8. Descriptor Management Services, Continued

8.3 Segment to Descriptor

This function is used to convert real mode segments into descriptors that are addressable by protected mode programs.

To Call

     AX = 0002h
     BX = Real mode segment address

Returns

     If function was successful:
     Carry flag is clear.
     AX = Selector mapped to real mode segment

     If function was not successful:
     Carry flag is set.

Programmer's Notes

• Since this function involves descriptor management, it will be rarely needed in 32-bit programs.
• Multiple calls to this function with the same segment will return the same selector.
• Descriptors created by this function should never be modified or freed. For this reason, you should use this function sparingly. If your program needs to examine various real mode addresses using the same selector you should allocate a descriptor and change the base using the Set Segment Base Address function instead of using this function.
• The descriptor's limit will be set to 64K.
• The intent of this function is to allow programs easy access to commonly used real mode segments such as 40h and A000h. Do not use this service to obtain descriptors to private data areas.

Code Sample:

The following function compiles with Watcom C/C++ version 11.0 or later, and Microsoft Visual C/C++ version 4.0 or later:

// allocate one descriptor

int dpmiSegmentToDescriptor(unsigned short realSegment,	// real-mode segment address
			dpmiSelector *pSel)	// (returned) the resulting selector
	{
	int result;			// zero if int 31h succeeds
	unsigned short errorCode;	// error code from DPMI host

	__asm {
		mov	bx, realSegment	// the real-mode segment to map

		mov	eax, 0x002	// call dpmi host to do mapping
		int	0x31

		mov	errorCode, ax	// save error code from DPMI, if any

		sbb	eax, eax	// record success/failure flag
		mov	result, eax
		}

	// if the dpmi call failed, return the dpmi error code
	if (result != 0)
		return errorCode;

	// if we succeeded, AX contained the selector
	*pSel = (dpmiSelector) errorCode;

	// indicate success
	return 0;
	}