Date: Mon, 10 Apr 2000 19:45:23 +0600 (LKT)
From: Kalum Somaratna aka Grendel <kalum AT lintux DOT cx>
X-Sender: root AT darkstar DOT grendel DOT net
To: djgpp AT delorie DOT com
Subject: Re: HARDWARE INTERRUPT HANDLING BY CWSDPMI
In-Reply-To: <Pine.SUN.3.91.1000410084111.19649G@is>
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On Mon, 10 Apr 2000, Eli Zaretskii wrote:

> 
> On Sun, 9 Apr 2000, Kalum Somaratna aka Grendel wrote:
> 
> > On Sun, 9 Apr 2000, Eli Zaretskii wrote:
> > 
> > > A DJGPP program indeed incurs additional overhead, because under DPMI,
> > > all hardware interrupts are reflected to protected-mode handlers
> > > first, and only if unhandled, they are passed to real-mode handlers.
> > > The mode switch that this involes takes up hundreds of CPU cycles.
> > > 
> > 
> > In such a case like handling com port interrupts, installing *both* a real
> > mode and a protected mode handler should avoid this overhead.
> 
> No, it won't, at least not with all DPMI providers.
> 
> A real-mode handler helps to avoid the overhead of the reflection from
> real to protected mode, which happens when the interrupt is raised
> while the CPU is in real mode (e.g., inside a DOS call), while the
> handler is a protected-mode one.
> 
> However, in this case, we have the opposite situation: the handler is
> a real-mode one (a real-mode TSR), while the CPU stays in protected
> mode most of the time because a DJGPP program runs in the foreground.
> A real-mode handler worn't help here, since most DPMI servers will
> call the protected-mode handler first, and only then reflect to the
> real mode.

Hmm.... I was commenting on a generic protected mode program that has to
do com port handling.

Would it be possible then for him to rewrite the 16-bit TSR as a
protected mode TSR  with a real mode handler too..then wouldn't that avoid
the problem...

Grendel

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