byte order of ethernet destination address

byte order of ethernet destination address
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Subject Author Date
byte order of ethernet destination address pranavtailor 07-26-2006
Posted by on July 26, 2006, 11:31 pm
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Hi people,
A simple and may be silly query for you. Can you tell me which byte
of the ethernet destination address follows the start frame
delimeter(SFD) on the gmii data bus....is it the byte 0(bits 7:0) or
byte 6 (bits 47:39)? I think its byte 0 but want to make sure so please
let me know.
Thanks


NMFall 20%
Posted by glen herrmannsfeldt on July 27, 2006, 12:09 am
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pranavtailor@gmail.com wrote:

> A simple and may be silly query for you. Can you tell me which byte
> of the ethernet destination address follows the start frame
> delimeter(SFD) on the gmii data bus....is it the byte 0(bits 7:0) or
> byte 6 (bits 47:39)? I think its byte 0 but want to make sure so please
> let me know.

The one on the left when you write the number on paper.

Most likely the one at a lower address when stored in memory, as it will
be the lower address in the output buffer.

Some people, and some machine architects, number bytes in a word with 0
on the left, some with 0 on the right. Some are little endian, some big
endian.

-- glen


Posted by on July 27, 2006, 9:30 am
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Hi Glen thanks for the reply. So do you mean that the dest addr
written as 48'hA2B012344567 on the paper will have the byte A2 showing
up first followed BO, 12 and so on on the gmii data bus?

glen herrmannsfeldt wrote:
> pranavtailor@gmail.com wrote:
>
> > A simple and may be silly query for you. Can you tell me which byte
> > of the ethernet destination address follows the start frame
> > delimeter(SFD) on the gmii data bus....is it the byte 0(bits 7:0) or
> > byte 6 (bits 47:39)? I think its byte 0 but want to make sure so please
> > let me know.
>
> The one on the left when you write the number on paper.
>
> Most likely the one at a lower address when stored in memory, as it will
> be the lower address in the output buffer.
>
> Some people, and some machine architects, number bytes in a word with 0
> on the left, some with 0 on the right. Some are little endian, some big
> endian.
>
> -- glen


Posted by Walter Roberson on July 27, 2006, 9:57 am
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>Hi Glen thanks for the reply. So do you mean that the dest addr
>written as 48'hA2B012344567 on the paper will have the byte A2 showing
>up first followed BO, 12 and so on on the gmii data bus?

The standards only talk about the transmission order once the
data gets on to the transmission media.

If for some reason you built your transmitter to buffer an octet
at a time and transmit the octet in reverse order, then for consistancy
the GMII data would have to be reversed as well.

There are standards about GBICs and SFP's, so you wouldn't pull a trick
like that if you were planning to use a -standard- GBIC or SFP,
but it wouldn't be the first time that a manufacturer customized a
transmission device to lock people in to their parts.

Posted by glen herrmannsfeldt on July 28, 2006, 5:39 am
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Walter Roberson wrote:


>>Hi Glen thanks for the reply. So do you mean that the dest addr
>>written as 48'hA2B012344567 on the paper will have the byte A2 showing
>>up first followed BO, 12 and so on on the gmii data bus?

> The standards only talk about the transmission order once the
> data gets on to the transmission media.

The MII for fast ethernet is part of the standard, and I thought
GMII was also, though it rarely exists outside of the chips.

In any case, that would be the expected ordering.

-- glen


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