[Simh] RX and Track 0

[Tim Shoppa]

"Alexander Voropay" <alec at sensi.org> wrote:
> 2008/9/7 Alan Frisbie <frisbie at flying-disk.com>:
> >
> >>  Does RX device on the PDP-11 use track 0 ?
> >
> > No, none of the DEC PDP-11 operating systems used track 0 on
> > 8" floppies, as far as I can tell.
>
> Should I add an emty track at the biginning of the floppy image to
> skip a Track=0 ?
>
>  SIMH's emulated RX-11 controller reads Sector=1 Track=1
> at the offset 0D00H = 3328 = 128*26 in the image, a middle
> of the RT-11 directory instead of the boot sector.

There are a number of different image formats for RX01/02 floppies.

(What was the Saturday night routine, "It's a dessert topping and a
floor wax!"?)

Some image formats are just the blocks as used by the DEC OS's.
Other image formats are the raw physical sectors. They differ
because DEC used an interleave to map non-contiguous physical
sectors into contiguous blocks.

Images that are the raw physical sectors will be 26*77*128 = 256,256
bytes. Twice that for a double density disk.

For images that are just the DEC OS accessible blocks, it's 494*512
bytes for a RX01, and 988*512 bytes for a RX02.

Some Unices that ran on DEC machines would start their interleave
at track 1, work through track 76, and then come back to track 0
for the end, using that last little bit of capacity that the DEC
OS's ignored. Complicated, huh?

John Wilson's PUTR knows how to deal with most of these different formats.

Now in the 12-bit world.... that's another story. I remember
Charles Lasner lecturing me about all the different PDP-8
floppy interleave issues!

Tim.
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