[sldev] PowerPC weirdness continues

Paul TBBle Hampson Paul.Hampson at Pobox.com
Sun May 20 04:41:02 PDT 2007


On Sat, May 19, 2007 at 10:38:51PM -0700, Ryan Williams wrote:
> Paul TBBle Hampson wrote:
> >OK, I've been trying to work out why my local PowerPC build
> >is stuffing up, yet at least one person has got a working
> >PowerPC build on this list...

> >Looking at ParcelOverlay again:

> >{
> >    ParcelOverlay Low Trusted Zerocoded
> >    {
> >        ParcelData      Single
> >        {   SequenceID  S32             }   // 0...3, which piece of region
> >        {   Data        Variable    2   }   // packed bit-field, (grids*grids)/N
> >    }
> >}

> >Based on my understanding of the packet format, I'm expecting

> >XX YY YY YY ff ff ZZ ZZ SE QU NC ID DL EN DATA...
> >  

> I'm just glancing over this, but I believe that the libsl folks
> discovered that the ordering of the variables in a block, and the
> ordering of blocks in a message, is determined by a hashtable
> ordering.   I.e. it is essentially random relative to the order
> specified in the template.  The reason this works at all is because
> the message templates are perfectly in sync, always, and the hash
> algorithm is always the same on all viewers.  One of the requirements
> of the message liberation project is to make the ordering consistent
> with the template, so once that is complete, this stuff will make more
> sense.

> >Then I get the 2-bytes I expected for DLEN, (the other packets concurred)
> >which is bizarre, since I'm expected four bytes of sequence ID here.

> This is explained if the hashtable ordering swapped SequenceID and
> Data relative to the template's ordering.  :-/  Hope that helps with
> your debugging.

*cries*

Yeah, that'd explain it. Thanks.

Oh well, the data going in is OK, I'll have to try and work out why it's
coming out wrong.

-- 
-----------------------------------------------------------
Paul "TBBle" Hampson, B.Sc, LPI, MCSE
On-hiatus Asian Studies student, ANU
The Boss, Bubblesworth Pty Ltd (ABN: 51 095 284 361)
Paul.Hampson at Pobox.com

Of course Pacman didn't influence us as kids. If it did,
we'd be running around in darkened rooms, popping pills and
listening to repetitive music.
 -- Kristian Wilson, Nintendo, Inc, 1989

License: http://creativecommons.org/licenses/by/2.1/au/
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