ppc LE questions (seeking help hand info pointers)

[Albert D. Cahalan]

Holger Bettag writes:
>"Albert D. Cahalan" <acahalan at cs.uml.edu> writes:

>> Now do an unconditional 64-bit byte swap on the motherboard.
>> Consider an array of four __u16 values affected by this.
>> They get converted from big-endian to little-endian, and they
>> get moved to different addresses. Excellent! All the weirdness
>> just cancels out, giving perfect little-endian. This works for
>> 8-bit, 16-bit, 32-bit, and 64-bit data. Life is good.
>
> Does it work for 128 bit data? The 74x0's most promiment feature,
> AltiVec, would expose a strange mixed endianness to programmers
> otherwise.

Yes. Motorola noticed this problem, and added a built-in swap
to fix the problem. See page 3-6, section 3.1.4, figure 3-5 in
ALTIVECPEM.pdf, the "AltiVec Technology Programming Environments
Manual". Unaligned vectors don't work in big-endian or little-endian
mode, and the code needed to load them is equally long.


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