FreeBSD boot manager customization for single-disk multiboot (4 x DOS)

Ian Smith smithi at
Mon Oct 15 08:29:18 UTC 2018

Hi there heroic wanderer,  it's been a while. 

I only rarely read questions@ these days, from the archives, but this one I thought maybe I could help with.   I'm in a care situation, tapping this out in K9mail on a small phone, so will try to be concise.
I've read the thread.  Manish is trying to help but his DOS familiarity doesn’t go back far enough.  I started at 2 and used 3.3 (with Desqview), later 5.0 for years running Fidonet mailer, BBS, message editor, DOS command space and other tasks in 6 MiB RAM .. so had to learn how to really squeeze things in - as did you, I gather. 

Correct, you cannot boot DOS from a non-primary partition.  Maybe you could use GRUB, it installs after the MBR on track 0 side 0 so shouldn't get in the way.

I dont think you need to install linux to install GRUB?  That said, I don't use it myself.  To add linux to the 4-DOS mix would need an extended partition, leaving only 3 primaries for 3 x DOS.  Same deal with OS/2, with its great boot manager. 

boot0.s MBR code is VERY tight with about ZERO free space as I  recall .. however there's code there - still was at 9.x I think - for a 1Ķ (2 sector) version written by the same author (Robert Nordier IIRC, I don't have a system to hand).  That should have plenty of room for more lavish labels, and may in fact include such code?  If you have a spare system you could assemble that and test.

Of course you'll have a backup MBR (directed more at other readers :)

One thing to watch is that esp.  earlier versions of DOS may want to use the first slice explicitly?  I'm not sure, but I'd start with earliest versions first, and test each as you go. 

Please CC me on any replies. 

cheers, Ian

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