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<?xml version="1.0" encoding="UTF-8"?>
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<?xml version="1.0" encoding="UTF-8"?>
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<chapter id="architecture">
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  <?dbhtml filename="arch.html"?>
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<chapter id="architecture"><?dbhtml filename="arch.html"?>
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    <title>Architecture overview</title>
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  <title>Architecture overview</title>
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    <section>
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  <section>
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        <title>Scheme</title>
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    <title>Scheme</title>
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        <para>
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    <para><mediaobject id="arch1">
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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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                </imageobject>
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        </imageobject>
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                <imageobject role="fop">
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        <imageobject role="fop">
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        </imageobject>
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            </mediaobject>
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      </mediaobject></para>
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        </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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        <title>Kernel primitives</title>
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    <title>Kernel primitives</title>
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        <para>
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        <termdef><glossterm>Thread</glossterm> is the basic execution primitive.</termdef>
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    <para><termdef><glossterm>Thread</glossterm> is the basic execution
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        </para>
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    primitive.</termdef></para>
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        <para>
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        <termdef><glossterm>Thread context</glossterm> represents state of the <emphasis>thread</emphasis>.
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    <para><termdef><glossterm>Thread context</glossterm> represents state of
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    the <emphasis>thread</emphasis>. Thread context is built of the context
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        Thread context is built of the context registers contents, FPU state and the stack.</termdef>
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    registers contents, FPU state and the stack.</termdef></para>
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        </para>
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        <para>
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        <termdef>
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            <glossterm>Task</glossterm> is a multi-purpose entity, serving to
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    <para><termdef> <glossterm>Task</glossterm> is a multi-purpose entity,
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            <itemizedlist>
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    serving to <itemizedlist>
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                <listitem>incorporate set if its threads</listitem>
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        <listitem>incorporate set if its threads</listitem>
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                <listitem>provide common address space to its threads</listitem>
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        <listitem>provide common address space to its threads</listitem>
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                <listitem>be an end-point in IPC</listitem>
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            </itemizedlist>
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        </termdef>
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        <listitem>be an end-point in IPC</listitem>
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        </para>
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      </itemizedlist> </termdef></para>
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        <para>
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        <termdef>
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    <para><termdef> <glossterm>Address space area</glossterm> is a mutually
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            <glossterm>Address space area</glossterm> is a mutually disjunctive range of memory with the code, stack and data.
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    disjunctive range of memory with the code, stack and data.
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        </termdef>
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    </termdef></para>
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        </para>
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        <para>
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        <termdef>
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    <para><termdef> <glossterm>Address space</glossterm> is a aggregating
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            <glossterm>Address space</glossterm> is a aggregating entity for address space areas, connecting them to the task.
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    entity for address space areas, connecting them to the task.
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        </termdef>
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    </termdef></para>
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        </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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        <title>Monolithic microkernel</title>
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    <title>Monolithic microkernel</title>
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        <para>
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            Though HelenOS was initially planned as a microkernel, we were trying to avoid several issues, connected
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    <para>Though HelenOS was initially planned as a microkernel, we were
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    trying to avoid several issues, connected with microkernels, such as much
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            with microkernels, such as much higher overhead during memory management and hardware operations. For this reason
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    higher overhead during memory management and hardware operations. For this
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            some of the subsystems, that are to be implemented as servers in classic microkernel design, were implemented
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    reason some of the subsystems, that are to be implemented as servers in
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            as a part of kernel, thus minimizing this overhead.
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    classic microkernel design, were implemented as a part of kernel, thus
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        </para>
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    minimizing this overhead.</para>
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        <formalpara>
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    <formalpara>
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        <title>Memory management</title>
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      <title>Memory management</title>
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        <para>
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      <para>Unlike the classic microkernel, HelenOS has all its memory
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            Unlike the classic microkernel, HelenOS has all its memory management functionality in the kernel, available to the memory
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      management functionality in the kernel, available to the memory
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            management server via the set of syscalls.
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      management server via the set of syscalls.</para>
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        </para>
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        </formalpara>
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    </formalpara>
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        <formalpara>
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    <formalpara>
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        <title>Kernel device drivers</title>
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      <title>Kernel device drivers</title>
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        <para>
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            HelenOS kernel has some of the very basic device drivers
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            <itemizedlist>
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            <listitem>ACPI</listitem>
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            <listitem>APIC</listitem>
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            <listitem>SMP configuration</listitem>
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            <listitem>System clock</listitem>
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            <listitem>Interrupt controllers</listitem>
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            <listitem>Console</listitem>
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            <listitem>VESA &amp; frame buffer</listitem>
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            </itemizedlist>
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      <para>HelenOS kernel has some of the very basic device drivers
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      <itemizedlist>
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          <listitem>
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            ACPI
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        </para>
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          </listitem>
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          <listitem>
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            APIC
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          </listitem>
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          <listitem>
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            SMP configuration
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          </listitem>
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          <listitem>
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            System clock
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          </listitem>
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          <listitem>
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            Interrupt controllers
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          </listitem>
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          <listitem>
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            Console
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          </listitem>
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          <listitem>
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            VESA &amp; frame buffer
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          </listitem>
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        </itemizedlist></para>
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        </formalpara>
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    </formalpara>
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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>IPC</title>
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    <title>IPC</title>
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        <para>HelenOS IPC is designed in analogy with telephone communication.
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    <para>HelenOS IPC is designed in analogy with telephone communication.
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            Each task has an <emphasis>answerbox</emphasis> and a set of <emphasis>phones</emphasis> to call another tasks' answerboxes.
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    Each task has an <emphasis>answerbox</emphasis> and a set of
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        </para>
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        <para>Communication
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            is possible after the connection is established, and can be either <emphasis>asynchronious</emphasis> or <emphasis>synchronious</emphasis>.
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    <emphasis>phones</emphasis> to call another tasks' answerboxes.</para>
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         </para>
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    <para>Communication is possible after the connection is established, and
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    can be either <emphasis>asynchronious</emphasis> or
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    <emphasis>synchronious</emphasis>.</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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        <title>Functionality model</title>
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    <title>Functionality model</title>
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        <para>
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    <para>As you know, microkernel design is very simple, just enough to
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        As you know, microkernel design is very simple, just enough to provide communication facility for tasks. Most of the OS functionality
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    provide communication facility for tasks. Most of the OS functionality is
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        is performed by server tasks, that are running in userspace.
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    performed by server tasks, that are running in userspace. Thus most of the
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        Thus most of the system calls in monolithic kernels, are the IPC calls on server tasks in microkernels.
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    system calls in monolithic kernels, are the IPC calls on server tasks in
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        </para>
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    microkernels.</para>
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        <para>
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        Moreover, problems experience the device drivers. Running in the user space, device driver still needs to recieve interrupts
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    <para>Moreover, problems experience the device drivers. Running in the
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    user space, device driver still needs to recieve interrupts and access
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        and access hardware directly.
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    hardware directly.</para>
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        </para>
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        <para>
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        This raises two major problems in microkernels:
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    <para>This raises two major problems in microkernels: <orderedlist
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            <orderedlist numeration="loweralpha">
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        numeration="loweralpha">
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        <listitem>
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            <listitem>What is the recipient address of the server (e.g. "memory manager" or a specific device driver) ?</listitem>
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          What is the recipient address of the server (e.g. "memory manager" or a specific device driver) ?
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        </listitem>
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        <listitem>
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            <listitem>How this server task is going to access hardware or kernel while running in the user mode?</listitem>
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          How this server task is going to access hardware or kernel while running in the user mode?
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            </orderedlist>
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        </listitem>
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        </para>
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      </orderedlist></para>
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        <formalpara id="intro_ns">
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    <formalpara id="intro_ns">
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        <title>Name server</title>
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      <title>Name server</title>
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        <para>As every microkernel, HelenOS has a "Name server" task with "well known" IPC address, that connects user task to any server
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        just by the string service indentification.</para>
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        </formalpara>
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      <para>As every microkernel, HelenOS has a "Name server" task with "well
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      known" IPC address, that connects user task to any server just by the
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      string service indentification.</para>
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    </formalpara>
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        <formalpara id="intro_ddi">
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    <formalpara id="intro_ddi">
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        <title>Device driver interface</title>
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      <title>Device driver interface</title>
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        <para>Device drivers use special syscalls to map physical memory areas into their address space, to map port regions (mostly ia32).
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        Interrupts are delivered to the device driver task by the standard IPC means.
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        </para>
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        </formalpara>
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      <para>Device drivers use special syscalls to map physical memory areas
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      into their address space, to map port regions (mostly ia32). Interrupts
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      are delivered to the device driver task by the standard IPC
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      means.</para>
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    </formalpara>
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    </section>
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  </section>
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</chapter>
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</chapter>
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