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  understand them completely. In addition, a microkernel based operating
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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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  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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  be used even without some traditional subsystems (e.g. block devices,
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  filesystems and networking).</para>
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  filesystems and networking).</para>
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  <figure>
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  <figure><mediaobject id="arch1" xreflabel="">
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    <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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        <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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    </mediaobject>
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    <title>HelenOS architecture overview.</title>
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    <title>HelenOS architecture overview.</title>
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    </figure>
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  </figure>
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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 the 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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  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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  tasks with different roles, capabilities and privileges. Some of the tasks
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  via kernel-provided IPC.</para>
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  via kernel-provided IPC.</para>
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  <section>
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  <section>
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    <title>Scheduling</title>
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    <title>Scheduling</title>
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    <indexterm>
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      <primary>thread</primary>
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    </indexterm>
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    <para>Kernel's unit of execution flow is a thread. A thread is an entity
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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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    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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    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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    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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    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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    they provide (i.e. several threads implement functionality of one task).
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    <indexterm>
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        <primary>task</primary>
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    Tasks serve as containers of threads, they provide linkage to address
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      </indexterm> Tasks serve as containers of threads, they provide linkage
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    space and are communication endpoints for IPC. Finally, tasks can be
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    to address space and are communication endpoints for IPC. Finally, tasks
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    holders of capabilities that entitle them to do certain sensitive
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    can be holders of capabilities that entitle them to do certain sensitive
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    operations (e.g access raw hardware and physical memory).</para>
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    operations (e.g access raw hardware and physical memory).</para>
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    <para>The scheduler deploys several run queues on each processor. A thread
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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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    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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    priority and its current processor, from where it is eventually picked up
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    <para>The virtual address translation layer currently supports two
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    <para>The virtual address translation layer currently supports two
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    mechanisms for mapping virtual memory pages to physical memory frames
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    mechanisms for mapping virtual memory pages to physical memory frames
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    (i.e. 4-level hierarchical page tables and global page hash table), and is
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    (i.e. 4-level hierarchical page tables and global page hash table), and is
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    further extensible to other mechanisms.</para>
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    further extensible to other mechanisms.</para>
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    <indexterm>
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      <primary>address space</primary>
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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 dijunctive address space
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    areas that group pages of common attributes. An address space area is
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    areas that group pages of common attributes. An address space area is
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    usually connected to, and backed by, anonymous memory, executable image of
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    usually connected to, and backed by, anonymous memory, executable image of
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    some program or continuous region of physical memory. However, swapping
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    some program or continuous region of physical memory. However, swapping
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    pages in and out to external memory is not supported. Address space areas
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    pages in and out to external memory is not supported. Address space areas
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    can be easily shared among address spaces.</para>
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    can be easily shared among address 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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      <primary>IPC</primary>
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    </indexterm>
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    <title>IPC</title>
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    <title>IPC</title>
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    <para>Due to the fact that HelenOS is a microkernel, strong emphasis is
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    <para>Due to the fact that HelenOS is a microkernel, strong emphasis is
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    put on its IPC (Inter-Process Communication<footnote>
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    put on its IPC (Inter-Process Communication<footnote>
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        <para>The term Inter-Process Communication is slightly confusing
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        <para>The term Inter-Process Communication is slightly confusing
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        to refer to communication among tasks.</para>
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        to refer to communication among tasks.</para>
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      </footnote>). Tasks communicate by passing very short messages to one
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      </footnote>). Tasks communicate by passing very short messages to one
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    another or by sending (i.e. sharing) address space areas when larger data
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    another or by sending (i.e. sharing) address space areas when larger data
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    is to be transfered.</para>
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    is to be transfered.</para>
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    <indexterm>
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      <primary>IPC</primary>
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      <secondary>- phone</secondary>
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    </indexterm>
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    <indexterm>
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      <primary>IPC</primary>
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      <secondary>- answerbox</secondary>
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    </indexterm>
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    <indexterm>
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      <primary>IPC</primary>
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      <secondary>- message queue</secondary>
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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 pretty similar to well-known abstraction of message queues. A task can
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    have multiple simultaneous simplex connections to several other tasks. A
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    have 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