long
Chk(s, n, ctrl)
register unsigned char *s;
register n;
unsigned ctrl;
{
    register unsigned long sum, x, t;

    sum = 0xffff;
    x = 0;
    do {
	sum <<= 1;
	if (sum & 0x10000)
	    sum ^= 0x10001;
	t = sum;
	sum = (sum + (*s++ & 0xff)) & 0xffff;
	x += sum ^ n;
	if (sum <= t)
	    sum = (sum ^ x) & 0xffff;
    } while (--n > 0);

    sum = 0xaaaaL - (sum ^ (ctrl & 0xffL));
    return 0xffffL & sum;
}