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README1.TXT
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1992-02-06
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DR-DOS & MS-DOS DUAL BOOT TECHNIQUE
Edward Welch 72361,620
Many users or potential users of DR-DOS 6.0 probably would like to
have the ability to occasionally run MS-DOS 5.0, without the need
to boot from a floppy. By having both systems available,
compatibility problems can be checked, and the user can remain
familiar with both systems. I was determined to find a way to do
this, and initially set up 2 partitions, using a small partition
activator utility to change the boot partition. This was very
awkward to configure, and caused a problem with the resultant
switching of C and D drive designations when the boot partition is
changed.
However, I found a way to solve the problem and have both
operating systems available on one partition. This required only
DEBUG, and the creation of several DEBUG scripts and batch files.
DEBUG is used to create two files with copies of the hard drive's
boot record, first with MS-DOS and then with DR-DOS, and then used
to write the appropriate file back to the hard drive's boot area
depending on the operating system desired. It also takes advantage
of several quirks of DR-DOS and MS-DOS: First, the fact that DR
calls their system files IBMBIO.COM and IBMDOS.COM, while MS calls
their files IO.SYS and MSDOS.SYS. Second, the fact that DR does
not require the system files to start at the beginning of the
drive, while MS requires IO.SYS to be first. Third, the fact that
DR-DOS, if installed after MS-DOS, will not disturb the MS-DOS
system files.
This method is potentially dangerous. If it fails, it can cause
the need for the drive to be SYS'd from a floppy, but much worse
could corrupt the FAT, causing severe data loss. Therefore, anyone
who tries this should MAKE A COMPLETE BACKUP FIRST. Two backups
would be better! I have tried this only on my system, and it has
worked with no problem, but it is entirely possible that it could
cause problems on another system. Try this at your own risk. It
might also work with earlier version of MS-DOS and DR-DOS, but I
haven't tried it. Also, someone with IBM-DOS, or an OEM MS-DOS,
using the file names IBMBIO.COM and IBMDOS.COM can't use this
method unless they rename the files and patch all references to
them within the files and the boot record to match the new names,
a complicated procedure, which would probably work, however.
One more thing, DEBUG, like most MS-DOS external utilities, does a
DOS version check, and will refuse to run under DR-DOS. It is easy
to patch it to eliminate the version check, however, and this must
be done to allow changing back to MS-DOS while running DR-DOS. It
is possible that DR's SID could be used instead, but I'm not
familiar enough with it to try it on a boot sector!
Once completely setup, changing operating systems involves simply
typing DRBOOT (or MSBOOT), pressing enter, then rebooting. The
necessary simple debug script files are included here. Two other
text files describe patching DEBUG (README2) and setting up the
system (README3). Good luck!