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From: "Charles Sandmann" <cwsdpmi AT earthlink DOT net>
Newsgroups: comp.os.msdos.djgpp
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Subject: Re: System()
Date: Mon, 7 May 2012 21:29:22 -0500
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To: djgpp AT delorie DOT com
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Reply-To: djgpp AT delorie DOT com

"Georg" <dosusb AT googlemail DOT com> wrote in message 
news:fea067ab-ee56-4b9e-bb73-ed3365fb9d59 AT ee2g2000vbb DOT googlegroups DOT com...
> 2. Real mode DOS 7.0:
> So a rough calculation: 572k minus 43k for CWSDPMI and minus 162k for
> testsys.exe's data gives 367k remaining. That is what DOS tells to be
> available for a program.

> Maybe one can load high CWSDPMI. But why are there 162k for
> testsys.exe's data while it is just 1936 bytes in Windows XP?

Under Windows XP, much of the memory for DPMI is implemented in
the virtual machine itself, not in the DOS session.

With CWSDPMI, all of the code and data structures are in DOS memory.
Excessive usage is a byproduct of new "huge" computers.  CWSDPMI will
try to provide lots of physical and virtual memory if it is needed.  But 
just the
bitmap for 2GB of 4KB pages would be 64KB of memory; there are
similar structures for swap space, etc.  In the DOS days, typically a 16MB
system was large, so the data structures were much smaller.  Any DOS user
also typically used EMM configs to provide UMBs (so DOS=HIGH,UMB).
In these cases all the typical data structures would fit in UMBs.  There are
some tuning settings to reduce CWSDPMI memory usage, but since recent
GCCs are such memory hogs, you may tune for one type of app over
another.  If most of your apps are 32-bit GCC apps, the 367KB remaining
is much more than you need.