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    <para>A thread is the basic executable entity with some code and stack.

    While the code, implemented by a C language function, can be shared by

    several threads, the stack is always private to each instance of the

    thread. Each thread belongs to exactly one task through which it shares

    address space with its sibling threads. Threads that execute purely in the

    kernel don't have any userspace memory allocated. However, when a thread

    has ambitions to run in userspace, it must be allocated a userspace stack.

    The distinction between the purely kernel threads and threads running also

    in userspace is made by refering to the former group as to kernel threads

    and to the latter group as to userspace threads. Both kernel and userspace

    threads are visible to the scheduler and can become a subject of kernel

    preemption and thread migration during times when preemption is

    possible.</para>

    <para>HelenOS userspace layer knows even smaller units of execution. Each

    userspace thread can make use of an arbitrary number of pseudo threads.

    These pseudo threads have their own synchronous register context,

    userspace code and stack. They live their own life within the userspace

    are completely implemented by the userspace library. This implies several

    things:</para>

    <itemizedlist>

      <listitem>

        <para>pseudothreads schedule themselves cooperatively within the time

        slice given to their userspace thread,</para>

      </listitem>

      <listitem>

        <para>pseudothreads share FPU context of their containing thread

        and</para>

      </listitem>

      <listitem>

        <para>all pseudothreads of one userspace thread block when one of them

        goes to sleep.</para>

      </listitem>

    </itemizedlist>

    <para></para>