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  special layer providing a user application developer a reasonably

  special layer providing a user application developer a reasonably

  synchronous multithreaded environment sufficient to develop complex

  synchronous multithreaded environment sufficient to develop complex

  protocols.</para>

  protocols.</para>

  <section>

  <section>

    <para>Every message consists of 4 numeric arguments (32-bit and 64-bit on

    <para>Every message consists of 4 numeric arguments (32-bit and 64-bit on

    the corresponding platforms), from which the first one is considered a

    the corresponding platforms), from which the first one is considered a

    method number on message receipt and a return value on answer receipt. The

    method number on message receipt and a return value on answer receipt. The

    received message contains identification of the incoming connection, so

    received message contains identification of the incoming connection, so

    primarily destined to deliver IRQ events to userspace device drivers.

    primarily destined to deliver IRQ events to userspace device drivers.

    These messages need not be answered, there is no party that could receive

    These messages need not be answered, there is no party that could receive

    such response.</para>

    such response.</para>

    <section>

    <section>

      <para>The whole IPC subsystem consists of one-way communication

      <para>The whole IPC subsystem consists of one-way communication

      channels. Each task has one associated message queue (answerbox). The

      channels. Each task has one associated message queue (answerbox). The

      task can call other tasks and connect it's phones to their answerboxes.,

      task can call other tasks and connect it's phones to their answerboxes.,

      send and forward messages through these connections and answer received

      send and forward messages through these connections and answer received

        </listitem>

        </listitem>

      </orderedlist>

      </orderedlist>

    </section>

    </section>

    <section>

    <section>

      <para>On top of this simple protocol the kernel provides special

      <para>On top of this simple protocol the kernel provides special

      services closely related to the inter-process communication. A range of

      services closely related to the inter-process communication. A range of

      method numbers is allocated and protocol is defined for these functions.

      method numbers is allocated and protocol is defined for these functions.

      The messages are interpreted by the kernel layer and appropriate actions

      The messages are interpreted by the kernel layer and appropriate actions

      as soon as the message is acknowledged.</para>

      as soon as the message is acknowledged.</para>

    </section>

    </section>

  </section>

  </section>

  <section>

  <section>

    <para>The conventional design of the asynchronous api seems to produce

    <para>The conventional design of the asynchronous api seems to produce

    applications with one event loop and several big switch statements.

    applications with one event loop and several big switch statements.

    However, by intensive utilization of user-space threads, it was possible

    However, by intensive utilization of user-space threads, it was possible

    to create an environment that is not necesarilly restricted to this type

    to create an environment that is not necesarilly restricted to this type

    of event-driven programming and allows for more fluent expression of

    of event-driven programming and allows for more fluent expression of

    application programs.</para>

    application programs.</para>

    <section>

    <section>

      <para>Each tasks is associated with only one answerbox. If a

      <para>Each tasks is associated with only one answerbox. If a

      multi-threaded application needs to communicate, it must be not only

      multi-threaded application needs to communicate, it must be not only

      able to send a message, but it should be able to retrieve the answer as

      able to send a message, but it should be able to retrieve the answer as

      well. If several threads pull messages from task answerbox, it is a

      well. If several threads pull messages from task answerbox, it is a

          </imageobject>

          </imageobject>

        </mediaobject>

        </mediaobject>

        <title>Single point of entry solution</title>

        <title>Single point of entry solution</title>

      </figure>

      </figure>

    </section>

    </section>

    <section>

    <section>

      <para>Unfortunately, the real world is is never so simple. E.g. if a

      <para>Unfortunately, the real world is is never so simple. E.g. if a

      server handles incoming requests and as a part of its response sends

      server handles incoming requests and as a part of its response sends

      asynchronous messages, it can be easily prempted and other thread may

      asynchronous messages, it can be easily prempted and other thread may

      start intervening. This can happen even if the application utilizes only

      start intervening. This can happen even if the application utilizes only

      where additional locking mechanism (futexes) should be used. The IPC

      where additional locking mechanism (futexes) should be used. The IPC

      framework ensures that there will always be enough free kernel threads

      framework ensures that there will always be enough free kernel threads

      to handle incoming answers and allow the application to run more

      to handle incoming answers and allow the application to run more

      user-space threads inside the kernel threads without the danger of

      user-space threads inside the kernel threads without the danger of

      locking all kernel threads in futexes.</para>

      locking all kernel threads in futexes.</para>

    </section>

    </section>

    <section>

    <section>

      <para>The interface was developed to be as simple to use as possible.

      <para>The interface was developed to be as simple to use as possible.

      Classical applications simply send messages and occasionally wait for an

      Classical applications simply send messages and occasionally wait for an

      answer and check results. If the number of sent messages is higher than

      answer and check results. If the number of sent messages is higher than

      kernel limit, the flow of application is stopped until some answers

      kernel limit, the flow of application is stopped until some answers