[Simh] Disk info request

[Mark Pizzolato]

On Wednesday, March 9, 2016 at 7:39 AM, Paul Koning wrote:
> > On Mar 8, 2016, at 8:47 PM, Timothe Litt <litt at ieee.org> wrote:
> >
> > In the process of debugging the ODS2 utility, it became clear that the
> > disk geometry really does matter.
> > And it's not available in simulator images -- or even for physical
> > devices -- at runtime.
> >
> > Below is a list of all the disks that have been used with FILES-11
> > (ODS2) for which I have reliable (I think) data.
> 
> I see a list in PDP11/pdp11_rq.c showing geometry information for a large
> number of MSCP drives.
> 
> One puzzle is the RD52.  In the PRO at least, there are several different drives
> (different manufacturers with different numbers of heads.  The entry you
> have seems to be the one for the Quantum (8 head) variant rather than Atasi
> (7 head).
> 
> > RD52       512     17     8   512   35651584      69632     34.000 MB   154

Well, there are many definitions for various MSCP drive types, but looking 
through the code shows that the only place the geometry info is actually 
referenced is in error paths which presumably don't normally happen.  The 
drive type and total size (in sectors) is passed into the simulated system 
when devices are probed for, but it seems that the OS gets to leverage 
that info by some programmatic or manual means as it sees fit.  I say 
'manual' here since I explicitly recall on Ultrix contriving entries in 
/etc/disktab to describe arbitrarily sized disks connected to 3rd party 
MSCP controllers.  The info in /etc/disktab is used by the file system 
creation code to lay down the file system data structures scattered 
around the disk surface based on some relationship of the presumed 
geometry that was described.

- Mark