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<HTML
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><HEAD
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><TITLE
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>Interfaces</TITLE
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"><LINK
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REL="HOME"
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TITLE="The eCos Component Writer's Guide"
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HREF="cdl-guide.html"><LINK
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REL="UP"
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TITLE="The CDL Language"
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HREF="language.html"><LINK
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TITLE="Updating the ecos.db database"
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><TH
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ALIGN="center"
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>The <SPAN
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>eCos</SPAN
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> Component Writer's Guide</TH
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>Chapter 3. The CDL Language</TD
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><A
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NAME="LANGUAGE.INTERFACE">Interfaces</H1
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><P
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>For many configurability requirements, options provide sufficient
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expressive power. However there are times when a higher level of
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abstraction is appropriate. As an example, suppose that some package
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relies on the presence of code that implements the standard kernel
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scheduling interface. However the requirement is no more stringent
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than this, so the constraint can be satisfied by the mlqueue
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scheduler, the bitmap scheduler, or any additional schedulers that may
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get implemented in future. A first attempt at expressing the
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dependency might be:</P
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><TABLE
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><PRE
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CLASS="PROGRAMLISTING"
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> requires CYGSEM_KERNEL_SCHED_MLQUEUE || CYGSEM_KERNEL_SCHED_BITMAP</PRE
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></TD
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></TR
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></TABLE
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><P
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>This constraint will work with the current release, but it is limited.
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Suppose there is a new release of the kernel which adds another
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scheduler such as a deadline scheduler, or suppose that there is a new
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third party package which adds such a scheduler. The package
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containing the limited constraint would now have to be updated and
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another release made, with possible knock-on effects.</P
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><P
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><SPAN
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CLASS="APPLICATION"
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>CDL</SPAN
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> interfaces provide an abstraction mechanism: constraints can be
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expressed in terms of an abstract concept, for example
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“scheduler”, rather than specific implementations such as
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<TT
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CLASS="VARNAME"
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>CYGSEM_KERNEL_SCHED_MLQUEUE</TT
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> and
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<TT
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CLASS="VARNAME"
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>CYGSEM_KERNEL_SCHED_BITMAP</TT
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>. Basically an interface
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is a calculated configuration option:</P
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><TABLE
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><PRE
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CLASS="PROGRAMLISTING"
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>cdl_interface CYGINT_KERNEL_SCHEDULER {
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display "Number of schedulers in this configuration"
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…
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}</PRE
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></TD
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></TR
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></TABLE
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><P
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>The individual schedulers can then implement this interface:</P
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><TABLE
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><TR
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><TD
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><PRE
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CLASS="PROGRAMLISTING"
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>cdl_option CYGSEM_KERNEL_SCHED_MLQUEUE {
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display "Multi-level queue scheduler"
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default_value 1
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implements CYGINT_KERNEL_SCHEDULER
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…
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}
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cdl_option CYGSEM_KERNEL_SCHED_BITMAP {
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display "Bitmap scheduler"
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default_value 0
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implements CYGINT_KERNEL_SCHEDULER
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…
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}</PRE
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></TD
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></TR
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></TABLE
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><P
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>Future schedulers can also implement this interface. The value of an
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interface, for the purposes of expression evaluation, is the number of
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active and enabled options which implement this interface. Packages
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which rely on the presence of a scheduler can impose constraints such
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as:</P
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><TABLE
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><TR
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><TD
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><PRE
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CLASS="PROGRAMLISTING"
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> requires CYGINT_KERNEL_SCHEDULER</PRE
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></TD
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></TR
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></TABLE
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><P
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>If none of the schedulers are enabled, or if the kernel package is not
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loaded, then <TT
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CLASS="VARNAME"
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>CYGINT_KERNEL_SCHEDULER</TT
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> will evaluate
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to <TT
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CLASS="LITERAL"
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>0</TT
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>. If at least one scheduler is active and
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enabled then the constraint will be satisfied.</P
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><P
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>Because interfaces have a calculated value determined by the
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implementors, the <SPAN
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CLASS="PROPERTY"
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>default_value</SPAN
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> and <SPAN
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CLASS="PROPERTY"
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>calculated</SPAN
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> properties are not
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applicable and should not appear in the body of a <TT
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CLASS="LITERAL"
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>cdl_interface</TT
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>
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command. Interfaces have the <TT
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CLASS="LITERAL"
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>data</TT
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> flavor by
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default, but the <TT
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CLASS="LITERAL"
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>bool</TT
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> and
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<TT
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CLASS="LITERAL"
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>booldata</TT
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> flavors may be specified instead. A
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<TT
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CLASS="LITERAL"
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>bool</TT
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> interface is disabled if there are no active
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and enabled implementors, otherwise it is enabled. A
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<TT
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CLASS="LITERAL"
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>booldata</TT
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> interface is disabled if there are no
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active and enabled implementors, otherwise it is enabled and has a
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value corresponding to the number of these implementors. Other
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properties such as <SPAN
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CLASS="PROPERTY"
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>requires</SPAN
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> and <SPAN
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CLASS="PROPERTY"
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>compile</SPAN
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> can be used as normal.</P
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><P
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>Some component writers will not want to use interfaces in this way.
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The reasoning is that their code will only have been tested with the
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existing schedulers, so the <SPAN
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CLASS="PROPERTY"
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>requires</SPAN
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> constraint needs to be
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expressed in terms of those schedulers; it is possible that the
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component will still work with a new scheduler, but there are no
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guarantees. Other component writers may take a more optimistic view
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and assume that their code will work with any scheduler until proven
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otherwise. It is up to individual component writers to decide which
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approach is most appropriate in any given case.</P
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><P
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>One common use for interfaces is to describe the hardware
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functionality provided by a given target. For example the <SPAN
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CLASS="APPLICATION"
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>CDL</SPAN
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>
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scripts for a TCP/IP package might want to know whether or not the
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target hardware has an ethernet interface. Generally it is not
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necessary for the TCP/IP stack to know exactly which ethernet hardware
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is present, since there should be a device driver which implements the
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appropriate functionality. In <SPAN
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CLASS="APPLICATION"
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>CDL</SPAN
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> terms the device drivers should
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implement an interface <TT
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CLASS="VARNAME"
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>CYGHWR_NET_DRIVERS</TT
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>, and the
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<SPAN
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CLASS="APPLICATION"
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>CDL</SPAN
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> scripts for the TCP/IP stack can use this in appropriate
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expressions. </P
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><DIV
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CLASS="NOTE"
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><BLOCKQUOTE
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CLASS="NOTE"
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><P
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><B
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>Note: </B
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>Using the term <SPAN
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CLASS="emphasis"
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><I
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CLASS="EMPHASIS"
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>interface</I
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></SPAN
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> for this concept is
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sometimes confusing, since the term has various other meanings as
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well. In practice, it is often correct. If there is a configuration
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option that implements a given <SPAN
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CLASS="APPLICATION"
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>CDL</SPAN
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> interface, then usually this
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option will enable some code that provides a particular interface at
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the C or C++ level. For example an ethernet device driver implements
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the <SPAN
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CLASS="APPLICATION"
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>CDL</SPAN
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> interface <TT
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CLASS="VARNAME"
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>CYGHWR_NET_DRIVERS</TT
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>, and also
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implements a set of C functions that can be used by the TCP/IP stack.
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Similarly <TT
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CLASS="VARNAME"
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>CYGSEM_KERNEL_SCHED_MLQUEUE</TT
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> implements
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the <SPAN
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CLASS="APPLICATION"
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>CDL</SPAN
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> interface <TT
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CLASS="VARNAME"
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>CYGINT_KERNEL_SCHEDULER</TT
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> and
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also provides the appropriate scheduling functions.</P
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></BLOCKQUOTE
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></DIV
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></DIV
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>Values and Expressions</TD
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WIDTH="34%"
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ALIGN="center"
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><A
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HREF="language.html"
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ACCESSKEY="U"
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>Up</A
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| 381 |
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></TD
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><TD
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WIDTH="33%"
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ALIGN="right"
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VALIGN="top"
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>Updating the <SPAN
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CLASS="DATABASE"
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>ecos.db</SPAN
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> database</TD
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></TR
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></TABLE
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></DIV
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></BODY
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></HTML
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>
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