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    <para>The most narrow sence refers to the the synchronous register
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    <para>The most narrow sence refers to the the synchronous register
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    context. It includes all the preserved registers as defined by the
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    context. It includes all the preserved registers as defined by the
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    architecture. To highlight some, the program counter and stack pointer
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    architecture. To highlight some, the program counter and stack pointer
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    take part in the synchronous register context. These are the registers
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    take part in the synchronous register context. These are the registers
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    that must be preserved across a procedure call and during synchronous
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    that must be preserved across a procedure call and during synchronous
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    context switches. </para>
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    context switches.</para>
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    <para>The next type of the context understood by the kernel is the
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    <para>The next type of the context understood by the kernel is the
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    asynchronous register context. On an interrupt, the interrupted execution
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    asynchronous register context. On an interrupt, the interrupted execution
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    flow's state must be guaranteed to be eventually completely restored.
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    flow's state must be guaranteed to be eventually completely restored.
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    Therefore the interrupt context includes, among other things, the scratch
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    Therefore the interrupt context includes, among other things, the scratch
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      returned.</para>
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      returned.</para>
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    </section>
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    </section>
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  </section>
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  </section>
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  <section>
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  <section>
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    <title>Scheduler</title>
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    <title>Threads</title>
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    <para>A thread is the basic executable entity with some code and stack.
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    While the code, implemented by a C language function, can be shared by
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    several threads, the stack is always private to each instance of the
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    thread. Each thread belongs to exactly one task through which it shares
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    address space with its sibling threads. Threads that execute purely in the
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    kernel don't have any userspace memory allocated. However, when a thread
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    has ambitions to run in userspace, it must be allocated a userspace stack.
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    The distinction between the purely kernel threads and threads running also
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    in userspace is made by refering to the former group as to kernel threads
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    and to the latter group as to userspace threads. Both kernel and userspace
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    threads are visible to the scheduler and can become a subject of kernel
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    preemption and thread migration during times when preemption is
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    possible.</para>
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    <para>HelenOS userspace layer knows even smaller units of execution. Each
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    userspace thread can make use of an arbitrary number of pseudo threads.
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    These pseudo threads have their own synchronous register context,
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    userspace code and stack. They live their own life within the userspace
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    <para>How scheduler designed and how it works.</para>
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    thread and the scheduler does not have any idea about them because they
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    are completely implemented by the userspace library. This implies several
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    things:</para>
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    <itemizedlist>
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      <listitem>
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        <para>pseudothreads schedule themselves cooperatively within the time
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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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        <para>pseudothreads share FPU context of their containing thread
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        and</para>
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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 them
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        goes to sleep.</para>
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      </listitem>
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    </itemizedlist>
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    <para></para>
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  </section>
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  </section>
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</chapter>
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</chapter>
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