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From: dj@ctron.com (DJ Delorie)
Newsgroups: comp.os.msdos.programmer
Subject: Re: Ms-Dos Source
Message-ID: <1434@balrog.ctron.com>
Date: 19 Apr 91 18:34:04 GMT
References: <1753@sjfc.UUCP> <91107.143308IO92203@MAINE.BITNET> <1991Apr19.150904.6241@netcom.COM>
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Reply-To: dj@ctron.com
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Lines: 36

> >   What
> >type of computer are you trying to multitask? Anything below a '386
> >doesn't allow multitasking anyway. It would be nearly impossible to make
> >DOS capable of multitasking anyway.
> > 
 
> You don't need an 80386 to multi-task. In fact, you don't need a chip
> designed for multi-tasking to multi-task. True preemptive tasking can
> be done (slowly) on an 8086. Although MS DOS doesn't inherently
> support multi-tasking, there are things which can be done to make it
> multi-task as well.

Side note (ie: no flames, please):

The 80386 is the first chip to support "true" multitasking.  This
means that it actually has a task register, task management opcodes
and hardware, and each process thinks it's the only one.  When the CPU
needs to handle exceptions (interrupts, system calls, faults), the
hardware *automatically* saves the ENTIRE state of the CPU and current
task in that task's Task State Segment and loads the ENTIRE state of
the new task to run.  This is normally done with no software
interaction at all.  This is true multitasking.

On lesser processors, "simulated" multitasking is used.  There is only
one processor state, and when you want to run a different task, the
currently executing task must, in it's own context, manually reload
all of the registers, stack, etc, and become the new task.  Interrupts
and exceptions are always serviced in the context of the running
program.

One might argue that the same thing is happening in both cases.  It
is.  The difference is that for the 386, it happens automatically, and
for lesser processors it must be done in software.

DJ
dj@ctron.com