DOCERR: AllocSelector & FreeSelector Documentation Incomplete

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The documentation in the Windows SDK for the AllocSelector and FreeSelectorfunctions does not mention that they can allocate and free multipleselectors. Each function uses the limit (length) of the selector passed toit to determine how many selectors to allocate or free. This articledescribes how AllocSelector allocates one or more selectors and howFreeSelector frees selectors. It also describes what you must know to usethese two functions correctly. The documentation should include all theinformation given in this article.

AllocSelector(UINT uSelector)

AllocSelector(UINT uSelector) allocates a new selector or array of tiledselectors and copies the base address, limit (length), and access rights ofuSelector to the new selector(s). If the limit of uSelector is less than orequal to 64K, a single selector is allocated. If the limit of uSelector islarger than 64K, an array of tiled selectors is allocated such that eachselector points to a 64K portion of the limit of uSelector.

If uSelector is NULL, AllocSelector allocates a single, completelyuninitialized selector. The selector cannot be used until its base address,limit (length), and access rights have been set. To set the access rightsof an unitialized selector, you must call the DOS Protected Mode Interface(DPMI) Set Descriptor Access Rights function (0x09). Furthermore, you mustset the selector's table bit and privilege bits by using a bitwise-OR maskof 0x0007 to set the lowest-order three bits of the selector.

You should copy an existing selector instead of allocating an uninitializedselector because it is easier to allow AllocSelector set the newly createdselector's access control bits. After creating a copy of an existingselector, use SetSelectorLimit and SetSelectorBase to make the copy pointto the desired location.

Selectors allocated by AllocSelector must be freed by FreeSelector becausethe system does not track or manage them automatically.

FreeSelector(UINT uSelector)

FreeSelector(UINT uSelector) frees either a single selector or an array oftiled selectors depending on the limit of uSelector. Frees one selector foreach 64K portion of the limit of uSelector. The selector or array of tiledselectors being freed must have been allocated previously by AllocSelector.Furthermore, the limit of uSelector must be the same as the selector usedas a parameter in the call to AllocSelector.

Example Array of Tiled Selectors

An example array of tiled selectors looks like this (assuming that theselector passed to AllocSelector has a limit of 256K):
  Sel 1     Sel 2      Sel 3      Sel 4   0         64K       128K       192K       256K   +----------+----------+----------+----------+   |          |          |          |          |   +----------+----------+----------+----------+   |          |          |          |--------->| (Limit = 64K)   |          |          |-------------------->| (Limit = 128K)   |          |------------------------------->| (Limit = 192K)   |------------------------------------------>| (Limit = 256K)				
Notice that each successive selector's base address starts 64K from theprevious selector's base address and has a limit that is 64K less than theprevious selector. What really makes these selectors tiled is that they arecontiguous in the local descriptor table (LDT). For example, if Sel 1 has avalue of 0x97, then Sel 2 will be 0x9F, Sel 3 will be 0xA7, and Sel 4 willbe 0xAF.

Only Sel 1 is used to create a huge pointer to the block. As you incrementthrough the block, the compiler generates the correct code to switch fromSel 1 to Sel 2 to Sel 3 to Sel 4 automatically. The code listed in the"Code Sample Two" section of this article demonstrates this.

Code Sample One

This example shows how to allocate exactly one data selector. Code segmentsin 16-bit code are always less than 64K, so you can ensure that you don'tallocate multiple selectors by allocating a copy of a code selector andconverting it into a data selector:
     UINT codeSelector, dataSelector;   _asm {        mov ax, cs        mov codeSelector, ax        }   dataSelector = AllocSelector(codeSelector);   if (!dataSelector)      return NULL;   // Change dataSelector from a code selector into a data selector   if (PrestoChangoSelector(codeSelector, dataSelector))      {      // Set the desired base address and limit      SetSelectorBase(dataSelector, dwLinearBase);      DPMISetSelectorLimit(dataSelector, dwLimit);      }   else      {      // If you get here, you couldn't change dataSelector so you need      // to free it because you can't use it as a code selector      FreeSelector(dataSelector);      return NULL;      }   // You now have a single data selector. Use it, and then free it by   // calling FreeSelector.				

Code Sample Two

This example shows how to automatically allocate an array of tiledselectors that points to a specified region of memory, and obtain a hugepointer from the selector array:
    // The application creates, uses, and frees a huge pointer here   char __huge * hpMem;   hpMem = CreateHugePointer (dwBaseAddress, dwLength);   // hpMem now points to a region of pre-allocated memory, such   // as a memory-mapped hardware device's buffer. Use it as you   // would any pointer.   FreeHugePointer (hpMem);// The following three functions show how to allocate an array// of tiled selectors that point to a specified region of memory./*--------------------------------------------------------------  This function creates a huge pointer with the proper number  of selectors to access physical memory. The huge pointer may be  used by C or C++ code. dwLinearBase is a 32-bit linear address,  and dwLength is the number of bytes that the huge pointer will  be able to access. This function returns the huge pointer if it  succeeds, or it returns NULL if it fails.--------------------------------------------------------------*/ void __huge * CreateHugePointer (DWORD dwLinearBase,                                 DWORD dwLength)   {   WORD tempSelector = NULL;   WORD codeSelector = NULL;   WORD dataSelector = NULL;   DWORD dwLimit;   /*     A segment's limit is defined as the last accessible offset in     the segment. Because the limit is the last accessible offset, it     is the desired length of the segment minus 1. For example, if     you want a 64K segment, then you need a limit of 0xFFFF, not     0x10000, because the segment contains byte offsets 0 to 0xFFFF.     Note that a segment with a limit of 0 is actually a single byte     in length. Thus, this function considers a length of zero     invalid.   */    if (dwLength == 0)      return NULL;   dwLimit = dwLength - 1;   /*     Allocate a single temporary selector by making a copy of the     code segment selector and converting the copy to a data     selector. Code segments are always less than or equal to     64K in length, so you are guaranteed to get a single temporary     selector and be sure to free a single selector.     Once you have the temporary selector, set its base address and     limit to the desired values, which may be larger than 64K.     Because the memory must be accessed by 16-bit code, you must     allocate an array of tiled selectors. The temporary selector is     used to force AllocSelector to allocate an array of the proper     number of tiled selectors, each with the proper base and limit.     Then you can free the single temporary selector.     If you fail anywhere along the way, clean up whatever has been     done, and return NULL.   */    _asm {        mov ax, cs        mov codeSelector, ax        }   tempSelector = AllocSelector (codeSelector);   if (!tempSelector)      return NULL;   /*     If you can successfully change the tempSelector into a     data selector, set its base address and limit to the     desired base and limit, and then allocate the real selector     array. Otherwise, prepare to return NULL.     SetSelectorLimit does not handle the granularity bit of     selectors properly, which limits its usefulness only to ranges     of addresses less than 1MB in length. This function calls     DPMISetSelectorLimit, a function defined below, to overcome     this limitation and allow you to create arrays of tiled     selectors that can access more than 1MB.   */    if (PrestoChangoSelector (codeSelector, tempSelector))      {      SetSelectorBase(tempSelector, dwLinearBase);      DPMISetSelectorLimit(tempSelector, dwLimit);      dataSelector = AllocSelector(tempSelector);      }   else      dataSelector = NULL;   // Clean up temp selector   DPMISetSelectorLimit(tempSelector, 0L);   FreeSelector(tempSelector);   // dataSelector will be NULL if it could not be allocated   // successfully, making this function return NULL.   return (void __huge *)MAKELONG(0, dataSelector);   }/*--------------------------------------------------------------  This function frees pointers allocated by CreateHugePointer.  It correctly frees all tiled selectors created to access the  block of physical memory. It is very important that you call  this on all pointers created by CreateHugePointer and that you  do not call this function on pointers allocated by any way  other than using CreateHugePointer.--------------------------------------------------------------*/ void FreeHugePointer (void __huge * hPtr)   {   if (hPtr)      FreeSelector (HIWORD(hPtr));   }/*--------------------------------------------------------------  This function sets the limit of a selector using DPMI Function  0008h (Set Segment Limit). This function is necessary if the  segment size is greater than 1 MB because the Windows  SetSelectorLimit() API function does not correctly set selector  limits 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, we need to make the limit a mulitple   // 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;   }				

Four Caveats to Keep in Mind

Keep the following in mind when using the suggestions in this article:

  • Allocating selectors does not actually allocate any memory. It merely creates a pointer that can be used to access existing memory (memory previously allocated or provided by a memory-mapped hardware device). Do not confuse allocating selectors with allocating memory.
  • Selectors that alias (point to) a memory block allocated by Windows are not updated if the memory block is moved. To ensure that the memory block is not moved, call GlobalFix on it before creating a selector that aliases it. However, if the allocated selector points to memory provided by a hardware device, there is no need to call GlobalFix because the device's memory was not allocated by Windows.
  • The Windows memory manager does not keep track of which task allocated selectors with these functions, so you must ensure that the task frees them correctly. In particular, make sure it does not free more or fewer selectors than it allocated. The sample code in this article demonstrates the proper way to allocate and free selectors with these functions.
  • Allocating large numbers of selectors is discouraged because selectors are a limited resource.

Article ID: 132005 - Last Review: 01/16/2015 17:57:24 - Revision: 1.0

  • Microsoft Windows Software Development Kit 3.1
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