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            </mediaobject>
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            </mediaobject>
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        </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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        <para>
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        <termdef><glossterm>Thread</glossterm> is the basic execution primitive.</termdef>
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        </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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        Thread context is built of the context registers contents, FPU state and the stack.</termdef>
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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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            <itemizedlist>
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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>be an end-point in IPC</listitem>
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            </itemizedlist>
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        </termdef>
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        </para>
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        <para>
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        <termdef>
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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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        </termdef>
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        </para>
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        <para>
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        <termdef>
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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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        </termdef>
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        </para>
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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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        <para>
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            Though HelenOS was initially planned as a microkernel, we where trying to avoid several issues, connected
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            Though HelenOS was initially planned as a microkernel, we were trying to avoid several issues, connected
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            with microkernels, such as much higher overhead during memory management and hardware operations. For this reason
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            with microkernels, such as much higher overhead during memory management and hardware operations. For this reason
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            some of the subsytems, that are to be implmented as servers in classic microkernel design, where implemented
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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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            as a part of kernel, thus minimizing this overhead.
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            as a part of kernel, thus minimizing this overhead.
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        </para>
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        </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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        </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>Kernel primitives</title>
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        <title>IPC</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>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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        </para>
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        </para>
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        <para>
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        <para>Communication
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        <termdef><glossterm>Thread context</glossterm> represents state of the <emphasis>thread</emphasis>. Contains registers state, FPU state, stack(s).</termdef>
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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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        </para>
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         </para>
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    </section>
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    <section>
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        <title>Functionality model</title>
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        <para>
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        <para>
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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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        <termdef><glossterm>Task</glossterm> </termdef>
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        is performed by server tasks, that are running in userspace.
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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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        </para>
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        </para>
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        <para>
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        <para>
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        <termdef>
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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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        Moreover, problems experience the device drivers. Running in the user space, device driver still needs to recieve interrupts
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        </termdef>
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        and access hardware directly.
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        </para>
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        </para>
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        <para>
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        <para>
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        <termdef>
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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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        </termdef>
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        </para>             1
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        This raises two major problems in microkernels:
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    </section>
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            <orderedlist numeration="loweralpha">
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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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            <listitem>How this server task is going to access hardware or kernel while running in the user mode?</listitem>
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            </orderedlist>
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        </para>
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    <section>
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        <title>IPC</title>
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        <para>
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        <formalpara id="intro_ns">
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        <title>Name server</title>
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            Due to the high intertask communication traffic, IPC becomes critical subsystem for microkernels, putting high demands on the
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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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            speed, latency and reliability of IPC model and implementation. HelenOS IPC is designed in analogy with telephone communication.
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        just by the string service indentification.</para>
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        </formalpara>
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        <formalpara id="intro_ddi">
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        <title>Device driver interface</title>
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            Each task has an <emphasis>answerbox</emphasis> and a set of <emphasis>phones</emphasis> to call another tasks' answerboxes. Communication
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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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            is possible after the link establishment, and can be either <emphasis>asynchronious</emphasis> or <emphasis>synchronious</emphasis>.
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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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        </para>
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        </formalpara>
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    </section>
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    </section>
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    <section>
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        <para>Task ID - unique 64 bit number. Used for syscalls.</para>
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        <para>Contains threads</para>
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        <para>Address space is created per task</para>
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        <para>Memory mapping is per task</para>
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        <para>Context per thread. (Note 2 stacks on IA64).</para>
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        <para>IPC answer box associated per task</para>
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        <title>Memory management</title>
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        <para>
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        Zones - linked list (not many zones, so we can afford it. Can be replaced with B-tree in the future)
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        Number of zones depends on HW tables. Describe zone allocation/deallocation algoritm
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        </para>
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        <para>Page tables. 4 level hierarchical and hash directly supported. B+ Tree can be implemented.</para>
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        <para>For paging there is an abstract layer</para>
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        <para>TLB shootdown implementation (update TLB upon mapping update/remove).
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        TLB shootdown ASID/ASID:PAGE/ALL.
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        TLB shootdown requests can come in asynchroniously
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        so there is a cache of TLB shootdown requests. Upon cache overflow TLB shootdown ALL is executed</para>
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        <para>
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            Address spaces. Address space area (B+ tree). Only for uspace. Set of syscalls (shrink/extend etc).
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            Special address space area type - device - prohibits shrink/extend syscalls to call on it.
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            Address space has link to mapping tables (hierarchical - per Address space, hash - global tables).
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        </para>
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    </section>
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</chapter>
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</chapter>
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