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<chapter id="architecture">

  <?dbhtml filename="arch.html"?>

  <title>Architecture overview</title>

  <section>

    <title>Scheme</title>

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  </section>

  <section>

    <title>Kernel primitives</title>

    <para><termdef><glossterm>Thread</glossterm> is the basic execution

    primitive.</termdef></para>

    <para><termdef><glossterm>Thread context</glossterm> represents state of

    the <emphasis>thread</emphasis>. Thread context is built of the context

    registers contents, FPU state and the stack.</termdef></para>

    <para><termdef><glossterm>Task</glossterm> is a multi-purpose entity,

    serving to incorporate set if its threads, provide common address space to its threads,

    be an end-point in IPC.</termdef></para>

    <para><termdef><glossterm>Address space area</glossterm> is a mutually

    disjunctive range of memory with the code, stack and data.

    </termdef></para>

    <para><termdef><glossterm>Address space</glossterm> is a aggregating

    entity for address space areas, connecting them to the task.

    </termdef></para>

  </section>

  <section>

    <title>Monolithic microkernel</title>

    <para>Though HelenOS was initially planned as a microkernel, we were

    trying to avoid several issues, connected with microkernels, such as much

    higher overhead during memory management and hardware operations. For this

    reason some of the subsystems, that are to be implemented as servers in

    classic microkernel design, were implemented as a part of kernel, thus

    minimizing this overhead.</para>

    <formalpara>

      <title>Memory management</title>

      <para>HelenOS has all its memory

      management functionality in the kernel, available to the memory

      management server via the set of syscalls.</para>

    </formalpara>

    <formalpara>

      <title>Kernel device drivers</title>

      <para>HelenOS kernel has some of the very basic device drivers

      <itemizedlist>

          <listitem>

            ACPI

          </listitem>

          <listitem>

            APIC

          </listitem>

          <listitem>

            SMP configuration

          </listitem>

          <listitem>

            System clock

          </listitem>

          <listitem>

            Interrupt controllers

          </listitem>

          <listitem>

            Console and frame buffer

          </listitem>

        </itemizedlist></para>

    </formalpara>

  </section>

  <section>

    <title>IPC</title>

    <para>HelenOS IPC is designed in analogy with telephone communication.

    Each task has an <emphasis>answerbox</emphasis> and a set of

    <emphasis>phones</emphasis> to call another tasks' answerboxes.</para>

    <para>Communication is possible after the connection is established, and

    can be either <emphasis>asynchronious</emphasis> or

    <emphasis>synchronious</emphasis>.</para>

  </section>

  <section>

    <title>Functionality model</title>

    <para>As you know, microkernel design is very simple, just enough to

    provide communication facility for tasks. Most of the OS functionality is

    performed by server tasks, that are running in userspace. Thus most of the

    system calls in monolithic kernels, are the IPC calls on server tasks in

    microkernels.</para>

    <para>Moreover, problems experience the device drivers. Running in the

    user space, device driver still needs to recieve interrupts and access

    hardware directly.</para>

    <para>This raises two major problems in microkernels: <orderedlist

        numeration="loweralpha">

        <listitem>

          What is the recipient address of the server (e.g. "memory manager" or a specific device driver) ?

        </listitem>

        <listitem>

          How this server task is going to access hardware or kernel while running in the user mode?

        </listitem>

      </orderedlist></para>

    <formalpara id="intro_ns">

      <title>Name server</title>

      <para>As every microkernel, HelenOS has a "Name server" task with "well

      known" IPC address, that connects user task to any server just by the

      string service indentification.</para>

    </formalpara>

    <formalpara id="intro_ddi">

      <title>Device driver interface</title>

      <para>Device drivers use special syscalls to map physical memory areas

      into their address space, to map port regions (mostly ia32). Interrupts

      are delivered to the device driver task by the standard IPC

      means.</para>

    </formalpara>

  </section>

</chapter>