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 > -----Original Message----- > From: The IBM z/VM Operating System > [mailto:IBMVM@LISTSERV.UARK.EDU] On Behalf Of Alan Altmark > Sent: Friday, May 20, 2011 10:45 AM > To: IBMVM@LISTSERV.UARK.EDU > Subject: Re: DASD HA & R0 ? > > On Friday, 05/20/2011 at 11:55 EDT, "Schuh, Richard" <rsc...@visa.com> > wrote: > > I think I remember R0 being referred to as the Track > Capacity record. > > It > was > > not a constant, but held the remaining capacity after the last > formatting write. > > It is possible that some OSes write things to R0, but CMS and > CP don't. > CMS FORMAT and DSF write R0 and HA with zeros. Use DDR > "PRINT 0 0 0" to > see them. Each time you WRITE R0, you erase the track. > > Track capacity is determined by using formulae described in > the book with > data from the READ DEVICE CHARACTERISTICS command. > > Alan Altmark > > z/VM and Linux on System z Consultant > IBM System Lab Services and Training > ibm.com/systems/services/labservices > office: 607.429.3323 > mobile; 607.321.7556 > alan_altm...@us.ibm.com > IBM Endicott >
