[PD-dev] examples of porting endianness?

[Me]

On Wednesday, February 26, 2003, at 01:16  AM, Hans-Christoph Steiner 
wrote:

>
> In effort to learn how to port stuff to MacOS X, I was hoping someone 
> could point me to a piece of code where endianness is an issue and has 
> been ported to PowerPC.  I always learn best by following an good 
> example.

hi hc,

...oddly enough, I don't there was any endian swapping necessary for 
the GEM OSX port...but I've done plenty of other ports where it's 
essential...all that is required is a set of functions to convert from 
(and to) int, short, and float:  char's usually don't need to be 
swapped...anyway, here are the equations I use, which are from the 
libSDL, except for the float stuff:

#ifndef SDL_Swap16
static __inline__ Uint16 SDL_Swap16(Uint16 D) {
	return((D<<8)|(D>>8));
}
#endif
#ifndef SDL_Swap32
static __inline__ Uint32 SDL_Swap32(Uint32 D) {
	return((D<<24)|((D<<8)&0x00FF0000)|((D>>8)&0x0000FF00)|(D>>24));
}
#endif
#ifdef SDL_HAS_64BIT_TYPE
#ifndef SDL_Swap64
static __inline__ Uint64 SDL_Swap64(Uint64 val) {
	Uint32 hi, lo;

	/* Separate into high and low 32-bit values and swap them */
	lo = (Uint32)(val&0xFFFFFFFF);
	val >>= 32;
	hi = (Uint32)(val&0xFFFFFFFF);
	val = SDL_Swap32(lo);
	val <<= 32;
	val |= SDL_Swap32(hi);
	return(val);
}
#endif
#endif

inline float LoadLEFloat( float *f )
{
	#if ! defined( __MWERKS__ )
		//Usage:  void __stwbrx( unsigned int, unsigned int *address, int 
byteOffsetFromAddress );
		#define __stwbrx( value, base, index ) \
			 __asm__ ( "stwbrx %0, %1, %2" :  : "r" (value), "b%" (index), "r" 
(base) : "memory" )
	#endif

	union
	{
		long		i;
		float		f;
	}transfer;
	
	//load the float into the integer unit
	unsigned int	temp = ((long*) f)[0];

	//store it to the transfer union, with byteswapping
	__stwbrx( temp,  &transfer.i, 0);	

	//load it into the FPU and return it
	return transfer.f;
}

...don't forget:  if you swap on load to disk, swap again on write to 
disk...otherwise it'll only work alternating times!  Also, if ya wanna 
be tricky, you can make a set of defines that work based on a test of 
the system for endianness:

#if SDL_BYTEORDER == SDL_LIL_ENDIAN
#define LoadLEFloat(X)	(X)
#else
#define LoadLEFloat(X)	LoadLEFloat(X)
#endif

...this way, the compiler makes the right decision, and there's less 
impact on code bloat...

l8r,
jamie