u16 __checksum_ex( u16 sum, void const * _data, u16 len )
{
u16 t;
const u08 * data = _data;
const u08 *dataptr;
const u08 *last_byte;

dataptr = data;
last_byte = data + len - 1;

while(dataptr < last_byte) { /* At least two more bytes */
t = (dataptr[0] << 8) + dataptr[1];
sum += t;
if(sum < t) ++sum; // carry
dataptr += 2;
}

if(dataptr == last_byte) {
t = (dataptr[0] << 8) + 0;
sum += t;
if(sum < t) ++sum; // carry
}

/* Return sum in host byte order. */
return sum;
}