3 Apr
2016
3 Apr
'16
11:43 a.m.
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
disks.
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.
paul