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From: aaron@jessica.Stanford.EDU (Aaron Wallace)
Newsgroups: comp.windows.ms
Subject: Differences between Standard and Enhanced modes
Message-ID: <1990Aug30.175103.7499@portia.Stanford.EDU>
Date: 30 Aug 90 17:51:03 GMT
Sender: Aaron Wallace
Reply-To: aaron@jessica.Stanford.EDU (Aaron Wallace)
Organization: Stanford University
Lines: 34


I'm working on a low-level DLL that has to accelerate the system clock (INT 8h)
to 22 KHz for a short time.  Basically, the DLL redirects the INT 8H interrupt
(using DOS call 25h), reprograms the appropriate timer channel, runs for a bit,
restores the timer frequency and interrupt and exits.  The interrupt handler
is contained within the DLL's code segment.

This works wonderfully under Standard mode--and it may be worth mentioning that
the interrupt routine is run in protected mode, so accessing a lot of memory,
etc, doesn't require GlobalDOSAlloc and such.

Under 386 Enhanced mode, it also works--kindof.  The clock runs at a much
slower pace than 22 KHz.  I assume that this has to do with running in a 
virtual machine, but it could also be that the interrupt routine is just run-
ning too slowly for 22 KHz and thus interrupts are being dropped.  My
question: is there a bug in the virtual timer that prevents accelerating the
clock to this speed (it *does* run faster than the normal 18.xx Hz.), or
does it take too long for the virtual I/O handlers to run at this speed.
I should mention that the interrupt routine does some very simple I/O
(two port outputs to reset the timer countdown).

Further (unrelated?) question:

In Standard mode, within the interrupt handler I can access BP-relative
parameters and locals with no problem (the BP is not changed by me ever).
In Enhanced mode, accessing such variables results in an Unrecoverable
System Error (*very* unrecoverable since almost all interrupts are masked
off).  Does Enhanced mode run interrupt handlers from a different stack,
a different stack frame, or what?

I'd hate to have to write a DOS-level driver for this since it seems to
*almost* work as a simple DLL.  Maybe I'll just live with Standard mode :-(.

Aaron Wallace