Path: utzoo!utgpu!news-server.csri.toronto.edu!helios.physics.utoronto.ca!ists!yunexus!xrtll!silver
From: silver@xrtll.uucp (Hi Ho Silver)
Newsgroups: comp.os.msdos.misc
Subject: Re: Multitasking?
Message-ID: <1990Dec22.193205.6537@xrtll.uucp>
Date: 22 Dec 90 19:32:05 GMT
References: <35.2771BC36@bohemia.metronet.org>
Reply-To: silver@xrtll.UUCP (Hi Ho Silver)
Organization: Not around here, pal!
Lines: 74

Greg, have you ever considered putting linefeeds at the end of your lines?

In article <35.2771BC36@bohemia.metronet.org> Greg.Smith@p11.f477.n104.z1.METRONET.ORG (Greg Smith) writes:
$In a message to All <18 Dec 90 01:47> Bruce Nelson wrote:
$ BN> Is there anyone in netland that might be able to tell me what
$ BN> "true" multitasking is, which microproccessors (8088, 80286,
$ BN> 80386sx, 80386, 80486) are able to multitask, and what type of
$ BN> software allows multitasking.
$True multitasking can only be achieved on the 80386 and up (486, soon 586
$etc..)  There is a virtual mode which allows the 386 to act like 4 8088s,
$and on the 486, I believe this mode can be extended even further.  To use
$these modes, OS/2 is a good operating system for it, or DESQview '386,
$Windows 3.0 '386.  These are the most common multitaskers that I know of.  

   Yes and no.  You can multitask on basically any processor; however, trying
to do it on a CPU which lacks support for it is far more difficult to do
and if the CPU lacks memory protection (e.g. the 8088), a poorly-written or
malicious program can easily take the whole system down.

   The 80286 has two modes.  Real mode basically simulates an 8086, while
protected mode is designed for multitasking.  It provides memory protection
so that each program can be prevented from doing anything antisocial (such
as clobbering someone else's memory, doing I/O, etc.)  It also provides
hardware support for switching between tasks.  Real mode on the 286 was
never really designed to be a fast 8086 (keep in mind that the 286 was
designed before the PC came along); rather, it was designed to provide a
way of initializing a multitasking system before jumping into protected
mode.  The 80286 doesn't provide an instruction to return from protected
mode to real mode; instead you have to reset the CPU.

   The 80386, in addition to the two modes above, adds two more:  virtual
8086, which allows any number (not just 4) of processes to be multitasked
while each thinks it's running on an 8086, plus another protected mode
that uses the features of the 80386.  In addition to the protection and
multitasking that came with the 80286, the 80386 also adds support for
virtual memory.  VM allows the computer to think it has more memory than
it really has.  The extra memory is actually the hard drive; areas of
memory which are not currently needed are swapped out to the hard drive,
freeing up main memory for other programs to use.  This happens trans-
parently to the application; the only difference is that obviously the
more you have to access the hard drive, the slower your program will run.

   The 80386SX is identical internally to the 80386DX.  It runs all the
same software and provides all the same operating modes.  The difference
is that externally, the SX has only a 16-bit wide data bus (as opposed
to 32 bits on the DX) and that it can't address as much physical memory.
The 80486 is basically an 80386, an 80387 and an 8K cache rolled into one
chip, with some fine-tuning done as well.

   As for operating systems ... DOS is purely an 8086 operating system.  The
only way it runs on higher machines is in modes which simulate an 8086.
The eventually-to-be-released DOS 5 will be aware of the 80286 and 80386
and will use those features to decrease the amount of conventional memory
(your usual 640K) it eats up, but other than that it's still an 8086 O/S.
The most common operating systems for 80286 machines are OS/2 version 1.xx
and Xenix 286; Coherent is also an 80286 operating system.  You can run a
single DOS task in the foreground in OS/2's DOS compatibility box; enough
said about that.  For the 80386, you can get numerous Unix implementations,
plus the eventually-to-be-released OS/2 2.0.  This will be an 80386 operating
system, including the ability to use more than the 16M of memory available
on the 80286 and a greatly improved DOS compatibility box (thanks the the
virtual 8086 mode, it will be able to be multitasked, run in the background,
etc.)  You can also get products for many 386 Unix-type systems that provide
the ability to run DOS programs as tasks under Unix.

   Other multitasking solutions are available which are based, in one way
or another, on DOS.  Windows and Desqview both run on DOS machines.  There
are also programs such as PC-MOS which allow you to run multiple DOS
sessions on a machine.
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