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From: gotwols@warper.jhuapl.edu (Bruce Gotwols)
Newsgroups: comp.sys.ibm.pc
Subject: NMI on EGA and VGA display cards
Message-ID: <4781@aplcen.apl.jhu.edu>
Date: 24 Feb 90 05:21:57 GMT
Sender: news@aplcen.apl.jhu.edu
Reply-To: gotwols@warper.jhuapl.edu (Bruce Gotwols)
Organization: JHU-Applied Physics Laboratory
Lines: 25

I recently acquired an analog to digital (A/D) board which uses DMA to transfer
the digitized data to an IBM AT.  In a short note in the instruction manual
it warned that data may be sporadically lost on machines with either EGA or VGA
screens if the user writes to the screen while DMA is in progress.  The claim is
that the act of writing to the screen (on EGA and CGA only) involves using the
processors Non Maskable Interrupt (NMI), and since the NMI has higher
priority than DMA it may keep things tied up long enough that the next A/D
sample comes in before the last one is safely stored in memory.

Can anyone out there confirm that writing to the EGA and VGA screens involves
using the NMI?  If this is true, under what conditions is the NMI used?  
(Surely it isn't necessary to invoke the Non Maskable Interrupt just to write a 
character to the screen?)

I would be interested to hear of any other experiences people have had using 
DMA to transfer A/D data to memory.  I'm particularly interested in experiences 
using the higher numbered DMA channels (>3) that are available on the AT but 
not the original PC or PC/XT.  
				Thanks,  Bruce Gotwols 
 

--
Bruce L. Gotwols
Johns Hopkins University, Applied Physics Lab., Laurel MD 20723
Internet:  gotwols@warper.jhuapl.edu   (128.244.176.48)