DOS 3.3 and DOS 5 on the same hard drive

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DOS 3.3 and DOS 5 on the same hard drive

Postby Brutman » Sun Nov 04, 2012 9:44 pm

I wasted the better part of the day on this, but I learned a bit.

First, PC DOS 3.3 is brain dead. Besides the limitation that you can only have 32MB partitions it also insists that the primary DOS partition is in the first 32MB of the drive. That part was news to me, and it is very true. So forget about all of the CHS crap that should let you put PC DOS 3.3 anywhere on the drive - if it is not in the first 32MB of the drive it will not work.

So first, boot from PC DOS 3.3, fdisk, and format. Remember when using fdisk that you need to set the partition as "Active" or "Bootable".

Next you are going to install PC DOS 5. If you boot PC DOS 5 from a floppy it will see the PC DOS 3.3 partition which makes things messy. The easiest way to deal with that is to hide the PC DOS 3.3 partition so that PC DOS 5 does not attempt to touch it. Hiding is accomplished by marking that partition as "something other" that PC DOS 5 does not understand. I have been using DEBUG.COM to read and write the boot sector directly; I am almost done with a tool that does this. You can try a 3rd party partition manager like Ranish instead but it may not run on the Jr. Moving the drive to another machine is the easiest way to do it for the moment - I used a Linux box.

Once the PC DOS 3.3 partition is hidden you can boot PC DOS 5 from a floppy. Fdisk, format, and patch as needed for the PCjr. Remember that DOS 5 needs "stacks=0,0" in CONFIG.SYS and probably needs JRCONFIG to recognize your extra memory.

Now hide the PC DOS 5 partition and make the PC DOS 3.3 partition active again. Use the PC DOS 3.3 FDISK to create an extended partition, and then create some logical drives in it. I created 3 32MB logical drives. Format them (no /s needed because they are not bootable.) Note that PC DOS 3.3 will create partitions with type 0x04, which is the original version of FAT16. PC DOS 5 uses type 0x06 which is the upgraded version of FAT16, or FAT16B. PC DOS 3.3 might not recognize those, so definitely use PC DOS 3.3 to create them.

If all goes well, you will have both PC DOS 3.3 and PC DOS 5 on your system. You keep the one that you are not using "hidden" so that the active one always shows up as the C: drive. The drives in the extended partition will be visible to both and will have the same drive letters assuming you use the "one hidden, one active" scheme.

I have the utility to make one partition active and hide the others done; it just doesn't let you hide a specific partition which is needed after installing PC DOS 3.3 and before installing PC DOS 5.0. If there is interest I will add that feature and make the program available. (With that program you don't need to use another machine or funny debug scripts - you can do everything right on the PCjr. It's a poor man's boot manager ...)

This technique works for other versions of DOS too .. PC DOS 3.3 is the minimum version that supports extended partitions though, so I would not bother with anything earlier. PC DOS 6.x or 7 should work exactly the same way.

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