Message-Id: <199703041954.NAA143876@audumla.students.wisc.edu>
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Date: Tue, 04 Mar 1997 13:55:26 -0600
To: djgpp AT delorie DOT com
From: Scott Fleischman <safleisc AT students DOT wisc DOT edu>
Subject: Re: __djgpp_base_address

>
>On Mon, 3 Mar 1997, Scott Fleischman wrote:
>
>> I am wondering why this code works fine in the Win95 Dos box, yet doesn't
>> work under Win95 msdos mode w/ cwsdpmi or in dos 6.22 w/ qemm unless I
>> change the line:
>>         info.address = (long) addr + __djgpp_base_address;
>> to
>>         info.address = (long) addr - __djgpp_base_address;
>> (or "... + __djgpp_conventional_base" since they are equivalent.)
>
>Why, because that's how you should use nearptrs!  The correct way is
>to add the `__djgpp_conventional_base', NOT `__djgpp_base_address' (it
>is documented that way in the libc reference).  You should never mess
>with `__djgpp_base_address' anyway.
>
>And btw, you should take the value of `__djgpp_conventional_base'
>immediately after a call to `__djgpp_nearptr_enable'.  Your code calls
>`sbrk' between these two, which is unsafe (`sbrk' will sometimes move
>the base address).
>

Thank you for your reply but I diagree with never using __djgpp_base_address,
and in this case it is the right thing to use.  If I use
__djgpp_conventional_base it doesn't work.  But if instead I wanted
something like the address for 0xa0000 I would add __djgpp_conventional_base
to it.
Anyway, I got it to work right and here is the code (just in case you wish
to see it).



#include <dpmi.h>
#include <sys/nearptr.h>
#include <stdio.h>

int main()
{
    int i, j, checksum;
    __dpmi_meminfo info;

    info.size = (__djgpp_selector_limit + 1) - 4096;
    info.address = __djgpp_base_address + 4096;

    if (__dpmi_lock_linear_region(&info))
    {
        printf ("Couldn't lock text and data\n");
        return 0;
    }

      /* touch the entire image */
    for (j=0 ; j<4 ; j++)
    {
        for(i=0x1000 ; i<(info.size - 16 * 0x1000) ; i += 4)
        {
            checksum += *(int *)i;
            checksum += *(int *)(i + 16 * 0x1000);
        }
    }
    printf("%d Kb locked.\n", info.size / 1024);
    return 0;
}