Date: Sat, 18 Aug 2001 10:26:28 +0300
From: "Eli Zaretskii" <eliz AT is DOT elta DOT co DOT il>
Sender: halo1 AT zahav DOT net DOT il
To: jp_prongs AT yahoo DOT com (Nick)
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CC: djgpp AT delorie DOT com
In-reply-to: <f5c79439.0108171412.689b2a6b@posting.google.com>
	(jp_prongs AT yahoo DOT com)
Subject: Re: Thoughts on physical address and DMA
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> From: jp_prongs AT yahoo DOT com (Nick)
> Newsgroups: comp.os.msdos.djgpp
> Date: 17 Aug 2001 15:12:58 -0700
> 
> Since you add `__djgpp_conventional_base' to a physical address to
> convert it to a linear address, could you possibly _subtract_ it from
> a linear address to convert it to a physical address? If so, could the
> physical address be used for DMA?

No and no.

You are mixing linear and physical addresses, which are two different
things.  Also, __djgpp_conventional_base is a negative value; see
section 18.6 of the DJGPP FAQ for more about that.