DEC Rainbow disks - help

[Allison]

>
>Subject: Re: DEC Rainbow disks - help
>   From: Jules Richardson <julesrichardsonuk at yahoo.co.uk>
>   Date: Fri, 29 Jul 2005 15:28:06 +0000
>     To: cctalk at classiccmp.org
>
>On Fri, 2005-07-29 at 09:14 -0400, Paul Koning wrote:
>> 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.
>
>Strange how they don't interleave the first couple of cylinders for the
>Rainbow. Why is that? It there some situation where it's necessary to
>write some form of boot sectors on the disk in order?

Those are reserved for "system" tracks and are not part of the data area.

>For RT11 it sounds like there are issues with the hardware finding the
>index hole and being able to react in time to read the first sector on
>the track, hence the offset within the track to the first logical block
>on that track?

No.  RT11 has no such problem.  It's possible the controller could but, that
is not true either.  The index hole and all are invisible to the OS as it's 
all a controller problem.

Allison