+++ /dev/null
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-<HTML
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->calculated</TITLE
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-HREF="ref.active-if.html"><LINK
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-TITLE="compile"
-HREF="ref.compile.html"></HEAD
-><BODY
-CLASS="REFENTRY"
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->The <SPAN
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-> Component Writer's Guide</TH
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->Next</A
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-><HR
-ALIGN="LEFT"
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-><H1
-><A
-NAME="REF.CALCULATED"><SPAN
-CLASS="PROPERTY"
->calculated</SPAN
-></H1
-><DIV
-CLASS="REFNAMEDIV"
-><A
-NAME="AEN3722"
-></A
-><H2
->Name</H2
->Property <SPAN
-CLASS="PROPERTY"
->calculated</SPAN
-> -- Used if the current option's value is not user-modifiable,
-but is calculated using a suitable CDL expression.</DIV
-><DIV
-CLASS="REFSYNOPSISDIV"
-><A
-NAME="AEN3726"><H2
->Synopsis</H2
-><TABLE
-BORDER="5"
-BGCOLOR="#E0E0F0"
-WIDTH="70%"
-><TR
-><TD
-><PRE
-CLASS="SYNOPSIS"
->cdl_option <name> {
- calculated <expression>
- …
-}</PRE
-></TD
-></TR
-></TABLE
-></DIV
-><DIV
-CLASS="REFSECT1"
-><A
-NAME="AEN3728"
-></A
-><H2
->Description</H2
-><P
->In some cases it is useful to have a configuration option whose value
-cannot be modified directly by the user. This can be achieved using a
-<SPAN
-CLASS="PROPERTY"
->calculated</SPAN
->, which takes a CDL expression as argument (see <A
-HREF="language.values.html#LANGUAGE.EXPRESSION"
->the Section called <I
->Ordinary Expressions</I
-> in Chapter 3</A
-> for a description of expression
-syntax). The configuration system evaluates the expression when the
-current package is loaded and whenever there is a change to any other
-option referenced in the expression. The result depends on the
-option's flavor:</P
-><P
-></P
-><DIV
-CLASS="VARIABLELIST"
-><DL
-><DT
-><TT
-CLASS="LITERAL"
->flavor none</TT
-></DT
-><DD
-><P
->Options with this flavor have no value, so the <SPAN
-CLASS="PROPERTY"
->calculated</SPAN
->
-property is not applicable.</P
-></DD
-><DT
-><TT
-CLASS="LITERAL"
->flavor bool</TT
-></DT
-><DD
-><P
->If the expression evaluates to a non-zero result the option is
-enabled, otherwise it is disabled.</P
-></DD
-><DT
-><TT
-CLASS="LITERAL"
->flavor booldata</TT
-></DT
-><DD
-><P
->If the result of evaluating the expression is zero then the option is
-disabled, otherwise the option is enabled and its value is the result.</P
-></DD
-><DT
-><TT
-CLASS="LITERAL"
->flavor data</TT
-></DT
-><DD
-><P
->The value of the option is the result of evaluating the expression.</P
-></DD
-></DL
-></DIV
-><P
->There are a number of valid uses for calculated options, and there are
-also many cases where some other CDL facility would be more
-appropriate. Valid uses of calculated options include the following:</P
-><P
-></P
-><UL
-><LI
-><P
->On some target hardware a particular feature may be user-configurable,
-while on other targets it is fixed. For example some processors can
-operate in either big-endian or little-endian mode, while other
-processors do not provide any choice. It is possible to have an
-option <TT
-CLASS="LITERAL"
->CYGARC_BIGENDIAN</TT
-> which is calculated in
-some architectural HAL packages but user-modifiable in others.</P
-></LI
-><LI
-><P
->Calculated options can provide an alternative way for one package to
-affect the behavior of another one. Suppose a package may provide two
-possible implementations, a preferred one involving self-modifying
-code and a slower alternative. If the system involves a ROM bootstrap
-then the slower alternative must be used, but it would be
-inappropriate to modify the startup option in every HAL to impose
-constraints on this package. Instead it is possible to have a
-calculated option whose value is
-<TT
-CLASS="LITERAL"
->{ CYG_HAL_STARTUP == "ROM" }</TT
->,
-and which has appropriate consequences. Arguably this is a spurious
-example, and it should be a user choice whether or not to use
-self-modifying code with a <SPAN
-CLASS="PROPERTY"
->default_value</SPAN
-> based on
-<TT
-CLASS="LITERAL"
->CYG_HAL_STARTUP</TT
->, but that is for the component
-writer to decide.</P
-></LI
-><LI
-><P
->Sometimes it should be possible to perform a particular test at
-compile-time, for example by using a C preprocessor
-<TT
-CLASS="LITERAL"
->#if</TT
-> construct. However the preprocessor has only
-limited functionality, for example it cannot perform string
-comparisons. CDL expressions are more powerful.</P
-></LI
-><LI
-><P
->Occasionally a particular sub-expression may occur multiple times in
-a CDL script. If the sub-expression is sufficiently complex then it
-may be worthwhile to have a calculated option whose value is the
-sub-expression, and then reference that calculated option in the
-appropriate places. </P
-></LI
-></UL
-><P
->Alternatives to using calculated options include the following:</P
-><P
-></P
-><UL
-><LI
-><P
->CDL <A
-HREF="language.interface.html"
->interfaces</A
-> are a form of
-calculated option intended as an abstraction mechanism. An interface
-can be used to express the concept of <SPAN
-CLASS="emphasis"
-><I
-CLASS="EMPHASIS"
->any
-scheduler</I
-></SPAN
->, as opposed to a specific one such as the bitmap scheduler.</P
-></LI
-><LI
-><P
->If a calculated option would serve only to add additional information
-to a configuration header file, it may be possible to achieve the same
-effect using a <A
-HREF="ref.define-proc.html"
-><SPAN
-CLASS="PROPERTY"
->define_proc</SPAN
-></A
->
-property or one of the other properties related to header file
-generation.</P
-></LI
-></UL
-><DIV
-CLASS="TIP"
-><BLOCKQUOTE
-CLASS="TIP"
-><P
-><B
->Tip: </B
->If the first entry in a <SPAN
-CLASS="PROPERTY"
->calculated</SPAN
-> expression is a negative
-number, for example <TT
-CLASS="LITERAL"
->calculated -1</TT
-> then this
-can be misinterpreted as an option instead of as part of the
-expression. Currently the <SPAN
-CLASS="PROPERTY"
->calculated</SPAN
-> property does not take any
-options, but this may change in future. Option processing halts at the
-sequence <TT
-CLASS="LITERAL"
->--</TT
->, so the desired value can be expressed
-safely using
-<TT
-CLASS="LITERAL"
->calculated -- -1</TT
-></P
-></BLOCKQUOTE
-></DIV
-><DIV
-CLASS="WARNING"
-><P
-></P
-><TABLE
-CLASS="WARNING"
-BORDER="1"
-WIDTH="100%"
-><TR
-><TD
-ALIGN="CENTER"
-><B
->Warning</B
-></TD
-></TR
-><TR
-><TD
-ALIGN="LEFT"
-><P
->Some of the CDL scripts in current <SPAN
-CLASS="APPLICATION"
->eCos</SPAN
-> releases make excessive use
-of calculated options. This is partly because the recommended
-alternatives were not always available at the time the scripts were
-written. It is also partly because there is still some missing
-functionality, for example <SPAN
-CLASS="PROPERTY"
->define_proc</SPAN
-> properties cannot yet access
-the configuration data so it may be necessary to use <SPAN
-CLASS="PROPERTY"
->calculated</SPAN
->
-properties to access the data and perform the desired manipulation via
-a <SPAN
-CLASS="APPLICATION"
->CDL</SPAN
-> expression. New scripts should use calculated options only in
-accordance with the guidelines.</P
-></TD
-></TR
-></TABLE
-></DIV
-><DIV
-CLASS="NOTE"
-><BLOCKQUOTE
-CLASS="NOTE"
-><P
-><B
->Note: </B
->For options with the booldata flavor the current CDL syntax does not
-allow the enabled flag and the value to be calculated separately.
-Functionality to permit this may be added in a future release.</P
-></BLOCKQUOTE
-></DIV
-><DIV
-CLASS="NOTE"
-><BLOCKQUOTE
-CLASS="NOTE"
-><P
-><B
->Note: </B
->It has been suggested that having options which are not
-user-modifiable is potentially confusing, and that a top-level
-<TT
-CLASS="LITERAL"
->cdl_constant</TT
-> command should be added to the
-language instead of or in addition to the <SPAN
-CLASS="PROPERTY"
->calculated</SPAN
-> property. Such
-a change is under consideration. However because the value of a
-calculated option can depend on other options, it is not necessarily
-constant.</P
-></BLOCKQUOTE
-></DIV
-></DIV
-><DIV
-CLASS="REFSECT1"
-><A
-NAME="AEN3799"
-></A
-><H2
->Example</H2
-><TABLE
-BORDER="5"
-BGCOLOR="#E0E0F0"
-WIDTH="70%"
-><TR
-><TD
-><PRE
-CLASS="PROGRAMLISTING"
-># A constant on some target hardware, perhaps user-modifiable on other
-# targets.
-cdl_option CYGNUM_HAL_RTC_PERIOD {
- display "Real-time clock period"
- flavor data
- calculated 12500
-}</PRE
-></TD
-></TR
-></TABLE
-></DIV
-><DIV
-CLASS="REFSECT1"
-><A
-NAME="AEN3802"
-></A
-><H2
->See Also</H2
-><P
->Properties <A
-HREF="ref.default-value.html"
-><SPAN
-CLASS="PROPERTY"
->default_value</SPAN
-></A
->,
-<A
-HREF="ref.flavor.html"
-><SPAN
-CLASS="PROPERTY"
->flavor</SPAN
-></A
-> and
-<A
-HREF="ref.legal-values.html"
-><SPAN
-CLASS="PROPERTY"
->legal_values</SPAN
-></A
->,</P
-></DIV
-><DIV
-CLASS="NAVFOOTER"
-><HR
-ALIGN="LEFT"
-WIDTH="100%"><TABLE
-SUMMARY="Footer navigation table"
-WIDTH="100%"
-BORDER="0"
-CELLPADDING="0"
-CELLSPACING="0"
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-HREF="ref.active-if.html"
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->Prev</A
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-WIDTH="34%"
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-VALIGN="top"
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