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

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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
>

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