[ECOS] REAL meaning of little-endian and big-endian?

Chris Gray chris.gray@acunia.com
Tue Dec 17 02:58:00 GMT 2002

On Tue, 17 Dec 2002, [gb2312] ÕÃ
 ÁÁ wrote:

> hello,every one!
>     I have a "old" and "simple" question (i think like this before):
>      1  both endian mode only differ in byte order? how  bit-order in one byte?

If we are talking about data stored in memory, this question is pretty 
well meaningless: for most CPUs the smallest amount of data that can be 
moved between CPU and memory is a byte, and all 8 bits move at the same 
time. So whether you call the most or the least significant bit 'bit 0' is 
purely a matter of convention: nowadays everybody calls the least 
significant bit 'bit 0', but at one time the US did this and the UK did 
the opposite! So an american would say `bit 0' and a brit would say `bit 
7', and they would be talking about the same bit ...

If we are talking about how data is transmitted `on the wire', then the 
question may or may not be meaningful (think of a parallel cable!), and if 
it is then the answer is only really useful if you are monitoring the wire 
with an oscilloscope: your software can only see the data as an array of 
bytes in memory, individual bits being handled by the hardware UART or MAC 
or whatever. I think that most serial communication actually sends the 
least significant bit first (and that ISO 3309 calls this 'bit 1'!), but I 
may be misremembering.

>      2   if differ in bit order,one byte with little-endian come from 
> net,then how me(receiver host with big-endian) to read it correctlly? from 
> IP processing,it simplely read it! why?

See above. Unless you are actually `bit-banging' in software, you never 
see the individual bits within a byte. In IP, fields of more than one byte 
(e.g. the network address) are _always_ transmitted with the most 
significant byte first, regardless of the `endianness' of the CPU 
involved. So `host byte ordering' may be big- or little-endian, but 
`network byte ordering' is always big-endian.

>     3 if identical in bit order,why so define:
> struct ip_hdr
> {
>  unsigned char ip_version;4,
>                ip_hlen:4;
> unsigned char ip_hlen :4,
>               ip_version:4;
> #endif

Very good question! The C standard doesn't specify the order of bitfields 
within a byte or word, so the interpretation is up to the writer of the 
compiler. The code you quote has probably been tested with gcc on both 
big- and little-endian machines, so I suppose that gcc uses a convention 
that bitfields are arranged according to the endianness of the CPU. Other 
C compilers may behave differently. (I got badly burned by this some 7 or 
8 years ago, and I've never used bitfields since. 8-0 So I neither know 
nor care what convention gcc is using ...)

Good luck


Chris Gray

VM Architect, ACUNIA

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