PDP-11 disk image question

Charles Anthony charles.unix.pro at gmail.com
Sat Feb 16 12:53:16 CST 2019

On Sat, Feb 16, 2019 at 10:04 AM Bill Gunshannon via cctalk <
cctalk at classiccmp.org> wrote:

> I used SIMH to build RSTS V9.6 on a simulated RA81 disk.  I wrote the
> disk as a file to a CDR in CD9660 format.  I moved the BA350 and the
> CD to a VS3100 running OpenBSD.  I was able to mount the CD under
> OpenBSD and see the file containing the disk image.  I used dd with
> the command given in my original message (and repeated above) to try
> to write the image to a real SCSI disk.  When I try to boot it I get
> the RSTS Message "INIT.SYS not found".  The disk was completely blank
> to start so the RSTS info must have been copied but apparently not
> copied correctly.
> Any more suggestions?

My wild speculation would be disk geometry. I don't know the specific
geometry of the RA81 disk, but it is possible that SIMH is writing a sparse
disk image.

As an arbitrary example, suppose that there were seven sectors per track,
and the RA81 accepts a 3 bit sector number. If SIMH is treating the sector
number as a 0-7, then the blocks would be written to the image file as:
   0/0  0/1  0/2  0/3  0/4  0/5  0/6  unused  1/0  1/1 .....
When the image file is copied to the drive, the "unused" block is written
to 1/0 on the drive, and 1/0 is written to 1/1. etc.

This could match the observed behavior; the data in the first track is call
correct, allowing RSTS to start, but the directory blocks containing
INIT.SYS are on the wrong locations, causing the 'can't find' failure.

I don't know if SIMH is doing this; one test would be to run something on
the SIMH RSTS that filled the disk with data (ensuring that the last
sectors are written to, and compare the size of the image file with the
size of the actual disk; if the image file is bigger, then it is probably
sparsely written.

-- Charles

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