DECmate II Questions
paulkoning at comcast.net
Sun Apr 3 13:43:25 CDT 2016
> On Apr 3, 2016, at 1:35 PM, Warner Losh <imp at bsdimp.com> wrote:
> DD certainly would be one way to image them. However, there's a caveat with
> that. DD
> will read the sectors labeled 1, 2, 3, ... 10 in that order. However, if
> you read the entire track
> off the disk you'll find they are stored in 1 6 2 7 3 8 4 9 5 10 order
> (again, IIRC, the order is
> different, but might not be this exactly). Back in the day, people (not me)
> reported success of
> duplicating via dd, only to find that the performance really sucked. My
> brain can't recall if
> this was on the PDP-11 / Professional formatted disks, or on the DECmate II
I was assuming that you'd use dd both to save and to restore the disks, or to create disk images to be read by something that expects sectors in physical order. That way, it doesn't matter what the interleave rule is.
On the PDP11, there is both 2:1 interleaving and track skew. Also, for some bizarre reason, logical track 0 is physical track 1, while physical track 0 holds logical track 79.
The equations are:
phys track = ((lba + 10) / 10) mod 80
phys sec = ((sec mod 5) * 2 + ((sec / 5) mod 2) + (sec / 10) * 3
if I translated from the original Macro-11 correctly.... :-)
On the Pro, the driver handles that translation; on other PDP11s where the RX50 is connected to an MSCP controller, the controller does the job, but the address mapping is the same in both cases.
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