From: nobody AT nowhere
Newsgroups: comp.lang.asm.x86,comp.os.msdos.djgpp,comp.os.msdos.programmer,comp.lang.pascal.borland,alt.msdos.programmer
Subject: Preemptive Multitasking under DPMI
Date: Mon, 28 Oct 96 17:37:08 GMT
Organization: LatNet news site
Lines: 29
Message-ID: <5538hu$2ue@news.latnet.lv>
NNTP-Posting-Host: 159.148.61.19
To: djgpp AT delorie DOT com
DJ-Gateway: from newsgroup comp.os.msdos.djgpp

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:

1) BETWEEN DPMI VIRTUAL MACHINES. 
This appears to be the way multitasking is intended in the DPMI specs.  This 
is most likely how multitasking is done in Multiuser DOS schemes.  We don't 
like this, because each multitasking object would have to have it's own copy 
of DOS, interrupts, etc.  We are dealing with a real-time application were 
many threads will be alive at any time.

2) BETWEEN DPMI CLIENTS WITHIN A SINGLE VIRTUAL MACHINE.
The DPMI specs clearly states that the only way to multitask clients is by 
passing the time-slice which means non-preemptive scheduling, so that doesn't 
appear as an option.

3) WITHIN A DPMI CLIENT BY A KERNEL LOCATED OUTSIDE THE DPMI HOST.
This is what we want. Unfortunately the DPMI host doesn't know about context 
switching between tasks and it's not supported in the DPMI specs.  The DPMI 
specs states that the host maintains *one* global host stack which makes it 
not reentrant.   What we want to achieve does not seem possible without a 
reentrant DPMI host.   Could we get some comments on this.

Is the lack of reentrancy something inherent to all DPMI hosts, or are there 
DPMI host implementations which are reentrant.  Is there source code available 
for DPMI server implementations which we might be able to modify to make it 
reentrant?

Any help would be greatly appreciated.