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<chapter id="architecture">
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  <title>Architecture Overview</title>

  <para>The HelenOS operating system is designed as a relatively small
  microkernel assisted with a set of userspace drivers and server tasks.
  HelenOS is not very radical in which subsystems should or should not be
  implemented in the kernel - in some cases, both kernel and userspace drivers
  exist. The reason for creating the system as a microkernel is prosaic. Even
  though it is initially more difficult to get the same level of functionality
  from a microkernel than it is in the case of a simple monolithic kernel, a
  microkernel is much easier to maintain once the pieces have been put to work
  together. Therefore, the kernel of HelenOS, as well as the essential
  userspace libraries thereof can be maintained by only a few developers who
  understand them completely. In addition, a microkernel based operating
  system reaches completion sooner than monolithic kernels as the system can
  be used even without some traditional subsystems (e.g. block devices,
  filesystems and networking).</para>

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    <title>HelenOS architecture overview.</title>
  </figure>

  <para>HelenOS is comprised of the kernel and the userspace server tasks. The
  kernel provides scheduling, memory management and IPC. It also contains
  essential device drivers that control the system clock and other devices
  necessary to guarantee a safe environment. Userspace communicates with the
  kernel through a small set of syscalls. The userspace layer consists of
  tasks with different roles, capabilities and privileges. Some of the tasks
  serve as device drivers, naming servers, managers of various kinds and some
  are just ordinary user programs. All of them communicate with other threads
  via kernel-provided IPC.</para>

  <section>
    <title>Scheduling</title>

    <indexterm>
      <primary>thread</primary>
    </indexterm>

    <para>Kernel's unit of execution flow is a thread. A thread is an entity
    that executes code and has a stack that takes up some space in memory. The
    relation between kernel and userspace threads is 1:1:n, meaning that there
    can be several pseudo threads running within one userspace thread that
    maps to one kernel thread. Threads are grouped into tasks by functionality
    they provide (i.e. several threads implement functionality of one task).
    <indexterm>
        <primary>task</primary>
      </indexterm> Tasks serve as containers of threads, they provide linkage
    to address space and are communication endpoints for IPC. Finally, tasks
    can be holders of capabilities that entitle them to do certain sensitive
    operations (e.g access raw hardware and physical memory).</para>

    <para>The scheduler deploys several run queues on each processor. A thread
    ready for execution is put into one of the run queues, depending on its
    priority and its current processor, from where it is eventually picked up
    by the scheduler. Special purpose kernel threads strive to keep processors
    balanced by thread migration. Threads are scheduled by the round robing
    scheduling policy with respect to multiple priority run queues.</para>
  </section>

  <section>
    <title>Memory Management</title>

    <para>Memory management is another large subsystem in HelenOS. It serves
    the kernel to satisfy its own memory allocation requests, provides
    translation between virtual and physical memory addresses and manages
    virtual address spaces of userspace tasks.</para>

    <para>Kernel allocates memory from the slab allocator, which itself
    allocates memory from a buddy system based allocator of physical memory
    frames.</para>

    <para>The virtual address translation layer currently supports two
    mechanisms for mapping virtual memory pages to physical memory frames
    (i.e. 4-level hierarchical page tables and global page hash table), and is
    further extensible to other mechanisms.</para>

    <indexterm>
      <primary>address space</primary>
    </indexterm>

    <para>Userspace tasks depend on support of address spaces provided by the
    kernel. Each address space is a set of mutually disjunctive address space
    areas. An address space area is usually connected to, and backed by,
    anonymous memory, executable image of some program or continuous region of
    physical memory. However, swapping pages in and out to external memory is
    not supported. Address space areas can be easily shared among address
    spaces.</para>
  </section>

  <section>
    <indexterm>
      <primary>IPC</primary>
    </indexterm>

    <title>IPC</title>

    <para>Due to the fact that HelenOS is a microkernel, strong emphasis is
    put on its IPC (Inter-Process Communication<footnote>
        <para>The term Inter-Process Communication is slightly confusing
        because in HelenOS terminology there are tasks instead of processes.
        However, its abbreviation, IPC, is being publicly used as a standard
        name for similar facilities. This book will therefore use the term IPC
        to refer to communication among tasks.</para>
      </footnote>). Tasks communicate by passing very short messages to one
    another or by sending (i.e. sharing) address space areas when larger data
    is to be transfered.</para>

    <indexterm>
      <primary>IPC</primary>

      <secondary>- phone</secondary>
    </indexterm>

    <indexterm>
      <primary>IPC</primary>

      <secondary>- answerbox</secondary>
    </indexterm>

    <indexterm>
      <primary>IPC</primary>

      <secondary>- message queue</secondary>
    </indexterm>

    <para>The abstraction uses terms like phones, calls and answerboxes, but
    is similar to well-known abstraction of message queues. A task can have
    multiple simultaneous simplex connections to several other tasks. A
    connection leads from one of the source task's phones to the destination
    task's answerbox. The phones are used as handles for making calls to other
    tasks. Calls are asynchronous and can be forwarded from one task to
    another.</para>
  </section>
</chapter>