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A basic question about data bus transfer

Note: This was originally posted on 29th August 2011 at http://forums.arm.com

In AMBA AHB spec2.0 or AHB lite,
for the data bus transfer with byte, there is a table (Active byte lanes for a 32-bit little-endian data bus) as shown in the attachment.
I am kind of confused with it.
1.
For byte transmission, I thought it was fine by just putting the byte data on data[7:0], why do we need different lanes?

2.
For the address offset, where does it come from?
In another words, how can I know which lane is active?

I googled but could not find some answers, it seemed that everyone knew how to deal with it except me.

Can someone give me some advice on those questions?

Thank you very much.