News Group Posts

floppy drive access, reading odd floppy

disks

From: Bela Lubkin <belal@sco.com>
Subject: Re: How to read differnt formated floppies
Date: Tue, 15 Jul 2003 00:18:20 GMT
References: <612ogv06j0dcdgi4i8edgqfhe2otgvqp1d@4ax.com>
<Xns93B67C5AE1845jajii1netscape@205.188.138.161>
<HHzCDD.11u8@wjv.com>
<kt15hv05h7s2r2cabaklu3ftqif36q36ci@4ax.com> 

Peter Michel wrote:

> >>> i have floppydisks written by a tar, but they are formated with 17
> >>> sectors/track and with 77 tracks per surface, each sector has 512
> >>> bytes on a 5.25" media.
> >>> How can i set parameters of the diskettes drivers to read (and perhaps
> >>> write) it, this stupid format?
> 
> the diskettes where written by an Xenix derivat on normal 5.25" Floppy
> media. The HW had a programmable device under /dev, which also was
> possible to use with the old ecma formats.
> Yes, they wrote it with 2*77*17*512 Bytes on a 5.25" HD disc  8-[:]
> But, the old HW isn't longer available :-(( and on the medias is
> source code we needed.
> Will try and see what will go.

The floppy driver isn't terribly picky about actual physical formats; at
least, not if the sector numbers written in the sector headers make
sense.  What happens if you:

  dd if=/dev/rfd0135ds18 bs=512 count=1 of=/dev/null

Do you get an I/O error or not?  If not, the physical sectors are
probably readable.  You just need to be sure to only read the sectors
that are actually present.

Let me digress for a moment about device names.  I used the "r" device
(raw) because it doesn't go into the buffer cache, and doesn't attempt
to do readahead.  If you try to read from the block device, readahead
will attempt to read sectors you didn't actually ask for (and which
aren't actually present), leading to failure.  I used the device name
"fd0135ds18" because that refers to an 80-track, 18-sector format.  Both
of those parameters are larger than your actual format.  The benefit is
that every sector that exists on _your_ format also exists somewhere in
this format.  (As do a few which don't exist on your format.)  I used
"fd0" but you'll probably actually be using "fd1" (whichever position
your 5.25" drive is on.)  "96" vs. "135" isn't important, they're
actually the same thing inside the driver.

So.  The trick is to request all the blocks that _do_ exist, while
avoiding the ones that don't.

When you read linearly from the 135ds18 format, you get:

  track 0 head 0 sectors 1..18
  track 0 head 1 sectors 1..18
  track 1 head 0 sectors 1..18
  track 1 head 1 sectors 1..18
  track 2 head 0 sectors 1..18
  track 2 head 1 sectors 1..18
  ...
  track 79 head 0 sectors 1..18
  track 79 head 1 sectors 1..18

and what you want is:

  track 0 head 0 sectors 1..17
  track 0 head 1 sectors 1..17
  track 1 head 0 sectors 1..17
  track 1 head 1 sectors 1..17
  track 2 head 0 sectors 1..17
  track 2 head 1 sectors 1..17
  ...
  track 76 head 0 sectors 1..17
  track 76 head 1 sectors 1..17

A "C" program to extract just those sectors wouldn't be too difficult.
But it can also be done in the shell (maybe with a bit of awk).  For
instance:

  awk 'BEGIN {
         dev = "/dev/rfd0135ds18"
         for (tk = 0; tk < 77; tk++)
           for (hd = 0; hd < 2; hd++)
             printf "dd if=%s bs=512 count=17 iseek=%d\n", dev,
                    (tk * 2 + hd) * 18
       }' > /tmp/get-image
  sh /tmp/get-image > /tmp/image
  tar tvf /tmp/image

If you look at /tmp/get-image, you'll see a bunch of separate `dd`
statements.  If this works partially but gets some I/O errors, you can
fiddle around with the specific contents of /tmp/get-image to work
around whatever goes wrong.  Of course if you can't read a single sector
then this isn't going to help...

>Bela<

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