byte order: the htonl (host-to-network-long) and htons (host-to-network-short) functions convert 32-bit and 16-bit values respectively from machine (''host'') to network order; whereas the ntohl and ntohs functions convert from network to host order. While the lowest network protocols may deal with sub-byte formatting, all the layers above them usually consider the ''byte'' (mostly meant as '' Octet '') as their atomic unit. "BIT ENDIANNESS" The terms ''bit endianness'' or ''bit-level endianness'' are seldom used when talking about the representation of a stored value, as they are only meaningful for the rare computer architectures which support addressing of individual bits. They are used however to refer to the transmission order of bits over a serial medium. Most often that order is transparently managed by the hardware and is the bit-level analogue of little-endian (low-bit first), although protocols exist which require the opposite ordering (e.g. I&2C ). In networking, the decision about the order of transmission of bits is made in the very bottom of the Data Link Layer of the OSI Model . NOTES EXTERNAL LINKS
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