From: "A. Sinan Unur" <asu1 AT cornell DOT edu>
Newsgroups: comp.os.msdos.djgpp
Subject: Re: how to convert real mode pointer into protected  mode ?
Date: Tue, 19 Aug 1997 08:54:55 -0400
Organization: Cornell University http://www.cornell.edu
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Wojciech Galazka wrote:
> 
> firewind wrote:
> >
> > I know very little about DOS stuff, having started with DJGPP. But, I do
> > know some. :)  Are you sure the _far* functions aren't what you want? For
> > example, here is the code to read the DOS InDOS flag:
> 
> Well, as I said <sys/farptr.h> probably cannot help. And why?
> Here's the problem description.
> 
> Upon return from a real mode ISR I get a pointer in ES:DI
> pointing to a structure in memory under 640 kB.
> The size of the structure is not known in advance so I
> cannot just call dosmemget to copy the struct into
> my own  one.
> The definition of the structure is something like
> 
> struct foo {
>    short size
>    struct {
>      short min
>      short max
>    } * minmax;  //holds a 'size' number of minmax structures
>  }
> 

don't think of this as a C structure. think of it as the layout of bytes
in memory. then you have:

WORD size   <--- pointed to by ES:DI
WORD off    offset
WORD seg    segment

where WORD stands for '16 bit integer'.

the code below (which will take some time to put into cmpilable form,
should do what you were doing with the for loop below.

unsigned addr1, addr2;
unsigned short size;
int i;

addr1 = ES << 4 +DI;
addr2 = _farpeekw(_dos_ds, addr1+2) + 16 * _farpeekw(_dos_ds, addr1+4);
size  = _farpeekw(_dos_ds, addr1); /* no of substructures */

while(size)
{
 printf("Min: %d, Max: %d\n", _farpeekw(_dos_ds, addr2) \
_farpeekw(dos_ds, addr2 + 2);
 addr2 += 4;
 size--;
}

now, you might want to transfer this stuff to virtual memory. in that
case, you _can_ use movedata or dosmemget/put function because at the
time you get the real mode address in ES:DI, you can compute the total
size of the structure as 

size = _farpeekw(_dos_ds, es << 4 + di);

as before the address of the minmax structures is:

addr2 = _farpeekw(_dos_ds, addr1+2) + 16 * _farpeekw(_dos_ds, addr1+4);

where addr1 = es << 4 + di.

now, suppose you have the following in your program:

typedef struct minmax
{
 short min;
 short max;
} minmax __attribute__((packed));

then, you can allocate

minmax* buffer = (minmax *) malloc(size*sizeof(minmax));

and use:

movedata(_dos_ds, addr2, _my_ds(), (unsigned) buffer);

hope this helps. i am writing this in a hurry before i leave for the
office, so there will be mistakes.
-- 
   Sinan
 
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