Re: [Dev-C++] Convert Little-Endian to Big-Endian (and vice versa)

You might also want to look at using "exclusive OR" on each byte.
  ----- Original Message -----=20
  From: Daniel K. O.<mailto:dan...@ya...>=20
  To: Joey Dala<mailto:jda...@ya...>=20
  Cc: =
dev...@li...<mailto:dev...@li...=
e.net>=20
  Sent: Thursday, November 11, 2004 10:34 AM
  Subject: Re: [Dev-C++] Convert Little-Endian to Big-Endian (and vice =
versa)


  Joey Dala wrote:

  > Does anybody know a faster algorithm to swap the bytes
  > of an 'unsigned long' than this:
  > =20
  > // SWAP 32-BITS
  > #define SWAP_LONG( A )  ((A << 24) | ((A & 0xff00L) << 8 ) | ((A &=20
  > 0xff0000L) >> 8 ) | ((unsigned long)A >> 24))


  Faster than this? Only if you use architecture-specific instructions.=20
  The 486 (and higher) family has the "bswap" instruction. Here's an =
example:

  ---
  #include <stdio.h>

  #define SWAP32(x)      asm volatile ("bswap %0" : "=3Dr" (x) : "0" =
(x))

  int main(void)
  {
          int foo =3D 1;
  =20
          printf("%08x\n", foo);
  =20
          asm volatile (
                  "bswap %0"
                  : "=3Dr" (foo)
                  : "0" (foo)
                  );
        =20
          printf("%08x\n", foo);

          SWAP32(foo);

          printf("%08x\n", foo);

          getchar();
          return 0;
  }
  ---

  Notice the use of the extended assembly syntax, so GCC can perform=20
  better optimizations around the asm beast.
  Also, the "volatile" keyword to make sure GCC won't move the assembly=20
  statement around in the optimization proccess.


  Before you ask, some information about using AT&T assembly syntax and=20
  extended syntax is available here:=20
  =
http://www.delorie.com/djgpp/doc/brennan/brennan_att_inline_djgpp.html<ht=
tp://www.delorie.com/djgpp/doc/brennan/brennan_att_inline_djgpp.html>


  Daniel K. O.



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