{
{      This request is used to control the subbuffer positioning
{     within a system buffer.  If the subbuffer position equals a 'trigger
{     read ahead' subbuffer position, the read ahead of the next system
{     buffer will be initiated if necessary.  If all subbuffers are used,
{     either a switch will be made to the next system buffer or the task
{     will be suspended until the next system buffer becomes available.
{
{  BAP$GET_NEXT_SUBBUFFER (FTD_PTR,SBD_PTR,LENGTH,STATUS)
{
{    KFT_DESCRIPTOR : (input) this parameter specifies the known
{                      file table entry associated with the file.
{
{    SBD_PTR : (input) this parameter specifies a pointer to the
{               system buffer descriptor of the system buffer to
{               position.
{
{               (output) this parameter specifies the new system
{                system buffer descriptor if a change in system
{                buffers occured.
{
{    LENGTH : (input) this parameter specifies the amount of space
{              to position within the buffer.
{
{    STATUS : (output) this parameter specifies request status.
