[Simh] DZ11 vs DZV/DZQ11

[Johnny Billquist]

On 2018-04-12 22:31, Johnny Billquist wrote:
> On 2018-04-12 21:24, Johnny Billquist wrote:
>> On 2018-04-12 02:41, Mark Pizzolato wrote:
>>> On Tuesday, April 10, 2018 at 10:15 AM, Johnny Billquist wrote:
>>>> On 2018-04-10 10:26, Mark Pizzolato wrote:
>>>>> On Monday, April 9, 2018 at 11:51 PM, Johnny Billquist wrote:
>>>>>> On 2018-04-10 00:25, Mark Pizzolato wrote:
>>> [...]
>>>>> Some OSes leverage the encoded value to correspond specifically with a
>>>>> particular model of DEC disk and run from there potentially 
>>>>> presuming details
>>>>> about disk size or geometry.
>>>>
>>>> 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.
>>
>> Some definitely allowed you to select it to some degree, and some 
>> might have been even more clever.
> 
> Ok, I decided to locate the documentation on how this value/string is 
> encoded, since Mark said he'd not seen it.
> It's in the MSCP Basic Disk Functions Manual, page 4-37 to 4-38.
> The media type identifier is a 32-bit number, and it's coded like this:
> The high 25 bits are 5 characters, each coded with 5 bits. The low 7 
> bits is a binary coded 2 digits.
> 
> Looking at it, you have:
> D0,D1,A0,A1,A2,N
> 
> For an RA81, it would be:
> 
> D0,D1 is the preferred device type name for the unit. In our case, that 
> would be "DU".
> A0,A1,A2 is the name of the media used on the unit. In our case "RA".
> N is the value of the two decimal digits, so 81 for this example.
> 
> And for letters, the coding is that A=1, B=2 and so on. 0 means the 
> character is not used.
> 
> So, again, for an RA81, we would get:
> 4,15,12,1,0,81
> 
> That's all in decimal, and you have the size of each bitfield.

And I'm an idiot. For the letters, that actually turned out to be hex, 
and not decimal...

   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