Date: Mon, 6 Jan 1997 13:55:57 +0200 (IST)
From: Eli Zaretskii <eliz AT is DOT elta DOT co DOT il>
To: John Eccleston <johne AT parallax DOT co DOT uk>
cc: djgpp AT delorie DOT com
Subject: Re: Displaying the VESA OEM string
In-Reply-To: <5aqgd9$h4i@red.parallax.co.uk>
Message-ID: <Pine.SUN.3.91.970106133749.24131H-100000@is>
MIME-Version: 1.0
Content-Type: TEXT/PLAIN; charset=US-ASCII


On Mon, 6 Jan 1997, John Eccleston wrote:

> I am trying to display the OEM string for the VESA graphics card
> in my system.  However this always causes the program to
> access violate.  Could someone please point me in the right
> direction.  By looking at the FAQ I think I need to use the
> _farpeekb (etc) functions but how do I convert the pointer given
> in the VESA info structure to the required (segment*16 + offset)
> value.

The pointers in the VBE structure are real-mode pointers, so you cannot
treat them as if they were protected-mode pointers to some buffer inside
your program's address space.  Instead, you should treat them as a far
pointer, i.e. a pair of 16-bit words seg:off and use farptr functions to
fetch the buffer contents to a protected-mode buffer.  First, the
structure should be declared like so: 

typedef struct VBEInfo
{
   char  VESASignature[4] PACKED; /* 'VESA' 4 byte signature      */
   short VESAVersion      PACKED; /* VBE version number           */
   short OEMStringPtr_Off PACKED; /* Pointer to OEM string - offset */
   short OEMStringPtr_Seg PACKED; /* Pointer to OEM string - segment */
   long  Capabilities     PACKED; /* Capabilities of video card   */
   short VideoModePtr_Off PACKED; /* Pointer to supported modes - offs */
   short VideoModePtr_Seg PACKED; /* Pointer to supported modes - segm */
   short TotalMemory      PACKED; /* Number of 64kb memory blocks */
   char  reserved[236]    PACKED; /* Pad to 256 byte block size   */
} VBEInfo;

Note how the pointers were converted into a seg:off pair, and in reverse 
order because the way Intel machines store bytes.  (Btw, it's best to 
declare the segment and offset to be unsigned short.)

Then, you should pull the OEM string from the real-mode buffer like so:

	char OEM_str[MAXOEM_SIZE];
	char *d = OEM_str;
	int  i  = OEMStringPtr_Seg*16 + OEMStringPtr_Off;

	_farsetsel (_dos_ds);
	while ( (*d = _farnspeekb (i++)) != '\0')
	  ;

Warning: the above code is untested!