UDMA requires VDS in V86 mode. Without XMS driver it can't do its tests and loads but fails.
HIMEM gives XMS without V86, so it theoretically should work in that situation.
Yes, but in practice it doesn't work everywhere. What chipset (south bridge) do you have?
For no HIMEM, UDMA doesn't say it doesn't work without an XMS driver and the ASM comments say that "DMA only" supported was added in version 1.7 for non-XMS environments. The UDMA.ASM code appears to support that remark. Perhaps the support is missing, but seems like crashing the machine is a bit more unfriendly than what is intended.
If it can't do its tests without XMS, it can either load and possibly crash or refuse to load without XMS at all. We chose the former.
VDS with EMM386, I didn't know how or if it would work. When I looked at the UDMA code, it appeared to check for a VDS flag and existence call that EMM386 would not have set and I thought it might work on its fallback to non-VDS mode.
UDMA is not perfect and Jack R. Ellis abandoned it when it became clear that it can never be made 100% compatible. He was 58 years old and became very tired of it. There was yet another reason - I proved to be a bad "PR guy" for him :-( I wish you keep programming when you are 58!
Lucho
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