PDP-11 disk image question

[Warner Losh]

On Tue, Feb 19, 2019, 7:24 AM Paul Koning <paulkoning at comcast.net wrote:

>
>
> > On Feb 18, 2019, at 10:29 PM, Warner Losh via cctalk <
> cctalk at classiccmp.org> wrote:
> >
> > On Mon, Feb 18, 2019 at 6:18 PM Bill Gunshannon via cctalk <
> > cctalk at classiccmp.org> wrote:
> >
> >> ...
> >> No, RX50 was a strange DEC format.  RX33 is a 1.2M floppy.
> >
> > The RX50 was a single sided 800 block floppy. The first two tracks had no
> > interleave. The rest has 2:1 interleave, though sometimes physical and
> > other times logical. Strange in some ways, kinda standard in others. But
> > it's still a floppy, and other than size, much like the RX33 with 1/3 the
> > number of blocks.
> >
> > Warner
>
> That's not correct.
>
> RX50 is 80 track, single sided, 10 sectors per track (not the PC-standard
> 9 per track).  All tracks are 2:1 interleaved.  There is a 3 sector skew
> from track to track.  And logical track 0 is physical track 1 (physical
> track 0 is logical track 79).
>

The interleave is logical on the Rainbow. The drive is formatted to 1, 2,
3, 4, 5, 6, 7, 8, 9, 10, however after track 2 they are logically
interlaved by the drivers in Venix, Dos and CP/M. There is no skew there,
except for Venix...  And on the Rainbow, tracks are purely sequential...
Guess that lead me astray for the pdp-11 users...

The MSCP controller does this; on a Pro it's done in the driver.  I've done
> it for RSTS, but it's easy to confirm by reading the source code of DEC
> drivers such as the one in RT-11.
>

I stand corrected. Yet another odd quirk of this quirky media. I recall
from back in the day there are other DECmate quirks...

Warner

>