[dfdl-wg] Bit/byte order

[Martin Westhead]

Hi all,

This is a discussion topic that came up in last weeks telcon.

Mike asserted that only byte order was needed. I was not completely 
comfortable with this and said I'd investigate. After talking it over 
with various folks and looking a couple of things up, I have reached the 
conclusion that I agree with Mike.

The key observation is that microprocessors handle bytes in parallel, 
there is no "bit pointer". So, within the context of data on a machine 
the only ordering over bits is their with respect to significance. 
Hence, to all intents and purposes, as Mike observes, bit-order==byte-order.

It is the case that hardware representations of binary data will have a 
particular bit-ordering - Ethernet for example is LSB first, 
representations of data on tapes (or disks) will have a bit ordering. 
However the conclusion I reached from discussions here was that this is 
a red-herring for DFDL. Bit-order in this context is one of a number of 
physical aspects of the data representation which would include 
frequencies, signal strengths etc. There may be some archival value in 
recording all that stuff, and you could put it in DFDL, but I think that 
  stuff is safely out of scope for where we are now. As soon as you are 
looking at your data from software you have bytes.

Cheers,

Martin