Re: Change endianess on G4?

[Wim Lewis <wiml%hhhh.org@localhost>]

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.