Big vs. Little endian was Re: Walnut user-space software problem

[Ralph Blach]

Jpe,

Dont forget that the PPC405 has MAC instructions!  These would have to
be coded into assembler but
they are very fast.

Also, on the 405gp, it is theoretically possible to allocate and create
Little endian TLB's although this is not in the port now. I would
recomend against this solution.

Chip


Joe Dery wrote:
>
> So is there a FAQ or HOWTO on the issues involved in porting
> little endian to big endian software somewhere ?  I haven't seen
> it yet after some looking.
>
> Thanks
> Joe Dery
>
> > Hi Joe,
> >
> > > My senior design group is working with a IBM Walnut board
> > > with the 200 MHz 405GP r3.  We have the linux kernel and
> > > target filesystem taken from the Nov. 26, 2000 sources.
> > > The board boots Linux from NFS flawlessly after compilation
> > > using the CDK 1.0 8xx toolkit.
> > >
> > > Our project objectives include  compiling the ITU-T G.723.1
> > > (Voice over IP) standard C source code.  We are (again) using
> > > the powerpc-linux-gcc included with the MV CDK 1.0 tools for
> > > PowerPC 8xx-based machines.
> > >
> > > I understand that an updated devkit is available, but since
> > > our budget cannot touch the subscription fees needed to
> > > obtain the MontaVista toolkit for the 405GP.
> > >
> > > Enough background already.  The problem arises in that
> > > when we compile the VoIP source code and run it on the
> > > PowerPC, the output of the encoder and decoder is wrong.
> > > When the same code is compiled for our host machine
> > > (using i386 gcc) the output is ok.  This C code involves a
> > > lot of speed-oriented fixed-point mathematics - that is, a lot
> > > of multiplies and shifts (to avoid divides)  Just searching for the
> > > >> and << operators in the kernel source doesn't bring up much.  So
> > > to me this may be the culprit.  Sorry to make this so vague - I am
> > > just hoping that someone has seen problems with math/DSP oriented
> > > programs before and can suggest something!
> >
> > If I had to put money on a bet where the problem might be located I
> > would say your VoIP en-/decoder program is not written properly for a
> > big endian machine. Especially reading data from/writing to memory via
> > pointers can cause all kinds of errors. Please find a known good big
> > endian machine somewhere and see if the algorithms work properly. If
> > not kick the developer of the beast and fix it. I've seen a lot of
> > integer DSP code to break when ported from little to big endian.
> >
> > Don't blame CPU or compiler too early, C doesn't abstract endianess
> > issues completely - so they might visible.
> >
> > > Also, I wanted to verify the size of the variable types in the
> > > gcc compiler for the PowerPC.  Please correct me if I'm
> > > wrong.
> > >
> > > int             - 32 bit signed integer
> > > short   - 16 bit signed integer
> >
> > correct for PPC.
> > short always 16
> > long always 32
> > long long always 64
> > int = machine word size
> >
> > I hope I could point you in the proper direction,
> > Rolf
> >
>
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