Path: utzoo!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!uwm.edu!zaphod.mps.ohio-state.edu!think!mintaka!chaos.cs.brandeis.edu!topgun
From: topgun@chaos.cs.brandeis.edu (Chandra Bajpai)
Newsgroups: comp.sys.ibm.pc
Subject: Re: NMI on EGA and VGA display cards
Message-ID: <1990Feb28.194455.15889@chaos.cs.brandeis.edu>
Date: 28 Feb 90 19:44:55 GMT
References: <4781@aplcen.apl.jhu.edu>
Organization: Brandeis University Computer Science Dept
Lines: 14

In article <4781@aplcen.apl.jhu.edu> gotwols@warper.jhuapl.edu (Bruce Gotwols) writes:
>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 

I read a Chips & Tech application note that said that they use NMI
to signal a program is writing to video controller addresses and switch
mode modes automatically (It doesn't happen by magic!)