Many BIOSes use only F7000-FFFFF as resident space, freeing 28 KB starting at F0000. That's a well-kept BIOS secret which I learned from the UDMA author Jack R. Ellis ;-)
Although on most of the machines I use I am able to find some place in the ROM BIOS area where I can place UMBs (really useful with the amount of drivers I usually load), it is in no case at F0000.
In no case the space F0000-F6FFF is "all ones" (FFs), but the BIOS just "forgets" to set it so after its initialisation, since it's officially reserved for it. But in fact it doesn't really use that space, so you can force EMM386 to use it by /I=F000-F6FF and it will work! :-) The upper limit varies (is not necessarily F6FFF) and can be determined by (1) trial-and-error, and (2) looking at the interrupt vector table to see where is the lowest interrupt handler addres.
This is very vendor/model-specific.
Of course, and is valid for Award BIOS only.
Lucho
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