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<chapter id="architecture">
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<chapter id="architecture">
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  <?dbhtml filename="arch.html"?>
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  <?dbhtml filename="arch.html"?>
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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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  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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  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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  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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  microkernel is much easier to maintain once the pieces have been put to work
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  together. Therefore, the kernel of HelenOS, as well as the essential
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  userspace libraries thereof can be maintained by only a few developers who
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  <section>
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  understand them completely. In addition, a microkernel based operating
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  system reaches completion sooner than monolithic kernels as the system can
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  be used even without some traditional subsystems (e.g. block devices,
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    <para>
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  filesystems and networking).</para>
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      <mediaobject id="arch1">
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  <para><mediaobject id="arch1" xreflabel="">
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        <imageobject role="html">
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      <imageobject role="html">
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          <imagedata fileref="images/arch1.png" format="PNG" />
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        <imagedata fileref="images/arch1.png" format="PNG" />
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        </imageobject>
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      </imageobject>
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        <imageobject role="fop">
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      <imageobject role="fop">
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          <imagedata fileref="images.vector/arch1.svg" format="SVG" />
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        </imageobject>
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      </imageobject>
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      </mediaobject>
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    </para>
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  </section>
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  <section>
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    <para>The HelenOS operating system is designed as a relatively small
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      <caption>HelenOS architecture overview</caption>
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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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    implemented in the kernel - in some cases, both kernel and userspace
 - 
 
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    drivers exist. The reason for creating the system as a microkernel is
 - 
 
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    prosaic. Even though it is initially more difficult to get the same level
 - 
 
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    of functionality from a microkernel than it is in the case of a simple
 - 
 
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    monolithic kernel, a microkernel is much easier to maintain once the
 - 
 
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    pieces have been put to work together. Therefore, the kernel of HelenOS,
 - 
 
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    as well as the essential userspace libraries thereof can be maintained by
 - 
 
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    only a few developers who understand them completely. In addition, a
 - 
 
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    microkernel based operating system reaches completion sooner than
 - 
 
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    monolithic kernels as the system can be used even without some traditional
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    subsystems (e.g. block devices, filesystems and networking).</para>
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    </mediaobject></para>
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    <para>HelenOS is comprised of the kernel and userspace server tasks. The
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  <para>HelenOS is comprised of the kernel and userspace server tasks. The
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    kernel provides scheduling, memory management and IPC. It also contains
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  kernel provides scheduling, memory management and IPC. It also contains
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    essential device drivers that control the system clock and other devices
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  essential device drivers that control the system clock and other devices
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    necessary to guarantee a safe environment. Userspace communicates with the
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  necessary to guarantee a safe environment. Userspace communicates with the
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  kernel through a small set of syscalls. The userspace layer consists of
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  tasks with different roles, capabilities and privileges. Some of the tasks
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  serve as device drivers, naming servers, managers of various kinds and some
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  are just ordinary user programs. All of them communicate with other threads
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    kernel through </para>
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  via kernel-provided IPC.</para>
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  <section>
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    <title>Scheduling</title>
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    <para>Kernel's unit of execution flow is a thread. A thread is an entity
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    that executes code and has a stack that takes up some space in memory. The
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    relation between kernel and userspace threads is 1:1:n, meaning that there
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    can be several pseudo threads running within one userspace thread that
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    maps to one kernel thread. Threads are grouped into tasks by functionality
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    they provide (i.e. several threads implement functionality of one task).
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    Tasks serve as containers of threads, they provide linkage to address
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    space and are communication endpoints for IPC.</para>
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    <para>The scheduler deploys several run queues on each processor. A thread
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    ready for execution is put into one of the run queues, depending on its
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    priority and its current processor, from where it is eventually picked up
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    by the scheduler. Special purpose kernel threads strive to keep processors
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    balanced by thread migration. Threads are scheduled by the round robing
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    scheduling policy with respect to multiple priority run queues.</para>
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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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