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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 Compaq's 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 load-
  ing request to the host computer whenever the	device encounters a UDC	whose
  font information is not present in the device. The host computer then	com-
  pletes 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 infor-
  mation 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

  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 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 follow-
  ing limitations:

    +  Some local language devices have	to have	the hardware On-Demand Load-
       ing flag	disabled before	the devices can	be used	with the SoftODL

    +  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 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 session and	the terminal is
       accidentally powered off, users must issue the following	command
       immediately after turning on the	power:

       % stty odlreset
	   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:

       % stty odlreset
	   This	command	flushes	the internal tty cache when you	return to the
	   terminal session where SoftODL is being used.

       % stty odlsize 0
	   This	command	instructs the tty driver to use	SoftODL	for all	the
	   user-defined	characters.  However, this option also prevents	cach-
	   ing 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.


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

  Files: atty(7), utx(7)