DEC Rainbow disks - help

[Paul Koning]

>>>>> "Allison" == Allison  <ajp166 at bellatlantic.net> writes:

 >> ...
 >> Ok, so where physically is LBA 0?  On most disks it is cyl 0 track
 >> 0 sec 0.  But not on the RX50; there it is cyl 1 track 0 sec 0.
 >> 
 >> So what LBA lives on cyl 0 track 0 sec 0?  Answer: I'm getting too
 >> confused in the arithmetic, but it's an LBA in the range 790 to
 >> 799.

 Allison> snipped...

 Allison> OK, that applies to NON CP/M and DOS disks and I can see how
 Allison> it would be a PITA to create DOS or CP/M media from other
 Allison> DEC systems.  Having done it in the past those memory bits
 Allison> were lost. ;) Mostly due to trying to do the reverse (DEC
 Allison> OSs from CP/M hardware).

 Allison> I see no other impact to the Rainbow from the RX50.

I don't actually know the Rainbow, but I have a bit of code elsewhere
that contains this comment:

/* Convert block number to cylinder, head, sector for RT11-RX50.
 * This is different for RT11 than for Rainbow DOS:
 * For DOS, for cylinders 2 through 79, the sectors are interleaved 2:1.
 * (DOS capability is not supported in this RT11 version).
 * For RT11, all sectors are interleaved 2:1, and each subsequent 
 * track has the first logical block offset by 2 more sectors.
 */

So the implication is that Rainbow RX50s are still weird, but weird
differently than PDP11 ones.  I don't have more detail than that, and
I don't remember the origin of that comment.

  paul