From: shedd timothy allen <shedd AT ehsn8 DOT cen DOT uiuc DOT edu>
Newsgroups: comp.os.msdos.djgpp
Subject: Device driver needs segment:offset info
Date: Sun, 2 Mar 1997 18:39:04 -0600
Organization: University of Illinois at Urbana
Lines: 59
Message-ID: <Pine.Sola.3.91.970302175553.2191C-100000@ehsn8.cen.uiuc.edu>
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To: djgpp AT delorie DOT com
DJ-Gateway: from newsgroup comp.os.msdos.djgpp

I am trying to port some older C code (circa 1992, apparently for MS C 
compiler) to djgpp.  This code makes up a couple of libraries used to 
control a vintage TARGA+ framegrabber.

I thought I had successfully jumped through the "far" pointer hoops until 
I realized that these pointers were being passed to the device driver 
itself, and naively replacing FP_OFF and FP_SEG produced unpredictable 
results.

I have read the FAQ, browsed the DPMI docs and have done some dancing 
around in the mail archives.  Unfortunately, I have no experience with 
dealing with interrupts and machine-level memory issues (or memory issues 
at all, for that matter) and, to make matters worse, am a novice with 
C/C++ (So why am I doing this?  I'm a grad student, of course, trying to 
make old equipment work with new research projects.)  As a result, if the 
answer is in the documentation, I have missed it and I apologize in 
advance.  

Would anyone be able to help with the following scenario?

The code sets up an IOCommand structure to pass to the driver.  The 
IOCommand structure is passed to the driver by means of an INT 0x21.  The 
driver then writes (or reads) the registers of the framegrabber from (to) 
the structure TPlusRegs.  Along with misc. data, the IOCommand 
structure contains the following lines to pass along the pointer to the 
TPlusRegs structure:

unsigned short	IORegsOff;	/*Structure Buffer Ptr Offset*/
unsigned short	IORegsSeg;	/*Structure Buffer Ptr Segment*/

In the library functions, communication with the driver goes something 
like this:

>>SetRegs:

/*Create a pointer to the command structure: */
IOCommandPtr	fp;

/*Set up the registers for an interrupt call: */

r->x.dx = FP_OFF(fp); /*where r is of type REGS...offset of address of 
                       pointer to IOCommand structure */
s->ds   = FP_SEG(fp); /*where s is of type SREGS...segment of address 
                       of pointer to IOCommand structure  */
r->x.ax = 0x4403; /* IOCtl Write */
r->x.bx = TPlusHandle; 
r->x.cx = sizeof(IOCommand);

>>WriteAll:

TplusRegsPtr	p; /*Pointer to structure containing TPlus Register data 
                    to be written */

IOCommand.IORegsOff = FP_OFF(p) ;  /* offset of location of TPlusRegs Data */
IOCommand.IORegsSeg = FP_SEG(p);  /* segment of location of TPlusRegs Data */

int86x(0x21,&regs,&regs,&sregs);  /*where regs is of type REGS and set as 
                                  above and sregs is of type SREGS and 
                                  set as above */