Integer Square Root

There's a very handy and fast method of finding integer square roots; I'm sure you could get this to work with fixed point:

unsigned long isqrt(v)
unsigned long v;
{
    register long t = 1L<<30, r = 0, s;

#   define STEP(k) s = t + r; r>>= 1; if (s <= v) { v -= s; r |= t;}

    STEP(15);  t >>= 2;
    STEP(14);  t >>= 2;
    STEP(13);  t >>= 2;
    STEP(12);  t >>= 2;
    STEP(11);  t >>= 2;
    STEP(10);  t >>= 2;
    STEP(9);   t >>= 2;
    STEP(8);   t >>= 2;
    STEP(7);   t >>= 2;
    STEP(6);   t >>= 2;
    STEP(5);   t >>= 2;
    STEP(4);   t >>= 2;
    STEP(3);   t >>= 2;
    STEP(2);   t >>= 2;
    STEP(1);   t >>= 2;
    STEP(0);

    return (unsigned long)r;
}