From: stow AT skyfox DOT usask DOT ca
Newsgroups: comp.lang.asm.x86,comp.os.msdos.djgpp,comp.os.msdos.programmer,comp.lang.pascal.borland,alt.msdos.programmer
Subject: RE: Preemptive Multitasking under DPMI
Date: 28 OCT 96 22:08:36 GMT
Organization: University of Saskatchewan
Lines: 24
Message-ID: <28OCT96.22083606@skyfox.usask.ca>
References: <5538hu$2ue AT news DOT latnet DOT lv>
NNTP-Posting-Host: sask.usask.ca
To: djgpp AT delorie DOT com
DJ-Gateway: from newsgroup comp.os.msdos.djgpp

In a previous article, nobody AT nowhere wrote:
>We are trying to understand reentrancy issues related to preemptive 
>multitasking under DPMI.  We've read the DPMI specs carefully, but things are 
>not completely clear. We can see three multitasking schemes:

Lotsa luck getting preemptive multi-tasking under DOS, Windows, or Win95.
About the best you can hope for is cooperative multitasking.  For 
pre-emptive multi-tasking you are going to have to use OS/2, NT, some
variant of UNIX, NextStep, etc.  

Preemptive M.T. requires that the operating system (including extensions
to the OS, such as when you run Borlands DPMI interface) be able to stop
one process when the _OS_ decides it is a good time, and give a few
time slices to another process.  DOS, Win 3.x don't have that capability
at all.  Win95 is _supposed_ to, but fails miserably.   Instead those
OSes sit and wait for an app to "cooperate" by saying "This is a good
place for me to pause if you have someone else waiting for a little time."
A good example of the failure of all of those OSes to implement pre-emptive
multitasking is that once you start a floppy formatting you are shut out from 
doing anything else until the format is done, even though formatting a floppy
is a relatively cpu un-intensive task:  the cpu has lots of time to spare
for other things but the format program won't cooperate.  Formatting a 
floppy while running OS/2 doesn't slow anything else down noticeably because
the formatting requires so little attention from the cpu.