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</p><div class="sect2" title="Design"><div class="titlepage"><div><div><h3 class="title"><a id="allocator.bitmap.design"></a>Design</h3></div></div></div><p>
As this name suggests, this allocator uses a bit-map to keep track
of the used and unused memory locations for it's book-keeping
</p><p>
Consider a block of size 64 ints. In memory, it would look like this:
(assume a 32-bit system where, size_t is a 32-bit entity).
- </p><div class="table"><a id="id
575954"></a><p class="title"><b>Table 33.1. Bitmap Allocator Memory Map</b></p><div class="table-contents"><table summary="Bitmap Allocator Memory Map" border="1"><colgroup><col align="left" /><col align="left" /><col align="left" /><col align="left" /><col align="left" /></colgroup><tbody><tr><td align="left">268</td><td align="left">0</td><td align="left">4294967295</td><td align="left">4294967295</td><td align="left">Data -> Space for 64 ints</td></tr></tbody></table></div></div><br class="table-break" /><p>
+ </p><div class="table"><a id="id
376664"></a><p class="title"><b>Table 20.1. Bitmap Allocator Memory Map</b></p><div class="table-contents"><table summary="Bitmap Allocator Memory Map" border="1"><colgroup><col align="left" /><col align="left" /><col align="left" /><col align="left" /><col align="left" /></colgroup><tbody><tr><td align="left">268</td><td align="left">0</td><td align="left">4294967295</td><td align="left">4294967295</td><td align="left">Data -> Space for 64 ints</td></tr></tbody></table></div></div><br class="table-break" /><p>
The first Column(268) represents the size of the Block in bytes as
seen by the Bitmap Allocator. Internally, a global free list is
used to keep track of the free blocks used and given back by the
_S_refill_pool which does the following:
</p><div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem"><p>
Gets more memory from the Global Free List of the Required
- size.
+ size.
</p></li><li class="listitem"><p>
- Adjusts the size for the next call to itself.
+ Adjusts the size for the next call to itself.
</p></li><li class="listitem"><p>
Writes the appropriate headers in the bit-maps.
</p></li><li class="listitem"><p>
Sets the use count for that super-block just allocated to 0
- (zero).
+ (zero).
</p></li><li class="listitem"><p>
All of the above accounts to maintaining the basic invariant
for the allocator. If the invariant is maintained, we are
sure that all is well. Now, the same process is applied on
the newly acquired free blocks, which are dispatched
- accordingly.
+ accordingly.
</p></li></ul></div></li></ol></div><p>
Thus, you can clearly see that the allocate function is nothing but a
combination of the next-fit and first-fit algorithm optimized ONLY for
sizeof(size_t) x 8 which is the number of bits in an integer,
which can fit exactly in a CPU register. Hence, the term given is
exponential growth of the internal pool.
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