Description of large and small end mode

summary

In calculation , Byte order refers to the bytes in the binary representation of numbers （ Or sometimes bit ） The order of . It can also be used more generally to refer to the internal ordering of any representation , For example, the part of a number or date in a digital system .

In the most common usage , Byte order represents the byte order within a multibyte number . Small end sorting puts the least significant byte first , Put the most significant byte last , The big end sort is the opposite . for example , Consider unsigned hexadecimal numbers 0x1234, This requires at least two bytes to represent . In small end sorting , The bytes will be arranged as [0x34,0x12], In the big end sort, they will be [0x12,0x34].

In memory form

Such as the int Integer numbers , Its value is expressed in hexadecimal ：0x12345678, Then the storage in memory is shown in the following figure ：

Why are there big and small ends

The description given online is ： This is because in a computer system , We are in bytes , Each address corresponds to a byte , A byte is 8bit. But in C In language, except 8bit Of char outside , also 16bit Of short type ,32bit Of long type （ It depends on the compiler ）, in addition , For digits greater than 8 Bit processor , for example 16 Bits or 32 Bit processor , Because the register width is larger than one byte , Then there must be a problem of arranging multiple bytes . So it leads to big end storage mode and small end storage mode . For example, one 16bit Of short type x, The address in memory is 0x0010, for example ,x The value of is 0x1122, that 0x11 For high byte ,0x22 Is low byte . For big end mode , will 0x11 Put it in the low address , namely 0x0010 in ,0x22 Put it in a high address , namely 0x0011 in . The small end model , Just the opposite . That we use a lot X86 The structure is small end mode , and KEIL C51 It's the big end mode . A great deal of ARM,DSP It's all small end mode . There are some ARM The processor can also choose the big end mode or the small end mode by the hardware .

In a nutshell , There is no unified specification for early computer hardware design , So there are hardware designs with inconsistent size ends , And their respective development is very good , When everything is interconnected , Data interaction , The size end has become a problem that must be considered .

present situation

at present Intel Of 80×86 Series chip is the only chip that still insists on using small end ,ARM The chip adopts small end by default , But you can switch to big end ; and MIPS And other chips can be stored in all big end ways , Or provide options to support big end —— You can switch between big and small ends . in addition , The processing of the size end is also related to the implementation of the compiler , stay C In language , The default is small end （ But in some single-chip implementation is based on the big end , such as Keil 51C）,Java It's platform independent , The default is big end . The big end is widely used to transmit data on the network .

Pay attention to the scene at the size end

1. Different end modes must be considered when data transfer is carried out by processors with different end modes
2. Be careful Java and C/C++ The conversion of size end when data is exchanged between programs developed by language
3. When transmitting data over the network , Because the two ends of data transmission correspond to different hardware platforms , The storage byte order used may be inconsistent . So in TCP/IP The protocol stipulates that the network byte order must be adopted on the network , That's the big end mode . about char Type data only takes up one byte , It doesn't matter whether the big end or the small end . But not for char Type data , The data must be converted to big end mode before it is sent to the network . When receiving network data, it shall be received according to the environment of the receiving host .

Reference resources

Reference link  https://en.wikipedia.org/wiki/Endianness