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odl(5)									odl(5)

       odl, ODL - On-demand font loading

       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 user-defined characters.

       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 com‐
       puter 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 dis‐
       played 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	 Soft‐
       ODL 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 informa‐
	      tion  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 UDCs.

       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	 capa‐
       bility  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  power-up, 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  ses‐
       sion 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  con‐
       nected by a terminal server to several sessions, only one of those ses‐
       sions 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 char‐
       acter 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 unex‐
       pected  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.

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

       Files: atty(7), utx(7)


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