SS> I have 16 as well (don't want to go higher or I'm afraid I'll lose
SS> the bus-mastering ability of my Adaptec SCSI HA on my ISA bus),

TM>Bus-mastering will work, but the memory above 16 MB is only used for
TM>swapping by OS/2, not as "real" memory.

PH| You make this sound like an absolute.  Is it?  I have a basic ISA
  | machine (with a couple of VESA slots, but they don't count) with 20MB
  | installed and seem to get full use of all my memory.  OS2MEMU from the
  | DevCon tells me my physical memory is 20MB and I seem to need the proper

I think you may be confusing the DMA lines with the ISA bus memory lines.
Here's my understanding of the technology:

If you have a 24-bit DMA, as most ISA machines do, you are limited to 16
megs of *directly accessible* RAM by your hardware, regardless of the fact
that your 386+ machine has 32-bit addressing for a 4 gig memory space.
There are tricks around the 16-meg limit, but with a 24-bit DMA, it's
physical limit is 16 meg (geez, and I remember with I was hot-stuff with a
64K CP/M machine with two SSSD floppies! ).  Most 32-bit bus (e.g.
Microchannel, EISA) systems also have 32-bit DMAs, so they don't suffer
from this problem.

However, the ISA bus only has 24 address lines (remember, it was designed
for the 286 with it's 24 address lines).  No ifs, ands or buts.  Your
ISA bus-mastering adapter cannot access memory above 16 megs, period,
since it's addressing is limited to 24 bits.  It may be able to access it
by using the DMA, but then you lose the benefit of bus-mastering since the
CPU has to handle the memory access.
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