Date: Thu, 27 Apr 1995 09:47:00 +0500
From: ld AT netrix DOT com
To: s709 AT icslab DOT agh DOT edu DOT pl
Subject: Re: more on extended memory
Cc: djgpp AT sun DOT soe DOT clarkson DOT edu


> From djgpp-bounces AT sun DOT soe DOT clarkson DOT edu Thu Apr 27 09:29 EDT 1995
> Address: Mickiewicza 30, 30-059 Krakow, POLAND
> Date: Thu, 27 Apr 95 13:05:44 +0200
> From: s709 AT icslab DOT agh DOT edu DOT pl (chojnacki)
> To: djgpp AT sun DOT soe DOT clarkson DOT edu
> Subject: more on extended memory
> 
[snip]
> and I got:  i = 0x38
> 
> I think this is an index in the descriptor table which includes
                     ^^^^^
                     Yes, it's an index, but later, you use it as a pointer...
> among other things base addresses of the segments and their length.
> 
> Next I wanted to view contents of item in the descriptor table and ran this:
> 
> #include <stdio.h>
> #include <go32.h>
> main()
> {
> u_short i = _go32_conventional_mem_selector();
> printf("value = %x\n", *((int *)(int)i ));
                          ^^^^^        ^^^^
     This makes a pointer to 0x38      This is the 0x38th index into a table
                                       (which is NOT at 0x00000000)
> }
> 
> and I got:
> Segmentation violation in pointer 0x00000038 at d8:11ca ...

  This is an invalid address (0x38) 

> I think this area of memory is protected but is there any
> way to access the descriptor table to see what is real segment's
> base address and its length ?

  Look at the module perform.c in Ladybug. It has code to decipher an interrupt
entry. The code to lookup a segment will be pretty much the same, with some
minor changes to the structure. (Intel manual is recommended.)

> I understand that to access first 16M memory in DPMI mode (and also
> in non-DPMI) I can use _go32_conventional_mem_selector()
> which gives me a selector which is used as pointer to an item
                                     ^^^^^^^^^^^^^^^
                                     It's used as an index to a table. The base
address of this table can be found by the instruction:
  SGDT (If your program is using the global descriptor table, like under VCPI)
  SLDT (If your program is using a local descriptor table, like under DPMI)

To see if your program is using GDT or LDT, check the CR0 register.

> in the desriptor table from which I get base address of my segment.
> But for example if I have 32M memory - can't I access second half of 16M
> in DPMI without hand coding (only 16M of memory gets mapped to
> _go32_conventional_mem_selector() ?).

Hope that helps,
Long.