SetSelectorLimit() Crossing 1 Megabyte Boundary

The information in this article applies to:

• Microsoft Windows Software Development Kit (SDK) for Windows, versions 3.0 and 3.1

SUMMARY

The SetSelectorLimit() application programming interface (API) function will not properly modify the limit of a selector if the change is to cross the 1 megabyte (MB) (0x100000) boundary. That is, the change is improper going from less than 1 MB to greater than 1 MB and visa versa. In these cases, DPMI function 08h must be used to properly change the limit of a selector.

MORE INFORMATION

SetSelectorLimit() does not correctly handle setting segment sizes from less than 1 MB to greater than 1 MB because it will not set the granularity bit. The granularity bit needs to be set to indicate page granularity in the segment descriptor. When the granularity bit is set, the selector limit has the form xxxxxFFFh for page alignment. A page is 4K (4096 bytes).

Similarly, SetSelectorLimit() does not correctly handle setting segment sizes from greater than 1 MB to less than 1 MB because it will not clear the granularity bit. When the granularity bit is clear, the selector limit has the form xxxxxh for byte alignment.

DPMI function 08h properly sets or clears the granularity bit when appropriate. The limit must be in the form xxxxxFFFh when changed to greater than 1 MB, and must be in the form xxxxxh when changed to less than 1 MB.

Sample Code

//********************************************************************
// DPMISetSelectorLimit()
//
// This function sets a selector limit using DPMI Function AX=0008h
//(Set Segment Limit). This function is necessary if the segment size
// is greater than 1 MB because SetSelectorLimit() does not correctly
// set selector sizes greater than 1 MB.
//
// Segments that are larger than 1MB are actually page granular,
// meaning that in the descriptor, the limit field is actually
// stored as the number of 4K pages rather than bytes. When you
// specify a limit greater than 1MB, this function rounds it up
// to the nearest page boundary.
//
// No matter the size of the segment, this function always accepts
// selector limits in number of bytes, never pages. The conversion
// between bytes and pages is handled internally.
//
// Note that this function takes a segment limit, which is one less
// than the number of bytes in the segment.
//
//********************************************************************

   BOOL bRetVal=TRUE;

// If the limit is >= 1MB, you need to make the limit a multiple of
// the page size or DPMISetSelectorLimit will fail.

   if( dwLimit >= 0x100000 )
      dwLimit |= 0x0FFF;

   _asm
   {
      mov  ax, 0008h
      mov  bx, selector
      mov  cx, word ptr [dwLimit+2]
      mov  dx, word ptr [dwLimit]
      int  31h
      jnc  success
      mov  bRetVal, FALSE
   success:
   }

   return bRetVal;