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<chapter id="architecture">
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<chapter id="architecture">
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  <?dbhtml filename="arch.html"?>
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  <title>Architecture overview</title>
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  <title>Architecture overview</title>
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  <para>The HelenOS operating system is designed as a relatively small
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  <para>The HelenOS operating system is designed as a relatively small
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  microkernel assisted with a set of userspace drivers and server tasks.
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  microkernel assisted with a set of userspace drivers and server tasks.
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  HelenOS is not very radical in what subsystems should or should not be
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  HelenOS is not very radical in what subsystems should or should not be
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  implemented in the kernel - in some cases, both kernel and userspace drivers
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  implemented in the kernel - in some cases, both kernel and userspace drivers
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  exist. The reason for creating the system as a microkernel is prosaic. Even
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  exist. The reason for creating the system as a microkernel is prosaic. Even
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  though it is initially more difficult to get the same level of functionality
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  though it is initially more difficult to get the same level of functionality
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  from a microkernel than it is in the case of a simple monolithic kernel, a
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  from a microkernel than it is in the case of a simple monolithic kernel, a
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  microkernel is much easier to maintain once the pieces have been put to work
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  microkernel is much easier to maintain once the pieces have been put to work
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  together. Therefore, the kernel of HelenOS, as well as the essential
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  together. Therefore, the kernel of HelenOS, as well as the essential
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  userspace libraries thereof can be maintained by only a few developers who
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  userspace libraries thereof can be maintained by only a few developers who
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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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      <caption>HelenOS architecture overview</caption>
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      <caption>HelenOS architecture overview</caption>
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    </mediaobject></para>
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    </mediaobject></para>
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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 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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  serve as device drivers, naming servers, managers of various kinds and some
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  serve as device drivers, naming servers, managers of various kinds and some
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  are just ordinary user programs. All of them communicate with other threads
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  are just ordinary user programs. All of them communicate with other threads
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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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    <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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    Tasks serve as containers of threads, they provide linkage to address
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    Tasks serve as containers of threads, they provide linkage to address
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    space and are communication endpoints for IPC.</para>
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    space and are communication endpoints for IPC.</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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    by the scheduler. Special purpose kernel threads strive to keep processors
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    by the scheduler. Special purpose kernel threads strive to keep processors
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    balanced by thread migration. Threads are scheduled by the round robing
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    balanced by thread migration. Threads are scheduled by the round robing
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    scheduling policy with respect to multiple priority run queues.</para>
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    scheduling policy with respect to multiple priority run queues.</para>
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  </section>
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  <section>
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    <title>Memory management</title>
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    <para>Memory management is another large subsystem in HelenOS. It serves
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    the kernel to satisfy its own memory allocation requests, provides
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    translation between virtual and physical memory addresses and manages
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    virtual address spaces of userspace tasks.</para>
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    <para>Kernel allocates memory from the slab allocator, which itself
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    allocates memory from a buddy system based allocator of physical memory
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    frames.</para>
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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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    (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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    <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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    areas that group pages of common attributes. An address space area is
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    usually connected to, and backed by, an anonymous memory or an executable
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    image of some program. Anonymous memory address space areas can be easily
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    shared among address spaces.</para>
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  </section>
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  <section>
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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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    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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        because in HelenOS terminology there are tasks instead of processes.
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        However, its abbreviation, IPC, is being publicly used as a standard
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        name for similar facilities. This book will therefore use the term IPC
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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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    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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    <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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    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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    task's answerbox. The phones are used as handles for making calls to other
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    tasks. Calls can be synchronous or asynchronous and can be forwarded from
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    <para></para>
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    one task to another.</para>
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  </section>
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  </section>
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</chapter>
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</chapter>