rd on stack is not in canonical space
                                         THEN #SS(0); FI;
                         FI
              FI;
         IF return code segment selector is NULL
              THEN GP(0); FI;
         IF return code segment selector addresses descriptor beyond descriptor table limit
              THEN GP(selector); FI;
         IF return code segment selector addresses descriptor in non-canonical space
              THEN GP(selector); FI;
         Obtain descriptor to which retu