On Sat, Jan 4, 2014 at 12:19 PM, Tom Gardner <tom94022 at comcast.net> wrote:
There is nothing in a standard disk drive MFM
interface that signals the
drive size.
Agreed.
This is almost always a jumper setting and almost
always on the
controller; one set of jumpers for each drive controlled.
For some controllers, yes.
If the controller
also has some BIOS on it, it could be a BIOS jumper setting.
Not for DEC equipment. For RQDX (and MV2000) controllers, like the OP
is asking about ("RD disk size"), there was a formatting application
(supplied on tape for Qbus MicroVAX and in ROM for the MicroVAX 2000).
You interacted with the application and you told it what your drive
was if it couldn't "guess". There were only a few specific disks
supported by this controller (unlike ISA controllers or SCSI bridge
controllers of the same era), so the formatter code could make a few
assumptions about what drive was plugged on there, but ultimately, if
you had a factory-fresh drive that had never seen an RQDX-type
controller before, you need the Field Service formatter that let you
put in any geometry data. AFAIK, it was then written to a magic spot
on the drive that the controller would read back later.
The way DEC did things and the way personal computer manufacturers did
things were frequently quite different.
-ethan