Endianess – When it matters ?

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We all knew what is an endianess in computer architectures , the big endian and little endian architectures works different. Big endian where most significant bit stored at smallest address where as in little endian least significant bit stored at smallest address.

The world’s most popular processor Intel is little endian machine, where world’s most oldest motorola processors are all big endian. All the network protocols , routers works in big endian architecture.

Most of the time endianess doesn’t come into picture , because programmers doesn’t deal with very low level bits. Ideally when I say int number = 4; the program runs properly in both kind of architecture. The reason is when the value is processed , it will be always in processor registers, not directly into memory. Endianess comes into picture when we deal with memory bits.

For example – If we are applying some effects or transform on an image by using shift operators or some standard APIs, then the things might go wrong. OpenGL APIs internally deal with shifting the bits in memory to perform certain operations in such cases the entire image may look inverted.

The common convention followed in networking is big endian , all TCP/IP protocols and other network protocols deal with big endian data. Those who work on networking should be aware of endianness. 

 

Let us take an example how endianness might be a problematic

Assume that a System 1 is sending an ip address to System 2 , If system 1 is 80×86 processor which is a little endian and System2 is SPARC machine which is a big endian. An ip address 192.168.2.1 sent from System 1 , will be received by SPARC machine and converts it to big endian format.

 

If I consider 192.168.2.1 , let us convert this to Hex value

ip address to Hex a logic =  take each octet , multiply it by 256 ^ n, where n is zero based index

= (1 * 256 ^ 0) + (2 * 256 ^ 1) + ( 168 * 256 ^ 2) + (192 * 256 ^ 3)  

= 1 + 512 + 11010048 + 3221225472

= 3232236033 ( decimal )

= C0A80201 ( Hex )

System 1 is sending value C0 A8 02 01 , but System 2 will interpret it as 01 02 A8 C0

This kind of issues happens due to endianness.