Date: Thu, 29 Sep 1994 10:37:16 +22305931 (WAUST)
From: Ron Grunwald <rgrunwal AT bluering DOT cowan DOT edu DOT au>
Subject: Re: Physical memory access
To: djgpp AT sun DOT soe DOT clarkson DOT edu

Hi all,


A posting was put on the list recently regarding a problem accessing
physical memory with DJGPP. Unfortunately I deleted that message
and now I can't remember the author's name. Anyway, the problem was
that the following code generated a "segmentation violation" during
compilation:


   unsigned char  *mem;

   mem = 0x00400000;
   *(mem) = 0x4F;

I've tried it and it seems to compile fine under GCC V2.6.0.

The problem is running this code portion, which indeed generates
a segmentation violation.

You have to remember that in protected mode you cannot simply
access any memory location at random. If you require access to a
particular physical memory location, you have to create a descriptor
for a 16- or 32-bit segment containing this address. The memory addr.
is then accessed by specifying SELECTOR:OFFSET.

The selector is a 16-bit quantity that enables the 80386 to access
the descriptor for the defined memory segment.


Regards, Ron.




 ********************************************************************
 | Author.............. Ron Grunwald                                | 
 | Internet............ rgrunwal AT bluering DOT cowan DOT edu DOT au              | 
 | Phone............... (09)273 8027 or (09)273 8468                |
 |------------------------------------------------------------------|
 | Department.......... Information Technology                      | 
 | Division/Faculty.... Operations and Systems Management           | 
 | Institute........... Edith Cowan University, Churchlands         | 
 | Location............ Perth, Western Australia                    |
 ********************************************************************
 "I don't have any solution but I certainly admire the problem!"