[Simh] Disk sizing

[Johnny Billquist]

On 2017-02-06 00:40, Mark Pizzolato wrote:
> On Sunday, February 5, 2017 at 2:48 PM, Timothe Litt wrote:
>> On 05-Feb-17 16:15, Johnny Billquist wrote:
>>> On 2017-02-05 21:31, Mark Pizzolato wrote:
>>>> A concept related to this discussion is simh's auto sizing of disks.
>>>> In the PDP11 and
>>>> VAX simulators the RQ and RP disks use the sim_disk library to
>>>> perform I/O to disk Images and/or physical disk devices.
>>>
>>>> So, I'd like to expand the set of file system types that the auto
>>>> sizing logic can perform to include RSTS disks and whatever might be
>>>> commonly used with RSX and RT11. I can certainly give all the
>>>> information you need for ODS-1, which is what RSX use. The structures
>>>> are pretty similar to ODS-2, so if you already have that, it should
>>>> be very simple to add the ODS-1 stuff.
>>> Unfortunately, this will only cover RSX systems. For RT-11 (as well as
>>> RSTS/E and others), you'll have to get someone else to help. I suspect
>>> Paul should know RSTS/E, if he just have the time.
>>>
>>> Anyway, the gist in ODS-1 is that the size of the device is contained
>>> in a file called BITMAP.SYS (FID 2,2,0). In there, there is a 32-bit
>>> value saying what the size of the disk is in logical blocks.
>>>
>> For ODS-2, you want the SCB, which is contained in BITMAP.SYS.  It gets tricky,
>> especially where geometry is involved and where the emulated disk image is a
>> physical copy of media that doesn't match the SCB.
>>
>> I have code that navigates ODS-2 and attempts to identify the correct disk
>> based on the recorded geometry.  It's part of an ODS-2 reader/writer.  I need
>> to get back to that project and finish some higher-level stuff; when it's released
>> you can extract that code for SimH.  It's about 5 down on my stack, which I'm
>> unlikely to reach before April/May.
>
> I've already done the ODS2 case and the Ultrix case, so nothing needed for that.
>
> 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

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.


	Johnny


-- 
Johnny Billquist                  || "I'm on a bus
                                   ||  on a psychedelic trip
email: bqt at softjar.se             ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol