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<!-- Copyright (C) 2003 Red Hat, Inc.                                -->
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>Configuration Items</TITLE
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><TR
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>eCos User Guide</TH
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><TR
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><TD
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><A
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ACCESSKEY="P"
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>Prev</A
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></TD
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><TD
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WIDTH="80%"
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ALIGN="center"
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VALIGN="bottom"
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>Chapter 24. CDL Concepts</TD
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><TD
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ALIGN="right"
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VALIGN="bottom"
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><A
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>Next</A
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WIDTH="100%"></DIV
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><DIV
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CLASS="SECT1"
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><H1
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CLASS="SECT1"
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><A
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NAME="CDL-CONFIGURATION-ITEMS">Configuration Items</H1
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><P
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><SPAN
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CLASS="emphasis"
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><I
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CLASS="EMPHASIS"
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>Configuration items</I
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></SPAN
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> are the
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          individual entities that form a configuration.  Each item
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          corresponds to the setting of a C pre-processor macro (for
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          example,
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          <TT
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CLASS="LITERAL"
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>CYGHWR_HAL_ARM_PID_GDB_BAUD</TT
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>).
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          The code of <SPAN
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CLASS="PRODUCTNAME"
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>eCos</SPAN
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> itself is written to test such pre-processor
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          macros so as to tailor the code.  User code can do
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          likewise.</P
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><P
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>Configuration items come in the following flavors:</P
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><P
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></P
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><UL
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><LI
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><P
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><SPAN
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CLASS="emphasis"
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><I
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CLASS="EMPHASIS"
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>None</I
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></SPAN
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>: such entities serve only as
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place holders in the hierarchy, allowing other entities to be grouped
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more easily.</P
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></LI
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><LI
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><P
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><SPAN
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CLASS="emphasis"
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><I
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CLASS="EMPHASIS"
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>Boolean</I
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></SPAN
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> entities are the most common
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flavor; they correspond to units of functionality that can be either
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enabled or disabled.  If the entity is enabled then there will be
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a #define; code will check the setting using, for example, #ifdef</P
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></LI
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><LI
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><P
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><SPAN
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CLASS="emphasis"
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><I
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CLASS="EMPHASIS"
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>Data</I
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></SPAN
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> entities encapsulate some arbitrary
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data. Other properties such as a set or range of legal values can
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be used to constrain the actual values, for example to an integer
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or floating point value within a certain range.</P
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></LI
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><LI
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><P
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><SPAN
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CLASS="emphasis"
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><I
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CLASS="EMPHASIS"
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>Booldata</I
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></SPAN
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> entities combine the attributes
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of <SPAN
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CLASS="emphasis"
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><I
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CLASS="EMPHASIS"
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>Boolean</I
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></SPAN
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> and <SPAN
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CLASS="emphasis"
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><I
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CLASS="EMPHASIS"
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>Data</I
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></SPAN
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>: they
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can be enabled or disabled and, if enabled, will hold a data value.</P
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></LI
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></UL
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><P
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>Like packages, configuration items exist in a tree-based hierarchy:
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each configuration item has a parent which may be another configuration
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item or a package.  Under some conditions (such as when packages
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are added or removed from a configuration), items may be &#8220;re-parented&#8221; such
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that their position in the tree changes. </P
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><DIV
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CLASS="SECT2"
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><H2
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CLASS="SECT2"
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><A
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NAME="AEN1898">Expressions</H2
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><P
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>Expressions are relationships between CDL items.  There are
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three types of expression in CDL:</P
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><DIV
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CLASS="TABLE"
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><A
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NAME="CDL-EXPRESSIONS"><P
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><B
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>Table 24-1. CDL Expressions</B
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></P
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><TABLE
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BORDER="1"
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CLASS="CALSTABLE"
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><THEAD
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><TR
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><TH
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ALIGN="LEFT"
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VALIGN="TOP"
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>Expression Type</TH
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><TH
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ALIGN="LEFT"
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VALIGN="TOP"
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>Result</TH
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><TH
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ALIGN="LEFT"
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VALIGN="TOP"
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>Common Use (see <A
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HREF="cdl-configuration-items.html#TABLE-CONFIGURATION-PROPERTIES"
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>Table 24-2</A
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>)</TH
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></TR
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></THEAD
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><TBODY
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><TR
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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>Ordinary</TD
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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>A single value</TD
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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>legal_values property</TD
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></TR
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><TR
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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>List</TD
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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>A range of
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                values (for example &#8220;1 to 10&#8221;)</TD
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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>legal_values property </TD
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></TR
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><TR
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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>Goal</TD
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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>True or False</TD
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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>requires and active_if properties</TD
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></TR
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></TBODY
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></TABLE
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></DIV
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></DIV
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><DIV
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CLASS="SECT2"
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><H2
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CLASS="SECT2"
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><A
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NAME="AEN1923">Properties</H2
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><P
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>Each configuration item has a set of properties.  The following
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table describes the most commonly used:</P
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><DIV
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CLASS="TABLE"
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><A
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NAME="TABLE-CONFIGURATION-PROPERTIES"><P
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><B
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>Table 24-2. Configuration properties</B
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></P
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><TABLE
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BORDER="1"
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CLASS="CALSTABLE"
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><THEAD
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><TR
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><TH
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ALIGN="LEFT"
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VALIGN="TOP"
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><SPAN
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CLASS="emphasis"
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><I
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CLASS="EMPHASIS"
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>Property</I
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></SPAN
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></TH
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><TH
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ALIGN="LEFT"
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VALIGN="TOP"
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><SPAN
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CLASS="emphasis"
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><I
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CLASS="EMPHASIS"
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>Use </I
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></SPAN
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></TH
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></TR
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></THEAD
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><TBODY
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><TR
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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>Flavor</TD
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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>The &#8220;type&#8221; of the item, as
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                  described above </TD
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></TR
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><TR
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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>Enabled</TD
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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>Whether
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                  the item is enabled </TD
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></TR
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><TR
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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>Current_value</TD
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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>The current value of the item </TD
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></TR
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><TR
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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>Default_value</TD
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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>An ordinary expression defining the default value of the
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                    item</TD
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></TR
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><TR
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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>Legal_values</TD
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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>A
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                  list expression defining the values the item may hold (for example,
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                  1 to10) </TD
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></TR
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><TR
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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>Active_if</TD
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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>A
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                  goal expression denoting the requirement for this item to be active
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(see below: <SPAN
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CLASS="emphasis"
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><I
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CLASS="EMPHASIS"
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>Inactive Items</I
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></SPAN
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>) </TD
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></TR
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><TR
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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>Requires</TD
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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>A goal
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expression denoting requirements this item places on others (see
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below: <SPAN
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CLASS="emphasis"
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><I
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CLASS="EMPHASIS"
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>Conflicts</I
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></SPAN
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>) </TD
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></TR
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><TR
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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>Calculated</TD
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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>Whether
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the item as non-modifiable </TD
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></TR
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><TR
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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>Macro</TD
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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>The corresponding
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C pre-processor macro </TD
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></TR
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><TR
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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>File</TD
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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>The C header
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file in which the macro is defined </TD
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></TR
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><TR
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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>URL</TD
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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>The URL of
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a documentation page describing the item </TD
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></TR
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><TR
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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>Hardware</TD
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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>Indicates
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that a particular package is related to specific hardware</TD
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></TR
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></TBODY
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></TABLE
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></DIV
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><P
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>A complete description of properties is contained in the <I
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CLASS="CITETITLE"
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>Component
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Writer&#8217;s Guide</I
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>.</P
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></DIV
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><DIV
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CLASS="SECT2"
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><H2
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CLASS="SECT2"
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><A
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NAME="AEN1976">Inactive Items</H2
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><P
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>Descendants of an item that is disabled are inactive: their
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values may not be changed.  Items may also become <SPAN
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CLASS="emphasis"
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><I
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CLASS="EMPHASIS"
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>inactive</I
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></SPAN
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> if
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an active_if expression is used to make the item dependent
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on an expression involving other items. </P
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></DIV
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