The OS VTOC on a CKD device is on cylinder 0, head 0, record 3.                 
The format of record three is                                                   
                                                                                
0.4   Junk           CL4'VOL1'                                                  
4.6   Label          CL6          6 Character VOLID                             
A.1                  C' '         Don't know what this is for.                  
B.5   VTOC Pointer   5X           CCHHR of first record of VTOC                 
                                                                                
 ..... more junk                                                                
                                                                                
------------------------------------------------------------------------        
                                                                                
The format of the first VTOC record is                                          
                                                                                
key length  = 44 bytes = 44X'04'     Data set name (that's right, the           
                                                    VTOC DSN is 44X'04')        
                                                                                
data length = 96 bytes = X'F4'       This is a "Format 4" record.               
                         4X          Ending track (CCHH) of VTOC.               
                                                                                
                         ..... more junk                                        
                                                                                
------------------------------------------------------------------------        
------------------------------------------------------------------------        
                                                                                
For Fixed Block devices, the volid is on logical block 1, but I'm not           
sure where the VTOC pointer is.  It's got to be in there somewhere.             
Probably in the same place, I don't know.                                       
                                                                                
The format of the VTOC records are basically the same, 44 byte data set         
name, followed by 96 bytes of the "Format 4" data, for a total of 140           
bytes, packed three 140-byte records per 512-byte block (wasting 92             
bytes).