Message-Id: <199605151407.KAA08436@eelab.newpaltz.edu>
Comments: Authenticated sender is <fortin44 AT eelab DOT newpaltz DOT edu>
From: "John Fortin" <fortin44 AT eelab DOT newpaltz DOT edu>
To: djgpp AT delorie DOT com
Date:          Wed, 15 May 1996 10:09:40 +0000
Subject:       Re: 0xa0000 and dpmi_segment_to_descriptor

> 
> On Tue, 14 May 1996, John Fortin wrote:
> 
> > mail archives in order to understand why we must use near or far pointers
> > to gain access to the video buffer.  
> 
> Far pointers are required to access portions of memory that aren't in the 
> application's address space.  Near pointer method just makes all the 
> memory be inside your address space, which is bad memory-protection-wise, 
> but required if you need the speediest access.  Without these methods, 
> you'd get GPF if you try to access those addresses.
> 
> > I don't understand why we can't use a function such as 
> > dpmi_segment_to_descriptor to map this to a linear address without
> > the disadvantages of the 'fat ds' method.  What am I missing here??
> 
> They get linear address, yes, but is that linear address inside your 
> program's address space?  I think not.
> 
>

 Then I think the example in the info page for 
__dpmi_segment_to_descriptor is misleading.  ( 
video=__dpmi_segment_to_descriptor(0xa000).  This looks like it gives 
you a mapping to video memory.



John E. Fortin
fortin44 AT eelab DOT newpaltz DOT edu