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Re: Change endianess on G4?

On Jun 1, 2009, at 11:07 AM, jim smith wrote:
I remember reading somewhere that the endianness of some architectures [specifically big- to little-endian] can be changed by a software switch or jumper on a motherboard.

There is a bit in the MSR which determines whether the processor is in big- or little-endian mode. From a glance at the ppc-6xx and MPC74xx docs I have, it should even be possible to run individual user processes with different endian-ness, if the kernel were to support it.

Is this possible with a Mac G4?

The docs don't list the endianness bit as being one of the optionally implemented features, so I assume it's available on all the 6xx and G4 processors. On the other hand, it's very clear that the "native" byte ordering of the ppc family is big-endian, and lots of operations take a performance hit in little-endian mode. (As I understand it, this is unlike the either-endian MIPS and ARM processors which are comfortable in either mode.)

If so, would NetBSD macppc even run on such a machine?

It would probably take some work... for one thing, I don't know if gcc is even capable of compiling to a little-endian-ppc target.

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