From:	"Tim Bird" <tbird AT caldera DOT com>
Message-Id:	<9705121531.ZM5548@caldera.com>
Date:	Mon, 12 May 1997 15:31:54 -0600
In-Reply-To:	Pierre Phaneuf <pp@55-174.hy.cgocable.ca>
	"Re: Back on track... Opendos's Not Unix!" (May 12, 11:43am)
References:	<Pine DOT LNX DOT 3 DOT 95 DOT 970512112322 DOT 26363A-100000 AT 55-174 DOT hy DOT cgocable DOT ca>
To:	OpenDOS Mailing List <opendos AT delorie DOT com>
Subject:	Re: Back on track... Opendos's Not Unix!
Mime-Version:	1.0

Pierre Phaneuf wrote:
> On Mon, 12 May 1997, Mr M S Aitchison wrote:
> > OBJECTIVE 2: OpenDOS will continue to run old DOS applications; it will
> >              be about as compatible with PC-DOS/MS-DOS as one version
> >              of MSDOS is with another.
>
> This one is rather easy. The DOS platform as it is today is such a
> braindead piece of software, not very hard to emulate...
Not at all easy.  DOS was not written with any kind of clean code/data
separation.  Because Microsoft did not publish enough of an API in the
beginning, numerous programs have become dependent on internal DOS data
structures (essentially pegging the data at fixed offsets in the data
segment), and making modification of the data segment in a backwards
compatible fashion very difficult.  The hardest thing about DOS compatibility
is retaining all the "undocumented" features and data offsets, as well
as keeping OpenDOS "bug-compatible" with MSDOS.  DOS is VERY hard to
emulate.

Tim Bird

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