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:

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:

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