Path: utzoo!utgpu!attcan!uunet!cbmvax!daveh
From: daveh@cbmvax.UUCP (Dave Haynie)
Newsgroups: comp.sys.amiga.tech
Subject: Re: BridgeBoard & VGA problems
Message-ID: <4647@cbmvax.UUCP>
Date: 2 Sep 88 21:51:13 GMT
References: <1760001@hpccc.HP.COM>
Organization: Commodore Technology, West Chester, PA
Lines: 35

in article <1760001@hpccc.HP.COM>, baryeka@hpccc.HP.COM (Jarek Baryeka) says:
> 
> I have recently bought a VGA card and use it with the XT BridgeBoard.
> It works OK in EGA, CGA and other modes but I have some problems with
> high resolution VGA graphics modes - the PC screen freezes and I cannot
> access my hard disc card from amiga side anymore. I think that maybe
> the video memory (512KB on the VGA card) addresses collide with Janus 
> dual port memory but I have no documentation on that.

I should try to get one of our PC folks to comment on the tricks of VGA.
But what you just said sounds impossible.  At least as I understand it,
the PC-XT, being 8088 based, is limited to a total of 1 meg normal address
space, plus 64K or so I/O space.  The MDA, CGA, EGA, and the various other
cards sit up in higher memory and by default must take less than the space
that's left over in the PC.  If VGA really takes up 512K of directly
addressable memory, it must either be unsuitable to an XT compatible, or
use some generally unused addressing mode, like the 16 bit I/O addressing
that the Z-80 can do, but generally doesn't do.

> Can some jumpers on the bridge change that address ?

There are jumpers on the bridge card to turn off it's CGA or MDA emulation;
the manual should explain this.  

> All hints welcome.

This is all a prime example of what's broken on PC[lone] machines....

> Thanks.

>   Jarek Baryeka.
-- 
Dave Haynie  "The 32 Bit Guy"     Commodore-Amiga  "The Crew That Never Rests"
   {ihnp4|uunet|rutgers}!cbmvax!daveh      PLINK: D-DAVE H     BIX: hazy
		"I can't relax, 'cause I'm a Boinger!"