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      <constant>Sleeping</constant> state by going to sleep or enters the
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      <constant>Sleeping</constant> state by going to sleep or enters the
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      <constant>Exiting</constant> state when it reaches termination. When the
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      <constant>Exiting</constant> state when it reaches termination. When the
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      thread exits, its kernel structure usually stays in memory, until the
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      thread exits, its kernel structure usually stays in memory, until the
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      thread is detached by another thread using <code>thread_detach()</code>
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      thread is detached by another thread using <code>thread_detach()</code>
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      function. Terminated but undetached threads are in the
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      function. Terminated but undetached threads are in the
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      <constant>Undead</constant> state. When the thread is detached or
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      <constant>Lingering</constant> state. When the thread is detached or
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      detaches itself during its life, it is destroyed in the
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      detaches itself during its life, it is destroyed in the
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      <constant>Exiting</constant> state and the <constant>Undead</constant>
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      <constant>Exiting</constant> state and the
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      state is not reached.<figure float="1">
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      <constant>Lingering</constant> state is not reached.<figure float="1">
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          <title>Transitions among thread states.</title>
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          <title>Transitions among thread states.</title>
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          <mediaobject id="thread_states" xreflabel="">
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          <mediaobject id="thread_states" xreflabel="">
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            <imageobject role="pdf">
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            <imageobject role="pdf">
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              <imagedata fileref="images/thread_states.pdf" format="PDF" />
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              <imagedata fileref="images/thread_states.pdf" format="PDF" />
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          </mediaobject>
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          </mediaobject>
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        </figure></para>
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        </figure></para>
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    </formalpara>
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    </formalpara>
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    <formalpara>
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    <formalpara>
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      <title>Pseudo Threads</title>
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      <title>Fibrils</title>
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      <para>HelenOS userspace layer knows even smaller units of execution.
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      <para>HelenOS userspace layer knows even smaller units of execution.
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      Each userspace thread can make use of an arbitrary number of pseudo
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      Each userspace thread can make use of an arbitrary number of fibrils.
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      threads. These pseudo threads have their own synchronous register
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      These fibrils have their own synchronous register context, userspace
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      context, userspace code and stack. They live their own life within the
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      code and stack. They live their own life within the userspace thread and
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      userspace thread and the scheduler does not have any idea about them
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      the scheduler does not have any idea about them because they are
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      because they are completely implemented by the userspace library. This
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      completely implemented by the userspace library. This implies several
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      implies several things:<itemizedlist>
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      things:<itemizedlist>
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          <listitem>
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          <listitem>
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            <para>pseudothreads schedule themselves cooperatively within the
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            <para>fibrils schedule themselves cooperatively within the time
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            time slice given to their userspace thread,</para>
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            slice given to their userspace thread,</para>
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          </listitem>
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          </listitem>
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          <listitem>
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          <listitem>
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            <para>pseudothreads share FPU context of their containing thread
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            <para>fibrils share FPU context of their containing thread
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            and</para>
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            and</para>
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          </listitem>
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          </listitem>
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          <listitem>
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          <listitem>
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            <para>all pseudothreads of one userspace thread block when one of
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            <para>all fibrils of one userspace thread block when one of them
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            them goes to sleep.</para>
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            goes to sleep.</para>
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          </listitem>
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          </listitem>
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        </itemizedlist></para>
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        </itemizedlist></para>
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    </formalpara>
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    </formalpara>
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  </section>
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  </section>
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