On 21 September 2015 at 01:55, Jerome H. Fine <jhfined...@compsys.to> 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 :) http://www.netbsd.org/docs/bootcd.html#multiimage So much cool stuff no-one bothers with now days... David
