PDP-11 disk image question

Paul Koning paulkoning at comcast.net
Sun Feb 17 10:40:38 CST 2019

> On Feb 16, 2019, at 1:04 PM, Bill Gunshannon via cctalk <cctalk at classiccmp.org> wrote:
> ...
> let's try with a detailed explanation of what I did that didn't seem
> to work.
> First, my hardware.  I  have a PDP-11/93 with a CMD SCSI Module and
> a BA350 with 6 2GB hard drives.  The Module is set up to present RA81
> disks and the first 3 disks have 4 partitions each which should work
> out to 12 RA81 disks. (But that last part is unimportant right now!)
> 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.

That message is from the first stage in INIT.SYS.  It means the boot loader loaded INIT successfully, because you got into that code and it executed disk drivers and file system code in an attempt to look up the INIT.SYS file.  That operation failed.

The most likely cause is that your disk is the wrong size.  If the system was built on a simulated disk of size x, and is booted on a disk where the controller reports size y, that may work.  But if the two sizes rounded up to the next power of two are different, RSTS will not recognize the file system.  For example, if x is 500 GB and y is 600 GB, that will be the case.  This is because RSTS has a file system parameters "device cluster size" which is device size in blocks / 65535 rounded up to a power of 2.  And that number affects the file system layout.

The boot loader uses simple LBA addressing so it is unaffected by this, which explains why INIT.SYS was properly loaded by the boot loader.


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