Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!purdue!haven!umd5!cgs
From: cgs@umd5.umd.edu (Chris Sylvain)
Newsgroups: comp.sys.ibm.pc
Subject: Re: EXTENDED memory and INTERRUPTS, problems ?
Keywords: EXTENDED, INTERRUPTS, BIOS
Message-ID: <4529@umd5.umd.edu>
Date: 15 Feb 89 14:16:10 GMT
References: <11702@ihlpa.ATT.COM>
Reply-To: cgs@umd5.umd.edu (Chris Sylvain)
Organization: University of Maryland, College Park
Lines: 29

In article <11702@ihlpa.ATT.COM> wolfordj@ihlpa.ATT.COM (452is-Wolford) writes:
< QUESTION: Is there a problem with using EXTENDED memory and
< a communications program or any other program.
< 
<  [...]
< Is there a problem and why is it a problem ?

It is a potential problem caused by the fact that using the BIOS service 15h
to move a 64kbyte block (maximum) into extended memory will disable
interrupts for 1 msec, measured on a 8 MHz IBM-PC/AT. The return to real
mode itself takes 400 microsec.

The BIOS service itself disables all interrupts, rather than deciding for
the user which interrupts are OK in protected mode, and which are not.
It is said that to use the BIOS 15h service, one has already done nearly
all the work to skip using the BIOS altogether and enable those interrupts
which are desired to be serviced in protected mode.

My source is _The IBM-PC from the Inside Out/Includes the IBM-PC/AT_ by
Sargeant and Shoemaker III, pub. by Addison and Wesley.

---> To further examine the problem, I would like to know what the expected
---> frequency of interrupts is for the AT I/O chip (the 16450?) and the
---> XT I/O chip (8450B?) when operated at 9600 baud.
-- 
--==---==---==--
.. He chortled in his joy. ..
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