8. Descriptor Management Services, Continued

8.6 Get Segment Base Address

This function returns the 32-bit linear base address of the specified segment.

To Call

     AX = 0006h
     BX = Selector

Returns

     If function was successful:
     Carry flag is clear.
     CX:DX = 32-bit linear base address of 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.
• This function will fail if the selector specified in BX is invalid

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:

// 32-bit linear base address from descriptor for selector

int dpmiGetSegmentBaseAddress(dpmiSelector sel,	// selector for base address
			unsigned long *pLinaddr)	// (returned) address
	{
	unsigned long base;	// base linear address of segment
	int result;		// zero if int 31h succeeds
	unsigned short errorCode;	// error code from DPMI host

	__asm {
		mov	bx, sel		// the selector to get base address of

		mov	eax, 0x006	// call dpmi host to do getting
		int	0x31

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

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

		mov	eax, ecx		// store cx:dx into base
		shl	eax, 16
		mov	ax, dx
		mov	base, eax
		}

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

	// return obtained base address
	*pLinaddr = base;

	// indicate success
	return 0;
	}