Backups [was Re: Is tape dead?]

David Brownlee abs at
Mon Sep 21 16:32:48 CDT 2015

On 21 September 2015 at 01:55, Jerome H. Fine <jhfinedp3k at> wrote:
>>Fred Cisin wrote:
>> On Sun, 20 Sep 2015, Jon Elson wrote:
>>> Well, one would assume this is also OS specific.  I would guess it would
>>> be incredibly hard to make a "disk" virus that would work on greatly
>>> differing OS's like Linux AND Windows.  No telling what would happen if one
>>> of these disk viruses got onto a hard drive on a Windows system and then the
>>> drive was reformatted and loaded with Linux.
>>> Most likely you'd have odd crashes or something.
>> It is possible to create an executable file that identifies the OS that it
>> is running on and does a conditional jump to different code, assuming that
>> the processor uses the same instruction set.
>> How different the OS's are would determine how much code could be shared.
>> If they are very different, then the executable file could be twice as
>> large, with no code in common.
>> It is even possible to make a disk that is readable as multiple disk
>> formats, so long as each is expecting the DIRectory tracks to be in
>> different places.
>> One of the many projects that I never got ready for market was to make a
>> multi-platform distribution format for software.  "Save a few cents on media
>> costs by putting all of your platforms on one disk"  But, after August 1981,
>> it eventually became apparent that the need for such was not going to be
>> around much longer.
>> If the boot code is short enough, it is even possible to have an FM, an
>> MFM, and a GCR boot sector in the same boot track, since each will not even
>> see any except its own.  Formatting/recording a track with mixed densities
>> and/or encodings and multiple sector sizes is not a supported function in
>> most operating systems, nor even FDCs, but can be done with some flux
>> transition controllers.
> I used the above example when I created a CD which had files to be used
> with RT-11 in addition to being a normal CD under Windows.  I found that
> for a normal CD under Windows, sectors 0 to 15 (hard disk blocks 0 to 63)
> on the CD were empty.  I don't know if that area is reserved for boot code
> under Windows when the CD is bootable, but my goal did not require the
> CD to be bootable under Windows.
> Under RT-11, the first six hard disk blocks (0 to 5) are reserved for boot
> code (when that is present) and hard disk blocks from 6 up to 67 are used
> for an RT-11 directory.  RT-11 rarely uses that large a directory and the
> minimum directory is only two hard disk block long.  For the CD, that
> allowed an RT-11 directory from hard disk blocks 6 to 63 or up to
> sector 15.
> What may have been unique was that only the RT-11 directory and the
> CD  ISO directory were distinct.  Otherwise, all the files were the same
> with each directory pointing to the same location on the ISO image.
> In practice, the same CD could be used as a data CD under Windows
> in addition to being a boot disk on a real DEC  RT-11 system which
> supported that operating system.  I was actually on the phone at one
> point when the first individual who received a copy of the CD used
> it to boot RT-11 on a CDROM drive configured to support 512 byte
> blocks using a CQD 220/TM host adapter.
> The same ISO image file can also be used under both SimH and Ersatz-11
> in the same manner, although it is STRONGLY recommended that the
> ATTACH or MOUNT command use the ISO image file as READ  ONLY.
> Ersatz-11 is also able to MOUNT the actual RAW CD on a CDROM
> SCSI drive and boot RT-11 from the CD.  Of course, the Windows
> operating system under which Ersatz-11 is also able to see all the same
> files on the CD as well, BUT  NOT  AT  THE  SAME  TIME - at
> least I never did attempt that possibility.
> If this can be done with Windows and RT-11 which have completely
> different file structures and instructions sets, it certainly seems likely
> that other operating systems and system hardware can also be supported.
> The one thing that seemed reasonable from a security point of view is
> that the CD is READ  ONLY, so no virus can be introduced on the
> CD after it is burned.
> Tim Shoppa did almost the same thing with his RT-11  Freeware CD
> when an RT-11 directory was added at the end of the ISO image file
> for the CD.
> If anyone finds this interesting and has additional questions, please ask.

Before the price of media and storage dropped so much NetBSD install
ISOs were multiboot - one image which booted on alpha, i386, pmax, and
sparc (and I think theoretically also macppc, vax and sun2, sun3 and
sun3x if it hadn't run out of room for the install files :)

So much cool stuff no-one bothers with now days...


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