Organization: University of Mining and Metallurgy
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

>>> How can I access the particular addresses of extended memory (above 1MB) ?
>>> I know the first 1MB is mapped to 0xE0000000 in the program's address space
>>> but how is it with above 1MB memory ?
>>> I need to access the memory of DT2862 frame grabber which grabs images
>>> into extended memory from address 0xA00000.
>>> How compiler handles that memory ?
>>> Please help !

>>The entire 256M of memory is mapped to 0xe0000000 (unless you don't have
>>that much... :->). If you want to access 0xa00000, just use 0xe0a00000.
>>BTW, that will not work under DPMI, unless you do some assembly coding to
>>map the memory yourself.

>I think only 16M of memory gets mapped to go32_conventional_memory_selector()
>(sp?) [which returns 0xe0000000 if you aren't in DPMI, but a value which may
>change from session to session otherwise].  No hand coding should be necessary.

Thanks to all who responded to my please.
I'd like to ask some more questions about it.

In my opinion _go32_conventional_mem_selector() returns selector
which pointers to an item in the descriptor table which includes
base addresses of segments.

For example I tried this:

#include <stdio.h>
#include <go32.h>
main()
{
u_short i = _go32_conventional_mem_selector();
printf("i = 0x%x\n", (int)i);
}

and I got:  i = 0x38

I think this is an index in the descriptor table which includes
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 ));
}

and I got:
Segmentation violation in pointer 0x00000038 at d8:11ca ...

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 ?

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
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() ?).

If my understanding is not correct at any point
please let me know of it.

Thanks,
Tom Chojnacki (s709 AT icslab DOT agh DOT edu DOT pl)