[Simh] DZ11 vs DZV/DZQ11

[Johnny Billquist]

On 2018-04-12 16:06, Paul Koning wrote:
> 
> 
>> On Apr 11, 2018, at 8:41 PM, Mark Pizzolato <Mark at infocomm.com> wrote:
>>
>> On Tuesday, April 10, 2018 at 10:15 AM, Johnny Billquist wrote:
>>>>
>>>> ...
>>>
>>> I certainly hope not. Like I said, this is cosmetic. MSCP reports disk size directly,
>>> and the id is just for information. Anything that is mad enough to assume size
>>> based on the id instead of the size reported by the device would be some
>>> seriously broken software.
>>
>> Well, most of the third party MSCP controllers provided a constant Media ID that
>> identified the drive as an RA81.  In general, since that was really cosmetic it shouldn't
>> have mattered.  I vaguely recall that some Ultrix file system generation logic used
>> the drive type to determine presumed values for the disk geometry for cylinder
>> boundary alignment, but no matter what that choice really didn't matter.
> 
> The ID string is just a string, for display.  RSTS displays it (in the "hardware list" command in the boot program "init").  But there is also in MSCP a way to ask for geometry, which MSCP views as sectors, tracks, cylinders, and "groups".  That too is informational since addressing is by LBA, but the intent was to allow an OS to do geometry-aware optimization if it wanted to.  Deriving geometry from the device ID would be wrong, but asking for it explicitly would be valid.

Right. Same story in RSX. It's purely cosmetic.

And I don't think anyone really tried to do something based on geometry, 
since it's mostly bogus anyway. MSCP address by LBA in the end, as you 
say, and newer disks did not have the same number of sectors per track 
anyway, so the geometry became just an approximation. Not to mention 
that replacement blocks could mean that you were not even close to the 
right place anyway.

>>>>> And that limitation of 7 is obviously because of limitations on SCSI,
>>>>> which can only have 8 devices on the bus, and the controller is one.
> 
> I thought it was 15 devices -- 4 bit LUN number with the value 7 reserved for the controller.

The LUN in SCSI is for something that looks like subunits or partitions. 
The SCSI ID, which is the primary identifier, is a 3 bit number, giving 
you a max of 8 devices on narrow SCSI, and it's a 4 bit number on wide 
SCSI. You often had three jumpers on the devices, to select the ID, in 
the old days.

Wide SCSI came pretty late in relation to PDP-11s, so there are very few 
wide SCSI controllers for PDP-11. Almost all that exists are narrow SCSI.

   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