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85-466: HDLC ensures reliable data transfer, allowing one device to understand data sent by another. It can operate with or without a continuous connection between devices, making it versatile for various network configurations. Originally, HDLC was used in multi-device networks, where one device acted as the master and others as slaves, through modes like Normal Response Mode (NRM) and Asynchronous Response Mode (ARM). These modes are now rarely used. Currently, HDLC

170-521: A hex dump ), little-endian representation of integers has the significance increasing from right to left. In other words, it appears backwards when visualized, which can be counter-intuitive. This behavior arises, for example, in FourCC or similar techniques that involve packing characters into an integer, so that it becomes a sequence of specific characters in memory. For example, take the string "JOHN", stored in hexadecimal ASCII . On big-endian machines,

255-431: A local network , repeater hubs or switches provide basic connectivity. A hub provides a point-to-multipoint (or simply multipoint) circuit in which all connected client nodes share the network bandwidth. A switch on the other hand provides a series of point-to-point circuits, via microsegmentation, which allows each client node to have a dedicated circuit and the added advantage of having full-duplex connections. From

340-435: A multidrop bus . The original "normal response mode" is a primary-secondary mode where the computer (or primary terminal ) gives each peripheral ( secondary terminal ) permission to speak in turn. Because all communication is either to or from the primary terminal, frames include only one address, that of the secondary terminal; the primary terminal is not assigned an address. There is a distinction between commands sent by

425-531: A point-to-point connection refers to a communications connection between two communication endpoints or nodes . An example is a telephone call , in which one telephone is connected with one other, and what is said by one caller can only be heard by the other. This is contrasted with a point-to-multipoint or broadcast connection, in which many nodes can receive information transmitted by one node. Other examples of point-to-point communications links are leased lines and microwave radio relay . The term

510-481: A 32-bit base address of the segment stored in little-endian order, but in four nonconsecutive bytes, at relative positions 2, 3, 4 and 7 of the descriptor start. Hardware description languages (HDLs) used to express digital logic often support arbitrary endianness, with arbitrary granularity. For example, in SystemVerilog , a word can be defined as little-endian or big-endian. The recognition of endianness

595-470: A byte being part of a "field" is its "significance". These attributes of the parts of a field play an important role in the sequence the bytes are accessed by the computer hardware, more precisely: by the low-level algorithms contributing to the results of a computer instruction. Positional number systems (mostly base 2, or less often base 10) are the predominant way of representing and particularly of manipulating integer data by computers. In pure form this

680-419: A combined station, it is important to maintain the distinction between P and F bits, because there may be two checkpoint cycles operating simultaneously. A P bit arriving in a command from the remote station is not in response to our P bit; only an F bit arriving in a response is. Both I and S frames contain a receive sequence number N(R). N(R) provides a positive acknowledgement for the receipt of I-frames from

765-433: A command S frame and a response S frame; when P/F is 0, the two forms are exactly equivalent. Unnumbered frames, or U-frames , are primarily used for link management, although a few are used to transfer user data. They exchange session management and control information between connected devices, and some U-frames contain an information field, used for system management information or user data. The first 2 bits (11) mean it

850-418: A distance, each endpoint would be fitted with a modem to convert analog telecommunications signals into a digital data stream. When the connection uses a telecommunications provider, the connection is called a dedicated , leased , or private line . The ARPANET used leased lines to provide point-to-point data links between its packet-switching nodes, which were called Interface Message Processors . With

935-468: A leading "10" indicating that it is an S-frame. This is followed by a 2-bit type, a poll/final bit, and a 3-bit sequence number. (Or a 4-bit padding field followed by a 7-bit sequence number.) The first (least significant) 2 bits mean it is an S-frame. All S frames include a P/F bit and a receive sequence number as described above. Except for the interpretation of the P/F field, there is no difference between

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1020-415: A line, a receiver on the line will see continuous idling 1-bits in the inter-frame period when no transmitter is active. HDLC transmits bytes of data with the least significant bit first (not to be confused with little-endian order, which refers to byte ordering within a multi-byte field). When using asynchronous serial communication such as standard RS-232 serial ports , synchronous-style bit stuffing

1105-472: A little-endian should start with FF FE 00 00 . Application binary data formats, such as MATLAB .mat files, or the .bil data format, used in topography, are usually endianness-independent. This is achieved by storing the data always in one fixed endianness or carrying with the data a switch to indicate the endianness. An example of the former is the binary XLS file format that is portable between Windows and Mac systems and always little-endian, requiring

1190-399: A new design is often arbitrary, but later technology revisions and updates perpetuate the existing endianness to maintain backward compatibility . A big-endian system stores the most significant byte of a word at the smallest memory address and the least significant byte at the largest. A little-endian system, in contrast, stores the least-significant byte at the smallest address. Of

1275-547: A number of hardware architectures where floating-point numbers are represented in big-endian form while integers are represented in little-endian form. There are ARM processors that have mixed-endian floating-point representation for double-precision numbers: each of the two 32-bit words is stored as little-endian, but the most significant word is stored first. VAX floating point stores little-endian 16-bit words in big-endian order. Because there have been many floating-point formats with no network standard representation for them,

1360-565: A parade route back to the studio. Little-endian In computing , endianness is the order in which bytes within a word of digital data are transmitted over a data communication medium or addressed (by rising addresses) in computer memory , counting only byte significance compared to earliness. Endianness is primarily expressed as big-endian (BE) or little-endian (LE), terms introduced by Danny Cohen into computer science for data ordering in an Internet Experiment Note published in 1980. The adjective endian has its origin in

1445-517: A positive acknowledge packet, the sender can retransmit the failed frame. The FCS was implemented because many early communication links had a relatively high bit error rate , and the FCS could readily be computed by simple, fast circuitry or software. More effective forward error correction schemes are now widely used by other protocols. Synchronous Data Link Control ( SDLC ) was originally designed to connect one computer with multiple peripherals via

1530-483: 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, though this is not always the case, such as on Intel's IA-64 -based Itanium CPU, which allows both. Some nominally bi-endian CPUs require motherboard help to fully switch endianness. For instance, the 32-bit desktop-oriented PowerPC processors in little-endian mode act as little-endian from

1615-413: A protocol field to the standard HDLC header. HDLC frames can be transmitted over synchronous or asynchronous serial communication links. Those links have no mechanism to mark the beginning or end of a frame, so the beginning and end of each frame has to be identified. This is done by using a unique sequence of bits as a frame delimiter, or flag , and encoding the data to ensure that the flag sequence

1700-411: A receiving modem to synchronize its clock via a phase-locked loop . If there are too many 1-bits in a row, the receiver can lose count. Bit-stuffing provides a minimum of one transition per six bit times during transmission of data, and one transition per seven bit times during transmission of a flag. When no frames are being transmitted on a simplex or full-duplex synchronous link, a frame delimiter

1785-402: A row. This is done by bit stuffing : any time that five consecutive 1-bits appear in the transmitted data, the data is paused and a 0-bit is transmitted. The receiving device knows that this is being done, and after seeing five 1-bits in a row, a following 0-bit is stripped out of the received data. If instead the sixth bit is 1, this is either a flag (if the seventh bit is 0), or an error (if

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1870-436: A setting which allows for switchable endianness in data fetches and stores, instruction fetches, or both; those instruction set architectures are referred to as bi-endian . Architectures that support switchable endianness include PowerPC / Power ISA , SPARC V9, ARM versions 3 and above, DEC Alpha , MIPS , Intel i860 , PA-RISC , SuperH SH-4 , IA-64 , C-Sky , and RISC-V . This feature can improve performance or simplify

1955-406: A single byte, so the complexity of the hardware is not affected by the byte ordering. Addition, subtraction, and multiplication start at the least significant digit position and propagate the carry to the subsequent more significant position. On most systems, the address of a multi-byte value is the address of its first byte (the byte with the lowest address). The implementation of these operations

2040-522: A single positional element (character) also has a positional value. Lexicographical comparison means almost everywhere: first character ranks highest – as in the telephone book. Almost all machines which can do this using a single instruction are big-endian or at least mixed-endian. Integer numbers written as text are always represented most significant digit first in memory, which is similar to big-endian, independently of text direction . When memory bytes are printed sequentially from left to right (e.g. in

2125-464: A time. It also allows operation over half-duplex communication links, as long as the primary is aware that it may not transmit when it has permitted a secondary to do so. Asynchronous response mode is an HDLC addition for use over full-duplex links. While retaining the primary/secondary distinction, it allows the secondary to transmit at any time. Thus, there must be some other mechanism to ensure that multiple secondaries do not try to transmit at

2210-404: A transmitter which transmits a narrow beam of microwaves with a parabolic dish antenna to a second parabolic dish at the receiver. It also includes technologies such as lasers which transmit data modulated on a light beam. These technologies require an unobstructed line of sight between the two points and thus are limited by the visual horizon to distances of about 40 miles (64 km). In

2295-491: A word in a register to the opposite endianness, that is, they swap the order of the bytes in a 16-, 32- or 64-bit word. Recent Intel x86 and x86-64 architecture CPUs have a MOVBE instruction ( Intel Core since generation 4, after Atom ), which fetches a big-endian format word from memory or writes a word into memory in big-endian format. These processors are otherwise thoroughly little-endian. There are also devices which use different formats in different places. For instance,

2380-501: Is a 16-bit CRC-CCITT or a 32-bit CRC-32 computed over the Address, Control, and Information fields. It provides a means by which the receiver can detect errors that may have been induced during the transmission of the frame, such as lost bits, flipped bits, and extraneous bits. However, given that the algorithms used to calculate the FCS are such that the probability of certain types of transmission errors going undetected increases with

2465-658: Is a U-frame. The five type bits (2 before P/F bit and 3 bit after P/F bit) can create 32 different types of U-frame. In a few cases, the same encoding is used for different things as a command and a response. The various modes are described in § Link configurations . Briefly, there are two non-operational modes (initialization mode and disconnected mode) and three operational modes (normal response, asynchronous response, and asynchronous balanced modes) with 3-bit or 7-bit (extended) sequence numbers. These frames may be used as part of normal information transfer. Point-to-point (telecommunications) In telecommunications ,

2550-400: Is also used in computer networking and computer architecture to refer to a wire or other connection that links only two computers or circuits, as opposed to other network topologies such as buses or crossbar switches which can connect many communications devices. Point-to-point is sometimes abbreviated as P2P . This usage of P2P is distinct from P2P meaning peer-to-peer in

2635-520: Is assigned a number, called its address , that the computer uses to access that data. On most modern computers, the smallest data group with an address is eight bits long and is called a byte. Larger groups comprise two or more bytes, for example, a 32-bit word contains four bytes. There are two possible ways a computer could number the individual bytes in a larger group, starting at either end. Both types of endianness are in widespread use in digital electronic engineering. The initial choice of endianness of

High-Level Data Link Control - Misplaced Pages Continue

2720-412: Is continuously transmitted on the link. This generates one of two continuous waveforms, depending on the initial state: [REDACTED] The HDLC specification allows the 0-bit at the end of a frame delimiter to be shared with the start of the next frame delimiter, i.e. "011111101111110". Some hardware does not support this. For half-duplex or multi-drop communication, where several transmitters share

2805-406: Is important when reading a file or filesystem created on a computer with different endianness. Fortran sequential unformatted files created with one endianness usually cannot be read on a system using the other endianness because Fortran usually implements a record (defined as the data written by a single Fortran statement) as data preceded and succeeded by count fields, which are integers equal to

2890-481: Is in principle a 16-bit little-endian system. The instructions to convert between floating-point and integer values in the optional floating-point processor of the PDP-11/45, PDP-11/70, and in some later processors, stored 32-bit "double precision integer long" values with the 16-bit halves swapped from the expected little-endian order. The UNIX C compiler used the same format for 32-bit long integers. This ordering

2975-431: Is inappropriate for several reasons: Instead asynchronous framing uses "control-octet transparency", also called " byte stuffing " or "octet stuffing". The frame boundary octet is 01111110, (0x7E in hexadecimal notation). A "control escape octet ", has the value 0x7D (bit sequence '10111110', as RS-232 transmits least-significant bit first). If either of these two octets appears in the transmitted data, an escape octet

3060-539: Is known as PDP-endian . UNIX was one of the first systems to allow the same code to be compiled for platforms with different internal representations. One of the first programs converted was supposed to print out Unix , but on the Series/1 it printed nUxi instead. A way to interpret this endianness is that it stores a 32-bit integer as two little-endian 16-bit words, with a big-endian word ordering: Segment descriptors of IA-32 and compatible processors keep

3145-434: Is marginally simpler using little-endian machines where this first byte contains the least significant digit. Comparison and division start at the most significant digit and propagate a possible carry to the subsequent less significant digits. For fixed-length numerical values (typically of length 1,2,4,8,16), the implementation of these operations is marginally simpler on big-endian machines. Some big-endian processors (e.g.

3230-406: Is meant as the extremity where the big resp. little significance is written first , namely where the field starts . The integer data that are directly supported by the computer hardware have a fixed width of a low power of 2, e.g. 8 bits ≙ 1 byte, 16 bits ≙ 2 bytes, 32 bits ≙ 4 bytes, 64 bits ≙ 8 bytes, 128 bits ≙ 16 bytes. The low-level access sequence to the bytes of such a field depends on

3315-451: Is never seen inside a frame. Each frame begins and ends with a frame delimiter. A frame delimiter at the end of a frame may also mark the start of the next frame. On both synchronous and asynchronous links, the flag sequence is binary "01111110", or hexadecimal 0x7E, but the details are quite different. Because a flag sequence consists of six consecutive 1-bits, other data is coded to ensure that it never contains more than five 1-bits in

3400-403: Is possible to establish a permanent circuit. One example might be a telephone in the lobby of a public building, which is programmed to ring only the number of a telephone dispatcher. "Nailing down" a switched connection saves the cost of running a physical circuit between the two points. The resources in such a connection can be released when no longer needed, for example, a television circuit from

3485-514: Is primarily employed in point-to-point connections , such as between routers or network interfaces , using a mode called Asynchronous Balanced Mode (ABM). HDLC is based on IBM 's SDLC protocol, which is the layer 2 protocol for IBM's Systems Network Architecture (SNA). It was extended and standardized by the ITU as LAP (Link Access Procedure), while ANSI named their essentially identical version ADCCP . The HDLC specification does not specify

High-Level Data Link Control - Misplaced Pages Continue

3570-442: Is redirected to the corresponding address and unaligned access is not allowed. ARMv6 introduces BE-8 or byte-invariant mode, where access to a single byte works as in little-endian mode, but accessing a 16-bit, 32-bit or (starting with ARMv8) 64-bit word results in a byte swap of the data. This simplifies unaligned memory access as well as memory-mapped access to registers other than 32-bit. Many processors have instructions to convert

3655-438: Is sent, followed by the original data octet with bit 5 inverted. For example, the byte 0x7E would be transmitted as 0x7D 0x5E ("10111110 01111010"). Other reserved octet values (such as XON or XOFF ) can be escaped in the same way if necessary. The "abort sequence" 0x7D 0x7E ends a packet with an incomplete byte-stuff sequence, forcing the receiver to detect an error. This can be used to abort packet transmission with no chance

3740-515: Is used throughout the file. Computer memory consists of a sequence of storage cells (smallest addressable units); in machines that support byte addressing , those units are called bytes . Each byte is identified and accessed in hardware and software by its memory address . If the total number of bytes in memory is n , then addresses are enumerated from 0 to n  − 1. Computer programs often use data structures or fields that may consist of more data than can be stored in one byte. In

3825-413: Is valid for moderate sized non-negative integers, e.g. of C data type unsigned . In such a number system, the value of a digit which it contributes to the whole number is determined not only by its value as a single digit, but also by the position it holds in the complete number, called its significance. These positions can be mapped to memory mainly in two ways: In these expressions, the term "end"

3910-527: The 6809 and the 68000 series of processors use the big-endian format. Solely big-endian architectures include the IBM z/Architecture and OpenRISC . The PDP-11 minicomputer, however, uses little-endian byte order, as does its VAX successor. The Datapoint 2200 used simple bit-serial logic with little-endian to facilitate carry propagation . When Intel developed the 8008 microprocessor for Datapoint, they used little-endian for compatibility. However, as Intel

3995-682: The Altera Nios II , the Atmel AVR , the Andes Technology NDS32, the Qualcomm Hexagon , and many other processors and processor families are also little-endian. The Intel 8051 , unlike other Intel processors, expects 16-bit addresses for LJMP and LCALL in big-endian format; however, xCALL instructions store the return address onto the stack in little-endian format. Some instruction set architectures feature

4080-650: The Cray T3E ). IBM AIX and IBM i run in big-endian mode on bi-endian Power ISA; Linux originally ran in big-endian mode, but by 2019, IBM had transitioned to little-endian mode for Linux to ease the porting of Linux software from x86 to Power. SPARC has no relevant little-endian deployment, as both Oracle Solaris and Linux run in big-endian mode on bi-endian SPARC systems, and can be considered big-endian in practice. ARM, C-Sky, and RISC-V have no relevant big-endian deployments, and can be considered little-endian in practice. The term bi-endian refers primarily to how

4165-463: The Intel Fortran compiler supports the non-standard CONVERT specifier when opening a file, e.g.: OPEN ( unit , CONVERT = 'BIG_ENDIAN' ,...) . Other compilers have options for generating code that globally enables the conversion for all file IO operations. This permits the reuse of code on a system with the opposite endianness without code modification. On most systems,

4250-471: The Internet protocol suite , where it is referred to as network order , transmitting the most significant byte first. Conversely, little-endianness is the dominant ordering for processor architectures ( x86 , most ARM implementations, base RISC-V implementations) and their associated memory. File formats can use either ordering; some formats use a mixture of both or contain an indicator of which ordering

4335-492: The OSI model 's layer perspective, both switches and repeater hubs provide point-to-point connections on the physical layer . However, on the data link layer , a repeater hub provides point-to-multipoint connectivity – each frame is forwarded to all nodes – while a switch provides virtual point-to-point connections – each unicast frame is only forwarded to the destination node. Within many switched telecommunications systems , it

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4420-454: The XDR standard uses big-endian IEEE 754 as its representation. It may therefore appear strange that the widespread IEEE 754 floating-point standard does not specify endianness. Theoretically, this means that even standard IEEE floating-point data written by one machine might not be readable by another. However, on modern standard computers (i.e., implementing IEEE 754), one may safely assume that

4505-531: The Add instruction of the IBM 1401 addresses variable-length fields at their low-order (highest-addressed) position with their lengths being defined by a word mark set at their high-order (lowest-addressed) position. When an operation such as addition is performed, the processor begins at the low-order positions at the high addresses of the two fields and works its way down to the high-order. Another important attribute of

4590-675: The BQ27421 Texas Instruments battery gauge uses the little-endian format for its registers and the big-endian format for its random-access memory . SPARC historically used big-endian until version 9, which is bi-endian. Similarly early IBM POWER processors were big-endian, but the PowerPC and Power ISA descendants are now bi-endian. The ARM architecture was little-endian before version 3 when it became bi-endian. Although many processors use little-endian storage for all types of data (integer, floating point), there are

4675-495: The C11 standard and commonly used in code interacting with hardware. Some operations in positional number systems have a natural or preferred order in which the elementary steps are to be executed. This order may affect their performance on small-scale byte-addressable processors and microcontrollers . However, high-performance processors usually fetch multi-byte operands from memory in the same amount of time they would have fetched

4760-720: The IBM System/360 and its successors) contain hardware instructions for lexicographically comparing varying length character strings . The normal data transport by an assignment statement is in principle independent of the endianness of the processor. Many historical and extant processors use a big-endian memory representation, either exclusively or as a design option. The IBM System/360 uses big-endian byte order, as do its successors System/370 , ESA/390 , and z/Architecture . The PDP-10 uses big-endian addressing for byte-oriented instructions. The IBM Series/1 minicomputer uses big-endian byte order. The Motorola 6800 / 6801,

4845-584: The P/F bit set), and the address field of a received frame must be examined to determine whether it contains a command (the address received is ours) or a response (the address received is that of the other terminal). This means that the address field is not optional, even on point-to-point links where it is not needed to disambiguate the peer being talked to. Some HDLC variants extend the address field to include both source and destination addresses, or an explicit command/response bit. Three fundamental types of HDLC frames may be distinguished: The general format of

4930-421: The address of a multi-byte value is the address of its first byte (the byte with the lowest address); little-endian systems of that type have the property that, for sufficiently low data values, the same value can be read from memory at different lengths without using different addresses (even when alignment restrictions are imposed). For example, a 32-bit memory location with content 4A 00 00 00 can be read at

5015-573: The context of file sharing networks or other data-sharing protocols between peers. A traditional point-to-point data link is a communications medium with exactly two endpoints and no data or packet formatting. The host computers at either end take full responsibility for formatting the data transmitted between them. The connection between the computer and the communications medium was generally implemented through an RS-232 or similar interface. Computers in close proximity may be connected by wires directly between their interface cards. When connected at

5100-439: The context of this article where its type cannot be arbitrarily complicated, a "field" consists of a consecutive sequence of bytes and represents a "simple data value" which – at least potentially – can be manipulated by one single hardware instruction . On most systems, the address of a multi-byte simple data value is the address of its first byte (the byte with the lowest address). There are exceptions to this rule – for example,

5185-410: The control field is: There are also extended (two-byte) forms of I and S frames. Again, the least significant bit (rightmost in this table) is sent first. Poll/Final is a single bit with two names. It is called Poll when part of a command (set by the primary station to obtain a response from a secondary station), and Final when part of a response (set by the secondary station to indicate a response or

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5270-561: The current standard, replaced all of these specifications. HDLC was the inspiration for the IEEE 802.2 LLC protocol, and it is the basis for the framing mechanism used with the PPP on synchronous lines, as used by many servers to connect to a WAN , most commonly the Internet . A similar version is used as the control channel for E-carrier (E1) and SONET multichannel telephone lines. Cisco HDLC uses low-level HDLC framing techniques but adds

5355-429: The end of transmission). In all other cases, the bit is clear. The bit is used as a token that is passed back and forth between the stations. Only one token should exist at a time. The secondary only sends a Final when it has received a Poll from the primary. The primary only sends a Poll when it has received a Final back from the secondary, or after a timeout indicating that the bit has been lost. When operating as

5440-446: The endianness is the same for floating-point numbers as for integers, making the conversion straightforward regardless of data type. Small embedded systems using special floating-point formats may be another matter, however. Most instructions considered so far contain the size (lengths) of their operands within the operation code . Frequently available operand lengths are 1, 2, 4, 8, or 16 bytes. But there are also architectures where

5525-437: The exception of passive optical networks , modern Ethernet is exclusively point-to-point on the physical layer – any cable only connects two devices. The term point-to-point telecommunications can also mean a wireless data link between two fixed points. The wireless communication is typically bi-directional and either time-division multiple access (TDMA) or channelized . This can be a microwave relay link consisting of

5610-542: The full semantics of the frame fields. This allows other fully compliant standards to be derived from it, and derivatives have since appeared in innumerable standards. It was adopted into the X.25 protocol stack as LAPB , into the V.42 protocol as LAPM , into the Frame Relay protocol stack as LAPF and into the ISDN protocol stack as LAPD. The original ISO standards for HDLC are the following: ISO/IEC 13239:2002,

5695-570: The interpretation of the P/F field, there is no difference between a command I frame and a response I frame; when P/F is 0, the two forms are exactly equivalent. Supervisory Frames, or 'S-frames', are used for flow and error control whenever piggybacking is impossible or inappropriate, such as when a station does not have data to send. S-frames in HDLC do not have information fields, although some HDLC-derived protocols use information fields for "multi-selective reject". The S-frame control field includes

5780-406: The late 1990s (SPARC v9 compliant processors) allow data endianness to be chosen with each individual instruction that loads from or stores to memory. The ARM architecture supports two big-endian modes, called BE-8 and BE-32 . CPUs up to ARMv5 only support BE-32 or word-invariant mode. Here any naturally aligned 32-bit access works like in little-endian mode, but access to a byte or 16-bit word

5865-588: The length of an operand may be held in a separate field of the instruction or with the operand itself, e.g. by means of a word mark . Such an approach allows operand lengths up to 256 bytes or larger. The data types of such operands are character strings or BCD . Machines able to manipulate such data with one instruction (e.g. compare, add) include the IBM 1401 , 1410 , 1620 , System/360 , System/370 , ESA/390 , and z/Architecture , all of them of type big-endian. Numerous other orderings, generically called middle-endian or mixed-endian , are possible. The PDP-11

5950-405: The length of the data being checked for errors, the FCS can implicitly limit the practical size of the frame. If the receiver's calculation of the FCS does not match that of the sender's, indicating that the frame contains errors, the receiver can either send a negative acknowledge packet to the sender, or send nothing. After either receiving a negative acknowledge packet or timing out waiting for

6035-497: The logic of networking devices and software. The word bi-endian , when said of hardware, denotes the capability of the machine to compute or pass data in either endian format. 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. Alpha, which runs only in big-endian mode on

6120-440: The number of bits in the sequence number, up to 7 or 127 I-frames may be awaiting acknowledgment at any time. Information frames, or I-frames , transport user data from the network layer. In addition they also include flow and error control information piggybacked on data. The sub-fields in the control field define these functions. The least significant bit (first transmitted) defines the frame type. 0 means an I-frame. Except for

6205-403: The number of bytes in the data. An attempt to read such a file using Fortran on a system of the other endianness results in a run-time error, because the count fields are incorrect. Unicode text can optionally start with a byte order mark (BOM) to signal the endianness of the file or stream. Its code point is U+FEFF. In UTF-32 for example, a big-endian file should start with 00 00 FE FF ;

6290-416: The operation to be performed. The least-significant byte is accessed first for addition , subtraction and multiplication . The most-significant byte is accessed first for division and comparison . See § Calculation order . When character (text) strings are to be compared with one another, e.g. in order to support some mechanism like sorting , this is very frequently done lexicographically where

6375-513: The other side of the link. Its value is always the first frame not yet received; it acknowledges that all frames with N(S) values up to N(R)−1 (modulo 8 or modulo 128) have been received and indicates the N(S) of the next frame it expects to receive. N(R) operates the same way whether it is part of a command or response. A combined station only has one sequence number space. This is incremented for successive I-frames, modulo 8 or modulo 128. Depending on

6460-444: The partial packet will be interpreted as valid by the receiver. The contents of an HDLC frame are shown in the following table: Note that the end flag of one frame may be (but does not have to be) the beginning (start) flag of the next frame. Data is usually sent in multiples of 8 bits, but only some variants require this; others theoretically permit data alignments on other than 8-bit boundaries. The frame check sequence (FCS)

6545-560: The point of view of the executing programs, but they require the motherboard to perform a 64-bit swap across all 8 byte lanes to ensure that the little-endian view of things will apply to I/O devices. In the absence of this unusual motherboard hardware, device driver software must write to different addresses to undo the incomplete transformation and also must perform a normal byte swap. Some CPUs, such as many PowerPC processors intended for embedded use and almost all SPARC processors, allow per-page choice of endianness. SPARC processors since

6630-416: The primary to a secondary, and responses sent by a secondary to the primary, but this is not reflected in the encoding; commands and responses are indistinguishable except for the difference in the direction in which they are transmitted. Normal response mode allows the secondary-to-primary link to be shared without contention , because it has the primary give the secondaries permission to transmit one at

6715-427: The same address as either 8-bit (value = 4A), 16-bit (004A), 24-bit (00004A), or 32-bit (0000004A), all of which retain the same numeric value. Although this little-endian property is rarely used directly by high-level programmers, it is occasionally employed by code optimizers as well as by assembly language programmers. While not allowed by C++, such type punning code is allowed as "implementation-defined" by

6800-406: The same time (or only one secondary). Asynchronous balanced mode adds the concept of a combined terminal which can act as both a primary and a secondary. Unfortunately, this mode of operation has some implementation subtleties. While the most common frames sent do not care whether they are in a command or response frame, some essential ones do (notably most unnumbered frames, and any frame with

6885-432: The seventh bit is 1). In the latter case, the frame receive procedure is aborted, to be restarted when a flag is next seen. This bit-stuffing serves a second purpose, that of ensuring a sufficient number of signal transitions. On synchronous links, the data is NRZI encoded, so that a 0-bit is transmitted as a change in the signal on the line, and a 1-bit is sent as no change. Thus, each 0 bit provides an opportunity for

6970-507: The two, big-endian is thus closer to the way the digits of numbers are written left-to-right in English, comparing digits to bytes. Bi-endianness is a feature supported by numerous computer architectures that feature switchable endianness in data fetches and stores or for instruction fetches. Other orderings are generically called middle-endian or mixed-endian . Big-endianness is the dominant ordering in networking protocols, such as in

7055-997: The value appears left-to-right, coinciding with the correct string order for reading the result ("J O H N"). But on a little-endian machine, one would see "N H O J". Middle-endian machines complicate this even further; for example, on the PDP-11 , the 32-bit value is stored as two 16-bit words "JO" "HN" in big-endian, with the characters in the 16-bit words being stored in little-endian, resulting in "O J N H". Byte-swapping consists of rearranging bytes to change endianness. Many compilers provide built-ins that are likely to be compiled into native processor instructions ( bswap / movbe ), such as __builtin_bswap32 . Software interfaces for swapping include: Some CPU instruction sets provide native support for endian byte swapping, such as bswap ( x86 — 486 and later, i960 — i960Jx and later ), and rev ( ARMv6 and later). Some compilers have built-in facilities for byte swapping. For example,

7140-420: The writings of 18th century Anglo-Irish writer Jonathan Swift . In the 1726 novel Gulliver's Travels , he portrays the conflict between sects of Lilliputians divided into those breaking the shell of a boiled egg from the big end or from the little end. By analogy, a CPU may read a digital word big end first, or little end first. Computers store information in various-sized groups of binary bits. Each group

7225-513: Was unable to deliver the 8008 in time, Datapoint used a medium-scale integration equivalent, but the little-endianness was retained in most Intel designs, including the MCS-48 and the 8086 and its x86 successors, including IA-32 and x86-64 processors. The MOS Technology 6502 family (including Western Design Center 65802 and 65C816 ), the Zilog Z80 (including Z180 and eZ80 ),

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