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Date: Wed, 08 Nov 2006 08:47:53 -0800
From: Ivor Bowden <ivorlist AT peritek DOT com>
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Newsgroups: comp.os.msdos.djgpp
Subject: Re: Accessing absolute address above 1MB
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This is the way I do it:

/* get the address */
laddr=addr=strtoul(argv[n],&chptr,0);

/* map the memory */
/* note map routine changes addr to base_addr */
if ((slctr=map(&addr,size))==-1) printf("failed\n");
laddr-=addr;

/* read the data */
_farpeekb(slctr,laddr);printf("0x%02x\n",data);

And here is the map routine:

/* map physical memory */
u_int map(u_int* addr, u_int size)
{
__dpmi_meminfo mi;
int selector;
u_int mapsize;

if (*addr < 0x100000)
	{
	if (*addr+size <= 0x100000)
		{
		*addr=0;
		return _dos_ds;
		}
	else
		{
		printf("address + size crosses DOS 0x100000 boundry - exiting\n");
		return -1;
		}
	}

mapsize=0x400;
while ((size+(*addr&(mapsize-1)))>mapsize)
	mapsize<<=1;
*addr &=~(mapsize-1);
/*printf("mapped addr = 0x%08x, mapped size = 0x%08x\n",*addr,mapsize);*/

mi.handle = 0; /* what should it be? */
mi.address = *addr;
mi.size = mapsize;
if (__dpmi_physical_address_mapping(&mi))
	{
	printf ("__dpmi_physical_address_mapping failed\n");
	return -1;
	}
selector = __dpmi_allocate_ldt_descriptors(1);
if (selector == -1)
	{
	printf ("__dpmi_allocate_ldt_descriptors failed\n");
	return -1;
	}
if(__dpmi_set_segment_base_address(selector,mi.address))
	{
	printf ("__dpmi_set_segment_base_address failed\n");
	return -1;
	}
if(__dpmi_set_segment_limit(selector,mi.size-1))
	{
	printf ("__dpmi_set_segment_limit failed\n");
	return -1;
	}
return selector;
}

I have a cool little program that can R/W any memory or IO location, as 
well as PCI config registers and probing the PCI bus. It used to be on 
the company web site, but with the new corporate it is gone now, but I 
can still send it to whoever wants it. It only works with DOS, Windows 
<= W98 (NOT NT,2K,XP,etc), Unix and VxWorks (separate compile versions). 
Program name is Peritool.

Ivor

on 11/7/2006 7:58 AM PDT countingtoten AT gmail DOT com wrote:
> I am trying to access the memory address 0x3FFF0000 in DOS using dpmi.
> When executed, I receive the following seg fault:
> 
> Exiting due to signal SIGSEGV
> General Protection Fault at eip=00003af6
> eax=000001cf ebx=00000299 ecx=00000000 edx=3fff0000 esi=00000054
> edi=0000ff70
> ebp=0008ff08 esp=0008ff08
> program=C:\DOCUME~1\WEINERT\DESKTOP\TEST\DPMITEST.EXE
> cs: sel=01a7  base=02990000  limit=0009ffff
> ds: sel=01af  base=02990000  limit=0009ffff
> es: sel=01af  base=02990000  limit=0009ffff
> fs: sel=01cf  base=3fff0000  limit=00000fff
> gs: sel=01bf  base=00000000  limit=0010ffff
> ss: sel=01af  base=02990000  limit=0009ffff
> App stack: [0008ff70..0000ff70]  Exceptn stack: [0000fed0..0000df90]
> 
> Call frame traceback EIPs:
>   0x00003af6
>   0x00001e8a
>   0x000036c8
> 
> from this code:
> 
> #include <go32.h>
> #include <sys/farptr.h>
> #include <dpmi.h>
> #include <stdio.h>
> 
> int selector;
> 
> int main(int argc, char** argv)
> {
>   __dpmi_meminfo mi;
>  int selector;
>  char buffer[4];
> 
>  /* Map the physical device address to linear memory.  */
>  mi.address = 0x3FFF0000;
>  mi.size    = 0x1000;
> 
>  if(mi.address < 0x100000)
>  {
> 	buffer[0] = _farpeekb(_dos_ds, mi.address);
>  }
>  else
>  {
> 	 __dpmi_physical_address_mapping (&mi);
>  	selector = __dpmi_allocate_ldt_descriptors (1);
>  	__dpmi_set_segment_base_address (selector, mi.address);
>  	__dpmi_set_segment_limit (selector, mi.size - 1);
>  
>  	buffer[0] = _farpeekb(selector, mi.address);
>  }
> }
> 

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