dave@westmark.UUCP (Dave Levenson) (09/14/87)
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