PDP-11 disk image question

Charles Anthony charles.unix.pro at gmail.com
Wed Feb 20 12:43:31 CST 2019

On Wed, Feb 20, 2019 at 9:21 AM Glen Slick via cctalk <cctalk at classiccmp.org>

> On Wed, Feb 20, 2019 at 8:57 AM Charles Anthony via cctalk
> <cctalk at classiccmp.org> wrote:
> >
> > However, in the original posters case, the SIMH disk image is being
> copied
> > to the RA81 drive without the benefit of the MSCP controller (if I
> > understand correctly). This would lead to track misalignment and could
> > result in the observed behavior.
> How could you possibly write to a real RA81 drive without going
> through a real KDA50 or UDA50 MSCP controller? Nothing like that is
> happening in the original post. Just the disk size was chosen to be
> that of an RA81, and the issue is to exactly match the emulated disk
> size to that configured by the hardware, which is an MSCP SCSI
> controller and drive in this case.

Re-reading the original post, it it not clear to me how the disk write was
accomplished; I am not familiar with the mentioned hardware. If the data
was written to the RA81 with a controller that correctly did the spare
sector mapping, then my spare sector hypothesis is wrong.

The reported symptoms sound like a disk geometry issue; the data passes
through several systems on the way to RSTS, and it seems likely to me that
one of the steps is damaging the data.

As I lack experience with or access to the specific hardware, I am
speculating about the exact failure mode. I do however believe that it is
highly likely that disk geometry is the root of the problem. Having no no
further specific ideas about the failure mode, I will shut up now.

-- Charles

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