When I start Freedos from the CD, it gives me four options. If I choose 1,3 or
4
it says it is loading an experimental UDMA driver for my hard disk and then
hangs with a divide by zero error and some hexadecimal garbage.

I am running an Athlon XP 2200 with two hard rives, one ATA66 20GB with only
Windows XP partition, one ATA 133 120GB with a linux partition (Morphix), a
linux swapfile partition, a Fat 32 partition and an NTFS partition. The FAT32
partition is not bootable as yet, so I boot Freedos from the CD. After choosing
option 2 I can still access the FAT32 partition as drive c:, but FreeDOS fails
to load high (about 400k conventional memory free).

Hi, UDMA should normally auto-detect systems
where it cannot load. About not having UMB
and other high memory free: We felt that the
installer would not need much free RAM, so we
did not activate EMM386 when you boot from the
CD. However, when you are later able to boot
the installed system on C:, EMM386 will be
active. You should also check www.freedos.org
news, as EMM386 is regularily updated.

I got some other comment which told that
UDMAJR would crash when used together with
FreeCOM XMS Swap on some computers, might
be a related problem.

Sorry, this is a case of 'everything or nothing'

IF extended memory available, then optimize memory usage, load a bunch of
drivers, among which UDMA, and continue.
If no extended memory available, you indeed get only 400KB or so.
FreeCOM (command.com , the shell) is pretty memory hungry.
MS COMMAND.COM on the other hand only uses 10KB in any situation it seems.
kernel memory usage is about 80KB if no XMS (and thus no HMA) available.

400KB is enough to install FreeDOS, I guess.
after installation you'll have a lot more RAM available.

I'm not sure I can get UDMA to load at any other later time in the bootsequence.

Please update to the latest version 2.7 and see if the bug still exist or not.

UDMA/XDMA is disabled by default now in the 1.0-Testing distributions, to
minimize possible failures.