{
{   The purpose of this request is to calculate the 16 bit ones complement,
{ left shift around checksum of a block of data on a computer that uses twos
{ complement arithmetic.  The data is allowed to be in multiple fragments.
{
{   IF a odd number of bytes are given in the data block, the last byte of
{ data is treated as the first byte of a 16 bit parcel with zero fill.
{
{   A computed checksum value of minus zero is converted to a plus zero.
{
{     NAP$XNS_CHECKSUM (DATA_FRAGMENTS)
{
{   DATA_FRAGMENTS: This parameter specifies the address and length of the
{         blocks of data to be checksummed.
{
{   Note the total length of the data fragment must be even
{
