Wolfgang writes:

>AFAIK, there isn't any "augmented DOS MBR bootstrap code" nor any "DOS TSRs/
>drivers" involved with secondary partitions.  Access to those secondary
>partitions is just part of the regular DOS code.

Oh. Okay.  I hadn't looked at it in detail since dos 2.11 or 3.x days.
Back then, extended partitions required installing nonstandard
bootstrap code into the MBR.

>The MBR is the very first sector of a harddisk.

Yes, but some people refer to any 512 bytes containing boostrap code
and an MBR-style parttition table as "an MBR". 
    i)  for a "secondary partition" as you describe it, and
    ii) When booting an active partition, the MBR (first 512 bytes of
        first disk) loads  the first 512 bytes of MBR partition into
	memory and jumps into it.  Some OSes (incl. netbsd)
	set this code up so that it can funciton as an MBR, too.
	(i.e., as first 512 bytes of entire disk, not just first 512
	bytes of OS's (primary) partition.)


I hope someone is collecting all this info, so it can go into an
appendix of the install docs...  rvb, maybe?