Source : Free On-Line Dictionary of Computing

IBM 3270
     
         A class of {terminals} made by {IBM} known as
        "Display Devices", normally used to talk to {IBM}
        {mainframes}.  The 3270 attempts to minimise the number of
        {I/O} {interrupts} required by accepting large blocks of data,
        known as datastreams, in which both text and control (or
        formatting functions) are interspersed allowing an entire
        screen to be "painted" as a single output operation.  The
        concept of "formatting" in these devices allows the screen to
        be divided into clusters of contiguous character cells for
        which numerous attributes (color, highlighting, {character
        set}, protection from modification) can be set.  Further,
        using a technique known as 'Read Modified' the changes from
        any number of formatted fields that have been modified can be
        read as a single input without transferring any other data,
        another technique to enhance the terminal throughput of the
        CPU.
     
        The 3270 had twelve, and later twenty-four, special Programmed
        Function Keys, or PF keys.  When one of these keys was
        pressed, it would cause the device to generate an I/O
        {interrupt} and present a special code identifying which key
        was pressed.  {Application program} functions such as
        termination, page-up, page-down or help could be invoked by a
        single key-push, thereby reducing the load on very busy
        processors.
     
        A version of the {IBM PC} called the "3270 PC" was released in
        October 1983.  It included 3270 {terminal emulation}.
     
        {tn3270} is modified version of {Telnet} which acts as a 3270
        {terminal emulator} and can be used to connect to an IBM
        computer over a network.
     
        See also {broken arrow}.
     
        (1995-02-07)