[Simh] 8" Floppy disk image getting HALT error

Tue Jul 11 14:02:41 EDT 2017

I think that the proper image size should be 256256, as 6656 is exactly
256x26, w which is a multiple of 13, as well as 256256.

On floppy disks with 77 tracks of 26 128-byte sectors each, or 13 256-byte
sectors, track 0 could be treated as the last one when the media was
accessed at the application level (probably because it tended to be less
reliable due to the heads spending more time there).

Moreover, as the media size is not a multiple of 512, reading the last one
or two sectors may be problematic in some systems if the system block size
is 512. This hindered my recovering of the Terak boot disk for several
years.

Leo

On Jul 11, 2017 10:26 AM, "Walker Sampson" <Walker.Sampson at colorado.edu>
wrote:

Hi everyone,

Many thanks for this feedback. A fair amount to chew on here. I’ve sent a
request to the donor to share the disk image, if I get a positive I’ll be
happy to throw up a Dropbox link to it for others to examine.

This may mean a problematic read of the disk itself, but addresses 00000 –
019D0 are all zeroed out, or about bytes 0 – 6655.

Data begins at byte 6657, and that is the document I mentioned. Last byte
of the document is 73080 and then just blocks of either zero or E5E5E5E5
till the end of the disk.

To Paul’s point of mounting a RK05 drive – which drive should I be mounting
here, assuming it is the RX02 disk it seems to be? “AT RX01 <disk_image>”
still gets a HALT error. Of course, as you all point out, perhaps this
isn’t a bootable disk, period. “SH RX01” gives “RX1, 256KB, attached to
test-decrx01.img, write enabled”

FYI as well, on a modern HFS+ system, the disk image file is coming in at
256 KB.

Any thoughts on the run of zeroes and E5E5E5E5?

Thanks again,

Walker

 On 7/11/17, 6:29 AM, "Simh on behalf of Johnny Billquist" <
simh-bounces at trailing-edge.com on behalf of bqt at softjar.se> wrote:

    Hi.

    On 2017-07-10 22:10, Walker Sampson wrote:
    > Hi everyone,
    >
    > Let me preface this by saying that I’m unfamiliar with the original
PDP
    > machines. I work as a digital archivist and have received 8” floppy
    > disks from which I need to recover data.
    >
    > I believe I have recovered at least partial data from these disks;
I’ve
    > connected a Y-E Data 8” floppy drive to a KryoFlux floppy disk
    > controller and gotten positive sector results setting the format to a
    > DEC RX02 sector image. When I investigate the resulting disk image in
a
    > hex editor, I am seeing clearly a report document, so I don’t believe
I
    > have a false positive.
    >
    > Outside of observing in a hex editor however, I don’t know how to
access
    > the disk or its contents. Using SIMH, I haven’t gotten the virtual
    > machine to boot the floppy disk image.
    >
    > Commands “AT RK01 <disk_image>” and then “BOOT RK01” give me a “HALT
    > instruction, PC: 000002 (HALT)” message for the PDP-11 program. The
    > PDP-8 stalls indefinitely and the PDP-10 outputs “Non-existent device”
    > as well.
    >
    > I can’t go back to the donors and ask what machines these 8” floppies
    > were used with, so I’m not sure how to begin troubleshooting.
    >
    > Any advice in that area is much appreciated!
    >
    > Thanks,
    >
    >
    > Walker

    To sum things up. You can probably ignore all the questions about if
    this really is some DEC floppy, what kind of format it has, and so on.
    If kryoflux managed to extract data that looks valid with RX02 parameter
    settings, then I'd say we can be sure it is an RX02 disk. And this
    format was unique to DEC, so it can't be anything else.

    Which also means, you already have managed to exact all the bits, and
    most probably correct. The next question is just about restoring the
    data in a more coherent form, which means getting it in the form of
    files, and understanding the format of the files.

    For this, we need to know what system the floppy was written on. Paul
    Koning gave the most useful advice. The first few blocks will usually be
    enough to find out what system the floppy was written on.

    RX02 floppies could certainly be bootable, but most are not. OS/8 (and
    derivatives) for the PDP-8, and RT-11 for the PDP-11 were the ones that
    supported RX02 as a bootable system. Other systems supported the
    floppes, but only as a way of carrying bits around, so not being able to
    boot from the floppy is probably to be expected.

    So, if you could give us just the first few blocks, it should be
    possible to tell what file system it has, and that gives us OS, file
    structure and probably the ability to work out the rest in quick order.

        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
    _______________________________________________
    Simh mailing list
    Simh at trailing-edge.com
    http://mailman.trailing-edge.com/mailman/listinfo/simh

_______________________________________________
Simh mailing list
Simh at trailing-edge.com
http://mailman.trailing-edge.com/mailman/listinfo/simh
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mailman.trailing-edge.com/pipermail/simh/attachments/20170711/fd68b633/attachment.html>