Reasons for a 128K Limit on Huge Arrays and halloc()s

 Last reviewed: July 17, 1997
Article ID: Q45705
5.10 6.00 6.00a 6.00ax 7.00 | 5.10 6.00 6.00a | 1.00 1.50 
MS-DOS                      | OS/2            | WINDOWS
kbprg

The information in this article applies to:

  • The C Run-time (CRT), included with: 

        - Microsoft C for MS-DOS, versions 5.1, 6.0, 6.0a, and 6.0ax
    - Microsoft C for OS/2, versions 5.1, 6.0, and 6.0a
    - Microsoft C/C++ for MS-DOS, versions 7.0
    - Microsoft Visual C++ for Windows, versions 1.0 and 1.5

SUMMARY

In Microsoft C , halloc() and huge arrays are limited to allocating 128K unless the size of the elements is a power of 2.

MORE INFORMATION

With huge pointers and arrays, the Microsoft C Compiler produces 32- bit pointer arithmetic only when accessing whole elements. Only the 16-bit offset of a pointer is used when accessing the individual members of elements. This technique is used in the interest of speed and smaller code size. However, it assumes that elements do not extend from the end of one segment into the beginning of the next. When crossing a 64K segment boundary, elements must end evenly with the end of that segment. The following diagram illustrates the conditions necessary for huge pointers and arrays: 

                             Seg1 Seg2
     Segment 1               FFFF 0000               Segment 2
     ----------------------------+----------------------------
    ......| Elem | Elem  | Elem  + Elem  | Elem  | Elem  |.....
    ......| X     | X+1  | X+2   + X+3   | X+4   | X+5   |.....
     ----------------------------+----------------------------
                                 ^ Elements must not cross
                                   a segment boundary.
If an element's size is not a power of 2, an array of that element will not fit evenly into a 64K segment. This is the root of the 128K limitation. To avoid breaking an element across a segment boundary in this case, extra space is left at the beginning of the first segment, pushing the entire array upwards in memory so that the element before the one that would have spanned the segment boundary ends exactly at the end of the segment. The element that would have spanned the segment boundary is pushed to the beginning of the second segment.

If the allocated elements do not fit evenly into a segment, there can be only one segment boundary onto which they fall evenly. The function halloc() uses the element size it is passed to calculate and return a pointer with an offset that results in the allocated elements falling evenly on this boundary. The following diagram demonstrates the way this is done and what can happen at the end of the second segment: 

        Segment 1                     Segment 2
        0000           FFFF 0000           FFFF
     +----------------------+-------------------+
     +Pad |Elem |Elem |Elem +Elem |Elem |Elem | +
     +----------------------+-------------------+
      ^   ^                                   ^
      ^   ^                                   Next element won't fall
      ^   ^                                   on segment bounds.
      ^   ^
      ^   Offset returned to allow elements to fall on segment bounds.
      ^
      Padding area used to force element boundary to fall on segment
      boundary.
These restrictions should also be considered when allocating memory for very large elements. For example, a request for three 33K structures will fail. Two of the structures could be allocated, but since each would go into a separate segment, neither of the segments would contain enough space for the third element.

Additional reference words: kbinf 5.10 6.00 6.00a 6.00ax 7.00 1.00 1.50 64K
KBCategory: kbprg
KBSubcategory: CRTIss
Keywords : kb16bitonly

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Last reviewed: July 17, 1997
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