Inside Macintosh: Memory
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The Memory Manager does its best to prevent situations in which nonrelocatable blocks in the middle of the heap trap relocatable blocks. When it allocates new nonrelocatable blocks, it attempts to reserve memory for them as low in the heap as possible. The Memory Manager reserves memory for a nonrelocatable block by moving unlocked relocatable blocks upward until it has created a space large enough for the new block. When the Memory Manager can successfully pack all nonrelocatable blocks into the bottom of the heap, no nonrelocatable block can trap a relocatable block, and it has successfully prevented heap fragmentation.
Figure 1-11 illustrates how the Memory Manager allocates nonrelocatable blocks. Although it could place a block of the requested size at the top of the heap, it instead reserves space for the block as close to the bottom of the heap as possible and then puts the block into that reserved space. During this process, the Memory Manager might even move a relocatable block over a nonrelocatable block to make room for another nonrelocatable block.
Figure 11 Allocating a nonrelocatable block
When allocating a new relocatable block, you can, if you want, manually reserve space for the block by calling the ReserveMem procedure. If you do not, the Memory Manager looks for space big enough for the block as low in the heap as possible, but it does not create space near the bottom of the heap for the block if there is already enough space higher in the heap.