The tape class extension area generated for a UCB is addressed by UCBCLEXT in the UCB common extension. It contains zeros at IPL, and is set to zeros whenever the volume label is ready to be verified and processed for accessibility (as in open, or next volume for EOV). The class extension holds volume label data across opens when there is no intervening volume label reverification (as is true after CLOSE LEAVE and another OPEN in the same job step). The UCBCX DSECT in the IECUCBCX macro maps the tape class extension area. If you use this DSECT to map the area, you get the version of the tape class extension area that you need. To see other fields for diagnosis purposes, go to z/OS MVS Data Areas in the z/OS Internet Library: at http://www.ibm.com/systems/z/os/zos/bkserv/.

The UCB tape class extension can reside above the 16 MB line, so your code should execute in 31-bit mode.