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  man pages->Tru64 Unix man pages -> odl (5)              



NAME    [Toc]    [Back]

       odl, ODL - On-demand font loading

DESCRIPTION    [Toc]    [Back]

       On-demand  font loading (ODL) is a mechanism through which
       a terminal or a printer can download the  relevant  bitmap
       font information for a user-defined character (UDC) at the
       time this character has to be displayed or printed by  the
       device.  A number of our local language terminals (such as
       the VT382 series) and printers support ODL  for  languages
       like  Japanese,  Chinese,  and Korean, which include userdefined

       ODL can be implemented either through  hardware  or  software.
  In the hardware implementation of ODL, the terminal
       or printer sends a font loading request to the  host  computer
  whenever  the  device  encounters  a UDC whose font
       information is not present in the device.  The  host  computer
  then  completes  the  transaction by sending to the
       device a font loading sequence  that  encodes  the  bitmap
       shape of the character. This font information stays in the
       device until the font information buffer is  full  or  the
       power is switched off.

       In the software implementation of ODL (SoftODL), the sending
 of the font loading request from the  device  is  disabled
  and  it  is  the  host computer that keeps track of
       which font information has been sent to the  device.  When
       the host computer encounters a new UDC or an old one whose
       font information must be flushed because the font information
  buffer is full, the host computer sends a font loading
 sequence to the device before sending the UDC  character
 code in the data to be displayed or printed.

       The  SoftODL  mechanism is supported only by the Asian tty
       driver, atty, and the odld daemon through the  utx  driver
       (see  atty(7),  odld(8),  and  utx(7)).Users run the cedit
       utility (see cedit(1)) to  create  and  edit  user-defined
       characters  in a UDC database. The cgen command is used to
       create from the UDC database an ODL database that is  used
       by the SoftODL mechanism.

       The  SoftODL mechanism can be tuned by setting the following
 parameters: The ODL buffer replacement strategy

              When the font information buffer is full, there are
              two ways to decide which piece of old font information
 is to be replaced by  information  for  a  UDC
              whose  information  is  not in the buffer. They are
              FIFO (First-In-First-Out) and LRU  (Least  Recently
              Used).  The size of the ODL font cache

              The  size of the font cache determines how much UDC
              font information can be kept in  the  device.  This
              value  is  device-specific. The value 256 is a safe
              maximum for most devices, although  some  terminals
              are  capable  of  storing  font information for 700

       Refer to the stty(1) reference  page  for  information  on
       setting  both  these  parameters  and  using  other  flags
       related to ODL.

       In general, ODL implemented in the hardware is more  accurate
  in  terms  of  fetching a UDC font only when needed.
       However, the overhead of  sending  font  loading  requests
       from  device  to host makes hardware ODL slower than software
 ODL.  SoftODL is faster and more flexible, but it has
       the  following  limitations:  Some  local language devices
       have to have the hardware On-Demand Loading flag  disabled
       before the devices can be used with the SoftODL mechanism.
       When a UDC is keyed in for some of  the  VT382  series  of
       local  language terminals, the character is also displayed
       on the 26th compose line  of  the  terminal  screen.  This
       capability  depends  on having the character's font loaded
       in the device. When SoftODL is being used  and  a  UDC  is
       entered  for the first time into the terminal after powerup,
 the correct font of that character does not be display
       on  the  compose  line.  Instead, a reversed wide question
       mark is displayed. However, the UDC will be displayed correctly
  on  other  parts  of  the  screen.   If SoftODL is
       enabled for a terminal session and the terminal  is  accidentally
  powered off, users must issue the following command
 immediately after turning on the power: This  command
       flushes the internal tty cache to avoid a mismatch between
       the font cache of the tty and that of the terminal.   When
       a  terminal  is  connected by a terminal server to several
       sessions, only one  of  those  sessions  should  have  ODL
       (software  or  hardware) enabled. If more than one session
       has ODL enabled, you may have to issue one of the  following
  commands  to avoid problems: This command flushes the
       internal tty cache when you return to the terminal session
       where  SoftODL  is being used.  This command instructs the
       tty driver to use SoftODL for all the user-defined characters.
   However,  this  option  also  prevents caching and
       slows the display of UDCs tremendously.  When users change
       session while SoftODL is enabled, their terminals may hang
       due to an incomplete escape sequence. This  condition  can
       be  corrected  by using the setup menu of the terminal and
       activating the "Reset Terminal" soft  key.   Changing  the
       terminal  line  discipline  while  SoftODL is active would
       cause all the SoftODL parameters stored in the tty  driver
       to be reset to their default values.  So, SoftODL is automatically
 deactivated if the line  discipline  is  changed
       and  must  be  restarted  after the change.  Users must be
       careful when setting the odlsize  parameter  of  the  stty
       command.  The optimal value for this parameter is the size
       of the character  font  cache  in  the  current  terminal.
       Users  can  set a value less than or equal to the hardware
       parameter, but must never set a value  larger  than  that.
       If  the stty parameter is larger than the hardware parameter,
 the SoftODL mechanism cannot operate properly.   Care
       must also be taken in setting the odltype parameter of the
       stty command. This parameter must be set to the same  type
       as  that  of  the  terminal.   A  mismatch between the tty
       driver  and  terminal  settings  can  produce   unexpected
       results.   If  the ODL database is changed, the change may
       not be reflected in any  SoftODL  terminal  sessions  that
       were  activated  prior  to  the  change.   For the already
       active sessions to recognize any new UDC characters in the
       ODL  database,  users  have  to  stop and then restart the
       SoftODL mechanism by through the stty command.  This problem
 also exists for hardware ODL.

SEE ALSO    [Toc]    [Back]

       Commands: cedit(1), cgen(1), stty(1), odld(8), utxd(8)

       Files: atty(7), utx(7)

[ Back ]
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