SetSelectorLimit() Crossing 1 Megabyte Boundary
ID: Q103950
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The information in this article applies to:
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Microsoft Windows Software Development Kit (SDK) versions 3.0, 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 DPMISetSelectorLimit (UINT selector, DWORD dwLimit)
{
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;
}
Additional query words:
gp-fault gpf uae gp fault
Keywords : kb16bitonly
Version : WINDOWS:3.0,3.1
Platform : WINDOWS
Issue type :