8. Descriptor Management Services, Continued

8.7 Set Segment Base Address

This function changes the 32-bit linear base address of the specified selector.

To Call

     AX = 0007h
     BX = Selector
     CX:DX = 32-bit linear base address for segment

Returns

     If function was successful:
     Carry flag is clear.

     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.
• Your program should only modify descriptors that were allocated through the Allocate Descriptors function.
• The high 8 bits of the base address (contained in CH) will be ignored by 16-bit implementations of DPMI. This is true even when running on ia32 (32-bit) machines.

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:

// Sets 32-bit linear base address in descriptor for selector

int dpmiSetSegmentBaseAddress(dpmiSelector sel,	// selector to set base address of
				unsigned long linaddr)	// new base address
	{
	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	ecx, linaddr	// put linear address into cx:dx
		mov	dx, dx
		shr	ecx, 16

		mov	eax, 0x007	// call dpmi host to do setting
		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;

	// indicate success
	return 0;
	}