Backups [was Re: Is tape dead?]

Johnny Billquist bqt at
Mon Sep 21 20:59:44 CDT 2015

On 2015-09-21 23:32, David Brownlee wrote:
> 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...

I actually did boot NetBSD/vax from CD at one point, so it worked in 
practice as well.


Johnny Billquist                  || "I'm on a bus
                                   ||  on a psychedelic trip
email: bqt at             ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol

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