Path: utzoo!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!usc!snorkelwacker!mintaka!ogicse!decwrl!amdcad!pepsi!phil
From: phil@pepsi.amd.com (Phil Ngai)
Newsgroups: comp.sys.ibm.pc
Subject: Re: NMI on EGA and VGA display cards
Message-ID: <29295@amdcad.AMD.COM>
Date: 26 Feb 90 20:41:08 GMT
References: <4781@aplcen.apl.jhu.edu>
Sender: news@amdcad.AMD.COM
Reply-To: phil@pepsi.AMD.COM (Phil Ngai)
Organization: Advanced Micro Devices, Inc. Sunnyvale CA
Lines: 13

The NMI situation on VGA adaptors is this: many cards do not
provide full hardware support for backwards compatibility.
Instead, they detect if a CGA type access is done and
put out an NMI. They then emulate the access using VGA
BIOS code. This is why NMI is used. If you are only doing
VGA access, NMI should not (I think) get asserted.

I don't know what happens if you do a CGA access on an
EGA adaptor.

--
Phil Ngai, phil@amd.com		{uunet,decwrl,ucbvax}!amdcad!phil
When guns are outlawed, only governments will have guns.