DEC Rainbow disks - help - test-drb@ccmp.vtda.org

29 Jul 2005

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Subject: Re: DEC Rainbow disks - help
   From: Paul Koning <pkoning at equallogic.com>
   Date: Fri, 29 Jul 2005 09:14:35 -0400
     To: cctalk at classiccmp.org

 >>>> "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 
No, the dos disk are structured not unlike the CP/M disks where the first
track(cylinder) is system area (loads a loader from there).  They are 
weird compared to nominal PC but in the CP/M system world they are 
not so strange.

FYI: some CP/M systems did not use interleave (skew) at the bios level 
and instead wrote the cylinders with non sequential (skewed) sector 
numbers.  The effect is the same but required less code to get skewing.
Ths was especially handy when 512byte sectors were used 
(4 logical cpm sectors).

Allison