.. _glossary:

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Glossary
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..   :sorted:

.. glossary::

    classic process
        A :term:`process` with only one thread.

    context switch
        Switching from running a user level process to the OS kernel and often
        to other user processes before the current process is resumed.  See
        :ref:`switch`

    CPU scheduling
        Determine which process in memory is executed by the 
        CPU at any given point.

    critical section
        A portion of a program that modifies a shared resources.  If other
        processes or threads also modify the shared resource at the same time,
        data inconsistencies could occur.

        .. seealso:: :ref:`race_condition`

    deadlock
        The condition where two or more processes are not able to continue
        because they are waiting on each other to either release a resource
        or do something, but that will not happen because they are waiting 
        on the other.

    demand paging
        A virtual memory management scheme where the pages of process are
        loaded only as they are needed.

    logical address
        The memory address used internally within each process.  This
        address space is contiguous and usually begins at zero and can 
        extend to the largest address allowed (4GB for 32 bit systems).
        Also called :term:`virtual address`.


    memory management
        Keeping track of what programs are in memory and where in 
        memory they reside.

    monitor
        An abstract data type for which only one process/thread may be
        executing of of its member procedures at any given time.

    multiprogramming 
        The technique of keeping multiple programs in main memory at the same
        time. These programs compete for access to the CPU so that they can
        execute

    mutual exclusion 
        Allowing only one process to access a :term:`critical section` at one
        time.

    page fault
        A CPU generated trap occuring a referenced physical memory frame is
        not currently loaded in physical memory.

    process 
        An instance of a program in execution. A thread may contain one or more
        :term:`thread`.

    process management 
        Keeping track the progress of processes and all of their
        intermediate states.

    process control block
        A data structure in the OS kernel's memory holding all data that the OS
        needs about a process.  All of the process control blocks are kept in
        an array indexed by the PID for each process.  See :ref:`switch`,
        :ref:`pcb`.

    process scheduling
        The act of determining which process in the ready state should be
        moved to the running state. That is, decide which :term:`process`
        should run on the CPU next.

    physical address
        The memory address used to locate data in the computer's physical
        memory banks.

    race condition
        When correct execution depends on the timing of processes or threads
        running a :term:`critical section` of code.

    semaphore
        A fundamental locking mechanism that can be used to allow a varied
        number (1 is the default) of processes or threads access to a
        :term:`critical section`.

    starvation
        When a lower priority item is unfairly delayed by higher priority items
        that arrive later.  This can occur in both :ref:`scheduler` and disk
        scheduling.

    system call
        A request from an application program for the operating system to
        perform some hardware action on behalf of the application.  System
        calls are initiated with a software interrupt assembly language
        instruction.  The argument to the interrupt instruction is a number,
        which corresponds to the index of an array in the OS kernel with
        pointers to the functions that provide the requested service.

        .. seealso::  :ref:`system_calls`

    thread 
        The basic unit to which the operating system allocates processor time.
        A thread can execute any part of the :term:`process` code, including
        parts currently being executed by another thread. 

    virtual address
        See :term:`logical address`.

    virtual machine
        The allusion provided to each process that they are the only process
        running on the computer.

    virtual memory
        Memory on a secondary storage device, such as a hard drive or solid
        state drive, that augments the computer's physical radom access
        memory.  Virtual memory is used to give the computer users the
        benefit of more memory than is physically present in the computer.