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    <section id="buddy_allocator">

      <title>Buddy allocator</title>

      <section>

        <title>Overview</title>

        <para>In buddy allocator, memory is broken down into power-of-two

        sized naturally aligned blocks. These blocks are organized in an array

        of lists in which list with index i contains all unallocated blocks of

        the size <mathphrase>2<superscript>i</superscript></mathphrase>. The

        index i is called the order of block. Should there be two adjacent

        equally sized blocks in list <mathphrase>i</mathphrase> (i.e.

        buddies), the buddy allocator would coalesce them and put the

        resulting block in list <mathphrase>i + 1</mathphrase>, provided that

        the resulting block would be naturally aligned. Similarily, when the

        allocator is asked to allocate a block of size

        <mathphrase>2<superscript>i</superscript></mathphrase>, it first tries

        to satisfy the request from list with index i. If the request cannot

        be satisfied (i.e. the list i is empty), the buddy allocator will try

        to allocate and split larger block from list with index i + 1. Both of

        these algorithms are recursive. The recursion ends either when there

        are no blocks to coalesce in the former case or when there are no

        blocks that can be split in the latter case.</para>

        <graphic fileref="images/buddy_alloc.eps" format="EPS" />

        <para>This approach greatly reduces external fragmentation of memory

        and helps in allocating bigger continuous blocks of memory aligned to

        their size. On the other hand, the buddy allocator suffers increased

        internal fragmentation of memory and is not suitable for general

        kernel allocations. This purpose is better addressed by the <link

        linkend="slab">slab allocator</link>.</para>

      </section>

      <section>

        <title>Implementation</title>

        <para>The buddy allocator is, in fact, an abstract framework wich can

        be easily specialized to serve one particular task. It knows nothing

        about the nature of memory it helps to allocate. In order to beat the

        lack of this knowledge, the buddy allocator exports an interface that

        each of its clients is required to implement. When supplied an

        implementation of this interface, the buddy allocator can use

        specialized external functions to find buddy for a block, split and

        coalesce blocks, manipulate block order and mark blocks busy or

        available. For precize documentation of this interface, refer to <link

        linkend="???">HelenOS Generic Kernel Reference Manual</link>.</para>

        <formalpara>

          <title>Data organization</title>

          <para>Each entity allocable by the buddy allocator is required to

          contain space for storing block order number and a link variable

          used to interconnect blocks within the same order.</para>

          <para>Whatever entities are allocated by the buddy allocator, the

          first entity within a block is used to represent the entire block.

          The first entity keeps the order of the whole block. Other entities

          within the block are assigned the magic value

          <constant>BUDDY_INNER_BLOCK</constant>. This is especially important

          for effective identification of buddies in one-dimensional array

      because the entity that represents a potential buddy cannot be associated

      with <constant>BUDDY_INNER_BLOCK</constant> (i.e. if it is associated

          with <constant>BUDDY_INNER_BLOCK</constant> then it is not a

          buddy).</para>

        </formalpara>

      </section>

    </section>

          <varname>frame_t::ref_count</varname> and