I don't think that there is any doubt about it being writable, There was a CCW for it. Writing record 0 was all that was needed to clear a track when you initialized a track (not a security write). There was also Write Home Address which marked the start of a track. It was needed for new disks. The vendor did not initialize them way back then.
Regards, Richard Schuh ________________________________ From: The IBM z/VM Operating System [mailto:IBMVM@LISTSERV.UARK.EDU] On Behalf Of Michael Harding Sent: Friday, May 20, 2011 12:39 PM To: IBMVM@LISTSERV.UARK.EDU Subject: Re: DASD HA & R0 ? If you're going back that far, ISTR that R0, if writeable at all, was used on an otherwise bad track to point to its alternate. -- Mike Harding z/VM System Support mhard...@us.ibm.com mike.b.hard...@kp.org mikehard...@mindless.com (925) 926-3179 (w) (925) 323-2070 (c) IM: VMBearDad (AIM), mbhcpcvt (Y!) The IBM z/VM Operating System <IBMVM@LISTSERV.UARK.EDU> wrote on 05/20/2011 11:56:14 AM: > From: "Schuh, Richard" <rsc...@visa.com> > To: IBMVM@LISTSERV.UARK.EDU > Date: 05/20/2011 11:56 AM > Subject: Re: DASD HA & R0 ? > Sent by: The IBM z/VM Operating System <IBMVM@LISTSERV.UARK.EDU> > > Must have been a thing from OS/360, then, probably from a time > before there was Sense id and RDC. Writing the capacity of the track > in R0 might have been helpful in those days. The fact that the Write > R0 CCW erases the entire track insures that the part about updating > it is incorrect. > > Regards, > Richard Schuh >
