The Frame Heap Manager

The Frame Heap Manager is a simple memory manager that can only allocate memory blocks of a specified size while all allocated memory blocks are freed simultaneously. Furthermore, the memory efficiency is good because the management information is not contained in the memory block. The following is an overview of the Frame Heap Manager.

Creating Heaps

To use the Frame Heap Manager you must create a frame heap. These functions create and destroy frame heaps.

Allocating Memory Blocks

Allocating and Freeing Memory Blocks

The Frame Heap Manager allocates memory blocks by filling the heap from the top and bottom without any gaps. Because this method is used to allocate memory blocks, there is no heap fragmentation. Furthermore, because there is no management region for the memory blocks that the Frame Heap Manager allocates, memory is efficiently used and processing for memory block allocation is reduced.

Figure 3-1 Allocating Memory for the Frame Heap

The following functions alblocate memory blocks.

To allocate memory blocks from the top of the heap region, pass a negative value to the alignment argument in the MEMAllocFromFrmHeapEx function.

Allocating the Minimum Memory Block Size

Even though the Frame Heap Manager does not have a memory block management region, allocated memory blocks must be aligned at 4-byte boundaries at a minimum. Allocating a 1-byte memory block consumes 4 bytes of memory.

Specifying Alignment

The Frame Heap Manager can specify alignment during memory block allocation. In the MEMAllocFromFrmHeapEx function, you can specify 4, 8, 16, or 32 as alignment values. If negative values (i.e., -4, -8, -16 or -32) are specified, memory blocks are allocated from the top of the heap. The MEMAllocFromFrmHeap function does not specify alignment; the alignment value is always a value of 4.

Freeing Memory Blocks

Because the Frame Heap Manager does not manage individual allocated memory blocks, it cannot free allocated blocks individually. The Frame Heap Manager uses one of the three following methods to free memory blocks.

The following function frees memory blocks.

The following methods can be specified in the MEMFreeToFrmHeap function.

The Frame Heap Manager can also save the memory block allocation status, simultaneously free the memory blocks that are subsequently allocated, and return to the status prior to saving. These features are described in the following section.

Saving and Restoring Memory Block Allocation Status

The Frame Heap Manager allows you to save the memory block allocation status for the heap region and to restore this status later. 20 bytes of memory are necessary for saving each memory block allocation status. The the memory block allocation status can be saved as many times as the heap capacity limit allows. When saving the memory block allocation status, a 4-byte tag can be attached. When restoring the memory block allocation status, the previous status or a status specified by a tag can be restored.

Figure 3-2 Mechanism for Saving and Restoring the Memory Block Allocation Status of a Frame Heap

The following functions save and restore memory block allocation status.

Adjusting the Heap Region Size

The Frame Heap Manager can reduce the heap region size to match the heap region content. Only use this functionality if memory blocks haven't been allocated from the top of the heap region. This functionality can be used to load an indefinite amount of data into memory without leaving gaps in the heap. First, create a heap that has a sufficient size, allocate memory blocks from the bottom of the heap region, and store the data. After all of the required data is stored, reduce the heap region size to match the heap content.

Figure 3-3 Adjusting the Size of the Frame Heap

Changing Memory Block Size

The memory block size can be changed with the Frame Heap Manager if the memory block is the last block allocated from the bottom of the free region in the heap. If the memory block becomes smaller than the current size, the remaining region is merged with the free region above the memory block after reduction. If the memory block becomes larger than the current size, the memory block expands into the free region above the memory block.

Figure 3-4 Changing the Size of Frame Heap Memory Blocks

If the free region size is smaller than the specified size when expanding a memory block, the MEMResizeForMBlockFrmHeap function fails and returns zero.

Getting the Allocatable Size

The Frame Heap Manager can find the size of the largest allocatable memory block. These functions are shown in the following table.

Revision History

03/01/2006 Initial version.
06/27/2006 Corrected an omission.

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