On Fri, 14 Nov 2008 18:12:45 -0500 Alan Altmark said: >On Friday, 11/14/2008 at 01:07 EST, "Schuh, Richard" <[EMAIL PROTECTED]> >wrote: >> Actually, it is record 3 that contains the volume label. It and records >> 1, and 2 are 80 byte records. > >Technically, the VOL1 label standard permits the label to exceed 80 >characters, up to the record length, but CPVOLs only use 80 bytes. > >> 1 and 2 are the IPL1 and IPL2 records if >> the volume can be IPLed. Records 4 and 5 are the VTOC that is written to >> prevent other systems from corrupting the volume. > >[Notation below is "cylinder/head(track)/record".] > >0/0/4 contains the allocation map. It is able to go there because the >label (0/0/3) points to the record (on 0/0) that contains the VTOC. CPVOLs >(i.e. CPFMTXA) place the VTOC on 0/0/5.
Well technically, the VOL1 has a CCHHR to the VTOC. It doesn't have to be on 0/0. See z/OS DFSMS: Using Data Sets SC27-7410 Appendix A. Something worthwhile understanding, and can be displayed easily with DDR. (Hey, it's gotta be useful for something) > >The VTOC contains two extents: >1. The extent of the VTOC itself. This is where it gets clever. The >label says it starts at 0/0/5, but VTOCs are measured in *tracks*, not >*records*. For CPVOLs, the VTOC consumes all of cyl 0, track 0. This >protects all the records on track 0 from the depredations of z/OS >(assuming it was so inclined). > >2. The list of available extents. Being oh so clever again, the list is >empty. > >If anyone wants to read more about VTOCs, and DSCBs, go look at the z/OS >DFSMSdfp Advanced Services book, chapter 1. > >> The system has, for a long time, protected the first records of cylinder >> 0, maybe all of 0/0/0, and used the rest of the cylinder for paging or >> spooling if cylinder 0 is so allocated. > >Note that this protection does not extend to T-disk. If you allocate cyl >0 as TDSK, CP will happily (for the moment) hand real cyl 0 to a guest. So >the rule is "don't do that". We plan to fix it in a future release. > >Alan Altmark >z/VM Development >IBM Endicott
