i386 Atomic Operations

The i386 provides a couple of ways to perform an atomic test-and-set. Here, I've provided two different (but mostly equivalent) functions to test-and-set, one that uses the BTS (bit test and set) instruction, and another that uses the XCHG (exchange) instruction.

Bit Test-and-Set

int testAndSet(unsigned char *b)
{
    int result;
    asm ("bts $1, %1; sbbl %0, %0":"=r" (result):"m" (*b):"memory");
    return result;
}

Exchange

int testAndSet(unsigned char *addr)
{
   unsigned char result = 1;
   asm ("xchgb %1, %0":"=m" (*addr),"=r" (result)
                      :"1" (result) :"memory");
   return result;
}