With an extended TMC we have new volume definition records between the VOLUME and DSNB records. These records can be identified by X'FF20' at offset +0 and needs to be skipped during sequential read.
1STVDEF - RELATIVE ADDRESS IST VOLUME DEFINITION RECORD
NUMVDEF - NUMBER OF VOLUME DEFINITION RECORDS
The VSN PTR points to the volume range after the last volume record and before the true first dsnb record and its the VDEF record.
If TMC Control Record #2 has a pointer to the new volume defintion records, it means that the TMC file is in a extended format.
The following methods can be used to check for that pointer:
* run TMSBINQ with HEADERS control statement. The pointer shows up
in the report as 'VSN PTR=nnnnnnnn'
* Inquire the TMC control records in the ISPF interface. The pointer
shows up in panel TMPITCTL as '1ST VOLDEF => nnnnnnnn'
The VSN PTR = 0000000 means that the TMC file didn't be extended.