{}
{   PURPOSE: }
{     The purpose of this module is to implement one of the access method}
{     program interfaces. This module contains a single [xdcl] procedure}
{     for ease in future maintenance and change of the interface. It is}
{     also a possibility that a procedure[inline] capability will be added}
{     to the CYBIL language. This would allow the contents of this module}
{     to physically be inserted in the user's program at the point of the}
{     user's call to the procedure contained herein. This would avoid the}
{     procedure call. Should CYBIL be changed, having one procedure per module}
{     will allow use of the language feature.}
{}
{   DESIGN: }
{     This module is called by the user of the access method, and it must}
{     execute in the ring of its caller. This module validates the}
{     file_identifier which is the key to the instance of open of the file}
{     to be accessed. Next the values of read parameters and the addresses}
{     of VAR parameters are packaged into a call_block which summarizes the}
{     user's request. Control is then given to the procedure which was}
{     selected when the file was opened. This procedure may be the access}
{     method, a user's file_access_procedure or a system file_access_procedure.}
{}
