Path: utzoo!utgpu!news-server.csri.toronto.edu!mailrus!cs.utexas.edu!usc!julius.cs.uiuc.edu!apple!vsi1!octopus!stever
From: stever@Octopus.COM (Steve Resnick )
Newsgroups: comp.os.msdos.programmer
Subject: Re: point to an address above 1M ?
Message-ID: <1990Sep24.204400.26052@Octopus.COM>
Date: 24 Sep 90 20:44:00 GMT
References: <14016@hydra.gatech.EDU>
Reply-To: steve.resnick@f105.n143.z1.FIDONET.ORG
Distribution: usa
Organization: Octopus Enterprises, Cupertino CA
Lines: 31

In article <14016@hydra.gatech.EDU> gb7@prism.gatech.EDU (Joe Bradley) writes:
>
>Can someone tell me how to set a pointer (in MS C) to an address above
>1M? I have a video card which maps its video RAM into the PC address
>space from 8M - 10M. I want to access video RAM directly, but I can't
>initialize a pointer to this address range presumably because I only
>have 16 bits of addressability with the segment portion of a huge pointer.
>I don't see how you can point to a physical address above FFFF:FFFF ?!
>I tried the following, but the compiler gives the error: "Constant too big."
>
>#define VIDEO_BASE 0x800000000
>
>main() {
>    unsigned char huge *ram_p;
>
>    ram_p = ( unsigned char huge * )VIDEO_BASE;
>}

You can't do that without entering protected mode, or by some "hidden" 
paging hardware on the video card. The FFFF:FFFF address is the top limit
on a far pointer. That address cannot be reached, normally, unless the A20
line is activated. 

Hope this helps....
Steve

-- 
----------------------------------------------------------------------------
steve.resnick@f105.n143.z1@FIDONET.ORG #include<std_disclaimer.h>
Flames, grammar errors, spelling errrors >/dev/nul
----------------------------------------------------------------------------