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From: hwb@texnix.stgt.sub.org (Harald Boegeholz)
Newsgroups: comp.sys.ibm.pc.misc
Subject: Re: Need Info: Slow I/O in protected mode
Summary: IN AX,DX slower in virtual 8086 mode
Keywords: virtual 8086 mode, 80386
Message-ID: <56786.901017@texnix.stgt.sub.org>
Date: 17 Oct 90 21:28:05 GMT
Reply-To: hwb@texnix.stgt.sub.org
Distribution: comp
Lines: 32

In article <4925@tuminfo1.lan.informatik.tu-muenchen.dbp.de>
lenz@charly.bl.physik.tu-muenchen.de (Udo Lenz) writes:
> ... I've found out,
> that the 'in ax,dx' command takes roughly a microsecond longer if I
> have installed Quarterdesks QEMM memory manager. The same delay
> appears, if I run it under Windows 3.0 in the 386 mode.
[...]
> The assembly reference manual says though, that the timing
> of the 'in ax,dx' command in protected mode is as fast as in real mode,
> except if encurring a protection fault.
Mine says something different:
IN AX,DX:  13   clocks in real mode
           7/27 clocks in protected mode (depending on CPL/IOPL)
    ****** 27   clocks in *Virtual 8086 Mode* ************
("Programming the 80386, John H. Crawford, Patrick P. Gelsinger, SYBEX 1987")

> What is the reason for the delay in the 'in ax,dx' command when using
> QEMM?
see below 

> Am I really in protected mode when installing QEMM as a device driver
> in config.sys?
No. You are (i.e., your processor is :-) ) in virtual 8086 mode.
        
> If in protected mode, why do my old assembly routines, which do segment
> register arithmetic and other dirty things, still run?
Well, you're not, and in virtual 8086 mode all dirty real mode tricks
work, as the name suggests.

Hope this helps
--
Harald Boegeholz    (hwb@texnix.stgt.sub.org)