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  </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>

    <para>Userspace tasks depend on support of address spaces provided by the

    kernel. Each address space is a set of mutually dijunctive address space

    areas that group pages of common attributes. An address space area is

    usually connected to, and backed by, an anonymous memory or an executable

    image of some program. Anonymous memory address space areas can be easily

    shared among address spaces.</para>

  </section>

  <section>

    <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>

    <para>The abstraction uses terms like phones, calls and answerboxes, but

    is pretty 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 can be synchronous or asynchronous and can be forwarded from