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  <title>Architecture Overview</title>
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  <title>Architecture Overview</title>
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  <para>The HelenOS operating system is designed as a relatively small
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  <para>The HelenOS operating system is designed as a relatively small
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  microkernel assisted with a set of userspace drivers and server tasks.
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  microkernel assisted with a set of userspace drivers and server tasks.
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  HelenOS is not very radical in what subsystems should or should not be
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  HelenOS is not very radical in which subsystems should or should not be
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  implemented in the kernel - in some cases, both kernel and userspace drivers
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  implemented in the kernel - in some cases, both kernel and userspace drivers
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  exist. The reason for creating the system as a microkernel is prosaic. Even
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  exist. The reason for creating the system as a microkernel is prosaic. Even
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  though it is initially more difficult to get the same level of functionality
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  though it is initially more difficult to get the same level of functionality
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  from a microkernel than it is in the case of a simple monolithic kernel, a
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  from a microkernel than it is in the case of a simple monolithic kernel, a
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  microkernel is much easier to maintain once the pieces have been put to work
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  microkernel is much easier to maintain once the pieces have been put to work
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    <indexterm>
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    <indexterm>
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      <primary>address space</primary>
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      <primary>address space</primary>
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    </indexterm>
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    </indexterm>
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    <para>Userspace tasks depend on support of address spaces provided by the
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    <para>Userspace tasks depend on support of address spaces provided by the
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    kernel. Each address space is a set of mutually dijunctive address space
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    kernel. Each address space is a set of mutually disjunctive address space
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    areas that group pages of common attributes. An address space area is
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    areas. An address space area is usually connected to, and backed by,
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    usually connected to, and backed by, anonymous memory, executable image of
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    anonymous memory, executable image of some program or continuous region of
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    some program or continuous region of physical memory. However, swapping
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    physical memory. However, swapping pages in and out to external memory is
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    pages in and out to external memory is not supported. Address space areas
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    not supported. Address space areas can be easily shared among address
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    can be easily shared among address spaces.</para>
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    spaces.</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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    <indexterm>
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    <indexterm>
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      <primary>IPC</primary>
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      <primary>IPC</primary>
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      <secondary>- message queue</secondary>
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      <secondary>- message queue</secondary>
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    </indexterm>
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    </indexterm>
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    <para>The abstraction uses terms like phones, calls and answerboxes, but
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    <para>The abstraction uses terms like phones, calls and answerboxes, but
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    is pretty similar to well-known abstraction of message queues. A task can
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    is similar to well-known abstraction of message queues. A task can have
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    have multiple simultaneous simplex connections to several other tasks. A
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    multiple simultaneous simplex connections to several other tasks. A
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    connection leads from one of the source task's phones to the destination
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    connection leads from one of the source task's phones to the destination
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    task's answerbox. The phones are used as handles for making calls to other
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    task's answerbox. The phones are used as handles for making calls to other
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    tasks. Calls can be synchronous or asynchronous and can be forwarded from
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    tasks. Calls are asynchronous and can be forwarded from one task to
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    one task to another.</para>
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    another.</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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