From:	"Bruce Hoyt" <bhoyt AT voyager DOT co DOT nz>
Newsgroups:	comp.os.msdos.djgpp
Subject:	Linear addressing using brk() and sbrk()
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Message-ID:	<ehYc3.1130$Dl.33483@news.clear.net.nz>
Date:	Sat, 26 Jun 1999 15:58:43 +1200
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To:	djgpp AT delorie DOT com
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Reply-To:	djgpp AT delorie DOT com

I am writing a loader for a Forth interpreter. The loader will
provide the system functions I need (memory allocation, file
handling, console operations, etc.). The loader will read a
binary image containing the Forth interpreter and hand control
over to it. I desire this loader to be as portable as possible
therefore I am using GCC (DJGPP) and so hope to be able to port
my Forth interpreter easily between Linux, Win98, DOS, and
possibly other systems.

I want to be able to specify a fixed memory location at which I
will place the binary image so that the Forth's internal
addressing can be done with absolute (linear) addresses. I know
this can be done using DPMI virtual addressing. It can also be
done in Linux and Win98 using specific system calls.

Question 1: Is there a way to do it using malloc? I think not.

Question 2: Is there a way to place a separate heap at a fixed
(linear) address using brk or sbrk?

Question 3: Is it very bad to do the following?

    unsigned desired_address = 0x100000;
    unsigned desired_size = 0x80000;
    if(sbrk((void *)desired_address - sbrk(0)) == (void *)-1) {
       printf("Can't adjust brk to %X\n", desired_address);
       return memory_error1;
    }
    if(sbrk(desired_size) == (void *)-1) {
        printf("Can't allocate %X bytes for image\n",
desired_size);
        return memory_error2;
    }

And what happens when I malloc some more memory? How do I keep
the heap from overlapping desired_address ?

Thanks for any help.

Bruce Hoyt

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