8. Descriptor Management Services, Continued

8.8 Set Segment Limit

This function sets the limit for the specified segment.

To Call

     AX = 0008h
     BX = Selector
     CX:DX = 32-bit segment limit

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 or the specified limit could not be set. 16-bit DPMI implementations can not set segment limits greater than 0FFFFh (64K) so CX must be zero when calling this function under these implementations of DPMI.
• Segment limits greater than 1 meg must be page aligned. That is, limits greater than one megabyte must have the low 12 bits set.
• Your program should only modify descriptors that were allocated through the Allocate Descriptors function.
• To get the limit of a segment you should use the instruction lsl (load segment limit) which is supported on 80286 and ia32 (32-bit) machines. Note that on ia32 (32-bit) machines you will need to use the 32-bit form of lsl if the segment has a limit greater than 64K.

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 limit (length) descriptor for selector

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

		mov	eax, 0x008	// 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;
	}