Erik McClure

inline unsigned char BSS_FASTCALL asmcas(int *pval, int newval, int oldval)
  {
      unsigned char rval;
      __asm {
#ifdef BSS_NO_FASTCALL //if we are using fastcall we don't need these instructions
        mov EDX, newval
        mov ECX, pval
#endif
        mov EAX, oldval
        lock cmpxchg [ECX], EDX
        sete rval // Note that sete sets a 'byte' not the word
      }
      return rval;
  }

This was an absolute bitch to get working in VC++, so maybe this will be useful to someone, somewhere, somehow. The GCC version I based this off of can be found here.

Note that, obviously, this will only work on x86 architecture.