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From: dave@westmark.UUCP (Dave Levenson)
Newsgroups: comp.sys.ibm.pc
Subject: MS-DOS in Protected Mode
Message-ID: <158@westmark.UUCP>
Date: Sun, 13-Sep-87 18:05:53 EDT
Article-I.D.: westmark.158
Posted: Sun Sep 13 18:05:53 1987
Date-Received: Tue, 15-Sep-87 02:57:21 EDT
Organization: Westmark, Inc., Warren, NJ, USA
Lines: 33
Keywords: Emulate Real Mode

The AT&T Personal Computer 6300PLUS runs SimulTASK OS-Merge, sold by
AT&T, and developed by Locus.  This version of UNIX uses the '286 in
protected mode, and runs MS-DOS as a process under UNIX.  Unlike
OS/2, SimulTASK does not switch the hardware into real mode when
passing control to MS-DOS.  It catches the traps that result when
MS-DOS or an MS-DOS application tries to address the hardware, and
emulates the real mode operation being performed (subject to UNIX
security and protection, of course).

The MS-DOS is not a special "protected-mode dos" but real Microsoft
MS-DOS -- it can run just as well on this machine without UNIX. 
This allows MS-DOS applications, even ill-behaved ones, to run
unmodified, while UNIX protects its kernel, and all of its other
applications.  An errant MS-DOS application can cause UNIX to
terminate the MS-DOS process, but cannot damage UNIX or any other
process.

This "real mode virtualization" costs a bit in performance, but buys
back a great deal of system safety and security.  To see its effect
on performance, this system, running plain MS-DOS (without UNIX)
turns up 7.3 Nortons.  Under UNIX, it turns up 5.6 Nortons.  (An IBM
PC-AT with the same clock rate, and running plain MS-DOS, turns up
5.2 Nortons.)  If two nroffs are running under UNIX, the MS-DOS
performance is degraded to about 3.5 Nortons.  I find this trade-off
acceptable.

I wonder why OS/2 tries so hard to do it the hard way?

-- 
Dave Levenson
Westmark, Inc.		A node for news.
Warren, NJ USA
{rutgers | clyde | mtune | ihnp4}!westmark!dave