Date:	Wed, 13 Nov 1996 16:09:21 GMT
From:	kvhk AT ivs4 DOT barco DOT com (Koen Van Herck)
Message-Id:	<9611131609.AA20023@ivs4.>
To:	djgpp AT delorie DOT com
Subject:	Accessing memory above 1MB
Reply-To:	Koen DOT VanHerck AT barco DOT com

I have a question about answer 18.7 in the FAQ:

18.7 Accessing absolute address above 1MB

**Q*: How can I access memory-mapped peripheral devices (or any other
absolute address) above 1 MByte mark?*

*A* :  Use the `__dpmi_physical_address_mapping' library function.  
...
If you prefer to create the mapping yourself,
these are the DPMI calls that you will have to use:

   - allocate an LDT descriptor (Int 31h/AX=0);
   - map selector to physical address (Int 31h/AX=0800h);
   - lock linear address (Int 31h/AX=0600h);
   - set segment base address (Int 31h/AX=7);
   - set segment limit (Int 31h/AX=8).

Now, what is meant exactly with "doing it yourself" ? Does this mean
that I use either __dpmi_physical_address_mapping or the five points above.

If so, then how do I do point 2 ? Isn't this what
__dpmi_physical_address_mapping does ?

Or is __dpmi_physical_address_mapping equivalent to all 5 points together?

As an illustration, I have include my memmap() function, which I use
to map a 32MB chunk on a PCI device. Please send me your comments on
this code.

/* memmap map's a chunk of memory */
/* returns -1 on error, else descriptor */
   
int memmap(unsigned int base, unsigned int size)
{
  int desc;
  __dpmi_meminfo meminfo;

  
  desc = __dpmi_allocate_ldt_descriptors(1);
  if (desc==-1)
  {
    fprintf(stderr, "ERROR: Unable to allocate descriptor\n");
    return -1;
  }

  meminfo.address = base;
  meminfo.size = size;

  if (__dpmi_physical_address_mapping(&meminfo))
  {
    fprintf(stderr, "ERROR: Unable to map physical address\n");
    return -1;
  }

  if (__dpmi_lock_linear_region(&meminfo))
  {
    fprintf(stderr, "ERROR: Unable to lock linear region\n");
    /* Don't return when this gives an error ! */
  }
    
  if (__dpmi_set_segment_base_address(desc, meminfo.address))
  {
    fprintf(stderr, "ERROR: Unable to set segment base\n");
    return -1;
  }

  if (__dpmi_set_segment_limit(desc, meminfo.size - 1))
  {
    fprintf(stderr, "ERROR: Unable to set segment limit\n");
    return -1;
  }

  return desc;
}

----
Koen Van Herck
Electronic Design Engineer
E-mail: Koen DOT VanHerck AT barco DOT com

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