Looking for i386 versions that support >16Meg memory

[Andrew H. Marrinson]

There are some things to watch out for when going beyond 16Meg even if
-- like the person running ESIX -- it seems to work.  In particular,
the AT bus DMA channels and all ISA-BUS cards support only 24-bit
addresses.  Now Unix since (at least) 3.2 has a mechanism to deal with
this a file (mentioned in a previous posting) allows you to indicate
which portions of memory support DMA and which don't.  When a DMA
would be performed to one of these addresses, a copy is performed
instead.

So end of problem, right?  Wrong.  Some drivers may not know about
this scheme and blindly truncate the address to twenty-four bits or
otherwise fail.  In particular, third party DMA controllers such as
Adaptec 1540 or WD7000 are more likely to suffer from this than
others, since they won't be using the supplied dma library routines.

This is a problem if you try to read or write raw partitions (e.g.
with a DBMS) from a user program that could be located above 16Meg.

I am not saying any particular OS/driver combination suffers from this
problem, just that you should be aware of it and find out for your
particular combination whether it works correctly or not.  This may be
the reason ISC and/or ESIX claim there OS's do not work with > 16 Meg.

Just another reason why AT/ISA architecture is gimping our hardware...
(Uh oh, stand by for flames on that one...)
--
		Andrew H. Marrinson
		Icom Systems, Inc.
		Wheeling, IL, USA
		(andy at icom.icom.com)