Path: utzoo!utgpu!watmath!clyde!att!rutgers!deimos!uxc!uxc.cso.uiuc.edu!uxg.cso.uiuc.edu!uxe.cso.uiuc.edu!mcdonald
From: mcdonald@uxe.cso.uiuc.edu
Newsgroups: comp.sys.ibm.pc
Subject: Re: *GA and TC
Message-ID: <45900173@uxe.cso.uiuc.edu>
Date: 1 Dec 88 14:37:00 GMT
References: <422@babbage.acc.virginia.edu>
Lines: 16
Nf-ID: #R:babbage.acc.virginia.edu:422:uxe.cso.uiuc.edu:45900173:000:666
Nf-From: uxe.cso.uiuc.edu!mcdonald    Dec  1 08:37:00 1988



>VGA is backwards compatible with EGA (i.e. supports EGA modes).  However,
>I don't think this means that you can hook up an EGA only monitor to a 
>VGA card (or a VGA monitor to an EGA card)  : not referring to the multi-
>frequency monitors.

VGA is NOT - repeat - NOT - entirely compatible at the programming
level with the EGA. It has one important gotcha. There is a bit,
the high order bit (128's bit) of the vertical retrace end register,
which is at 0x3d5 after 0x11 is sent to 0x3d4, which if set disables
writing to a whole bunch of other registers.

You have to clear this bit before reprogramming the VGA.

Otherwise, it seems to emulate the EGA fine.