[Simh] Disk sizing

[Mark Pizzolato]

On Sunday, February 5, 2017 at 4:41 PM, Johnny Billquist wrote:
> On 2017-02-06 01:11, Johnny Billquist wrote:
> > On 2017-02-06 00:40, Mark Pizzolato wrote:
> >> Now that Johnny pointed me to the ODS1 spec, I'm adding the
> >> appropriate file
> >> system structure definitions right now.   I have enough to solve this
> >> case.
> >
> > Mark, I found a slightly revised ODS-1 spec that gives an alternative
> > interpretation of the SCB block in BITMAP.SYS on ODS-1 for large disks.
> > DEC did a revision.
> > Putting that up as well, as http://mim.update.uu.se/ods1-1

Once you provided the earlier document, I looked to see if it was available
on Bitsavers since having it amongst all the other archived documents 
would seem to make sense.  What I found was this newer document 
which I actually worked from.

> > And the size limit mentioned is not correct. I seem to remember there
> > is also a file level 403, and the limit of ODS-1 with 3+1 retrieval
> > pointers are in fact 8G. I don't think that file level changes
> > anything on how you find the disk size, though.
> 
> I'll try to find if I have any documentation for really large ODS-1 volumes. But
> for now, I can report this:
> 
> They seem to still use 402 of H.VLEV
> In the SCB of BITMAP.SYS, all four initial bytes (number of storage bitmap
> blocks) are 0. And then comes disk size. And then the number of blocks for the
> bitmaps comes after the disk size instead.
> 
> I can provide example dumps of the SCB if you need any.

Rather than that, please just give the latest github code a spin to see if the 
ODS1 volumes you have at your fingertips are correctly recognized when they 
are attached to a RP or RQ device.  

Bringing this functionality to other disk types is a project for another day and
might not provide much value since most, if not all, of them had their last 
track written with BAD144 data.

An interesting effect is that now all of the ODS1 and ODS2 disk images which 
were originally created on other  device types (RK, RL, etc.) can be attached
to an RQ controller and be properly sized.

I looked at the RT11 specs and that's a project I won't be undertaking.

Meanwhile, we could use support for detecting RSTS disk sizes...

- Mark