[Simh] DEC floppy disk interleave questions

Sun Mar 8 06:13:56 EDT 2015

Potentially interesting for reading floppy disks at a low level: Kryoflyx
<http://kryoflux.com/>.

// Christian

On 8 March 2015 at 02:14, Howard M. Harte <hharte at hartetechnologies.com>
wrote:

> Dave Dunfield's ImageDisk (.IMD) format can preserve the floppy disk
> metadata.  He has some DEC disk images in this format on his site:
> http://www.classiccmp.org/dunfield/img/index.htm
>
> When I implemented several floppy disk controllers for the SIMH/AltairZ80
> simulator several years ago, I wrote a module for SIMH called sim_imd which
> can utilize the ImageDisk format within SIMH.  At that time, I had a patch
> to make it work as an alternate to the flat file format that is normally
> used for SIMH for the pdp_rx disk controller.  I tested sim_imd with the
> PDP-11 RT11 disk image from Dave's site, and it worked fine.
>
> I may be incorrect, but if I remember correctly, the RX02 disks have the
> sector header in single-density and the data field in double-density.  In
> that case, I don't think ImageDisk will be able to handle it.  If you hook
> up an 8" floppy drive to a semi-modern PC motherboard with the right disk
> controller, then you may be able to read at least RX01 disks with
> ImageDisk.  Last time I tried this was around 2008, and I believe I used an
> Intel Desktop Board with Pentium 4 CPU and Shugart 800 drives.
>
> Only certain PC floppy controllers can read single-density, and even fewer
> can write it.
>
> Preserving the sector headers is fairly important in my opinion.  It
> allows the image to be written back to a physical disk and used on real
> hardware.  That said, just getting the data off the disk in a flat file is
> still very useful for most purposes.
>
> -Howard
>
> -----Original Message-----
> From: Simh [mailto:simh-bounces at trailing-edge.com] On Behalf Of Johnny
> Billquist
> Sent: Saturday, March 7, 2015 4:52 PM
> To: Alan Frisbie; SIMH at trailing-edge.com
> Subject: Re: [Simh] DEC floppy disk interleave questions
>
> Hi, Alan.
>
> On 2015-03-08 00:47, Alan Frisbie wrote:
> > I have a large quantity of disks that I wish to copy to files that can
> > be directly used by the SIMH PDP-11 emulator and by the
> > E11 emulator.   They include 8" floppies (both RX01 and RX02),
> > RL01, and RL02.
> >
> > The issue is that the disks have sector sizes that differ from the
> > usual 512 bytes, as well as having interesting interleave
> > and stagger factors.   RX50 (and RX33?) disks have (I think)
> > 512-byte sectors, but some odd track usage.   I also believe
> > that RX02 disks have the first track in single-density mode, just to
> > complicate things, but it isn't used by most DEC O/S
> > software.   RL01 and RL02 disks also have bad-block sectors
> > at the end of the disk.
> >
> > I am assuming that SIMH and E11 emulate the device faithfully enough
> > that programs which are aware of the interleaving and
> > small sector sizes will work properly.   If this assumption is
> > wrong, please enlighten me.
> >
> > If my assumption is correct, what is the best way to copy the raw
> > disks (which are in a variety of O/S formats) to files which the
> > emulators will be happy with.   I can bring up a real PDP-11 with
> > RX02, but will probably be using a microVAX-II with an Andromeda
> > FDC11-B controller and Shugart 800 drives.   I don't mind writing
> > my own code with QIOs.
> >
> > I have a bunch more questions related to this, but this will
> > do for now.   :-)
> >
> > All of this is related to cleaning out my storage units and
> > de-cluttering my life.
>
> Hmm, I believe this is not absolutely straight forward. The problem is
> that simh (or E11) do not emulate the physical layer, but the logical one.
> As such, the image files of disks are assumed to always be containing
> sequential blocks, and no block headers are in the image file.
> So, it will work, in that, if you can dump out an image from a disk, where
> block #1 is block #1 on the image file, then things will just work fine.
> If you dump out the physical blocks raw, including block headers, then
> they are not usable by the emulators.
> Logical rearranging of blocks will work fine, though. So, the trick you
> normally see with RX01/RX02, where they remap block numbers to other blocks
> numbers in the device driver, is just fine. You just want the actual
> physical blocks, in the order they are on the disk. )As indicated by the
> disk block headers.) The actual layout, as created when formatting the
> disk, will not carry over, but it is also not important.
> (I hope I'm making sense here, I feel I might be overcomplicating my
> text...)
>
> Bad blocks, as described by the RL01/RL02 bad block tables, are totally
> under the device drivers and system software, so that is just fine.
> Systems will avoid those blocks, even on a dumped image of the disk,
> assuming you copy all blocks, including the bad block list.
>
>         Johnny
>
> --
> Johnny Billquist                  || "I'm on a bus
>                                    ||  on a psychedelic trip
> email: bqt at softjar.se             ||  Reading murder books
> pdp is alive!                     ||  tryin' to stay hip" - B. Idol
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