PDP-11 disk image question

Charles Anthony charles.unix.pro at gmail.com
Wed Feb 20 10:57:10 CST 2019

On Wed, Feb 20, 2019 at 5:38 AM Paul Koning <paulkoning at comcast.net> wrote:

> You're misinterpreting "spare".  MSCP exposes the user address space as
> contiguous LBAs, for which it uses 51 sectors per track.  The spare sector
> is used to do bad sector replacement.  That is invisible to users, it
> doesn't affect the LBA addressing.  dd, like any other host-resident code,
> sees the user address space.  It will copy MSCP devices correctly.

Yes; I agree that the MSCP conceals the spare sector when RSTS accesses the
disk through MSCP in the case of the *hardware*.

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.

The SIMH MSCP emulation is not doing the spare sector correctly; it is not
including the spare sectors on the RA81 disk image.

-- Charles

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