Re: Mplayer: "Little-Endian"

From: Peter T. Breuer (ptb_at_oboe.it.uc3m.es)
Date: 07/14/03 

Date: Mon, 14 Jul 2003 20:30:48 +0200

Paul Black <nospam@nospam.saturnine.org.uk> wrote:
> Peter T. Breuer wrote:
>>>Rather than "suspecting", why not suggest how to do it.
>>
>> Because you are saying that it is not possible to observe the internal
>> bitorder in a byte.

> Your sentence does not follow from what I wrote.

It seems to me to be exactly what you wrote, not only follow from it!
I.e. it seems to me to be exactly logically equivalent to whatever you
wrote. However, I'll just prove the one direction.

Consider what happens if (the converse of what I suggested you said) you
CAN observe the internal bitorder in a byte - then you can distinguish
between 0-7 and 7-0 ordering within a byte, by observation, and
therefore you can write programs which differ in their results according
to which way the internal format goes, and therefore you can indirectly
address a byte at any point within it, because you can for example
generate the result 1 for the third position of 0x7 if you (in an
earlier step) detect the machine ordering to be one way round and 0 if
you detected it to be the other way round.

Since you say that a byte cannot be addressed within its boundaries (I
said you meant "directly", but you seemed to resist this
interpretation), the above is in contradiction with what you say, and
therefore its negative follows from what you say. That is, my
suggestion sas to what you said ("it is not possible to observe the
internal bitorder in a byte") follows from what you said.

Clear?;)

>> All I'm saying is that your argument doesn't
>> establish it. You'd have to go through all the basic machine ops
>> "f" one by one and show that each satisfied the symmetry:
>>
>> f(inv(x)) = inv(f(x))
>>
>> where inv is the operation that writes the bits of a byte backwards.

> What inv is this? Provide one concrete example.

The position permutation: 0 1 2 3 4 5 6 7 -> 7 6 5 4 3 2 1 0

Or any random transposition, come to that, since you're saying that
any internal generic permutation of the bits within a byte in terms
of tehir physical position vs their logical postion is undetectable.

>>>Or have I been trolled?
>>
>> Dunno! Have you?

> Looks like it :-(

Peter

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