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<h2>
<a name="1204415"> </a><font color="#003366" face="Verdana, Arial, Helvetica, sans-serif">10.1 Introduction</font>
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<p>
<a name="1204416"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif"><strong>st20cc</strong> supports the creation of relocatable code units (RCUs) which may be loaded dynamically at run time and executed. Relocatable code units may be used in the following situations:</font>
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<p> <font size=2 face="Verdana, Arial, Helvetica, sans-serif"><li ><a name="1204417"> </a>when field upgrades to running applications are needed in real-time,</font></p>
<p> <font size=2 face="Verdana, Arial, Helvetica, sans-serif"><li ><a name="1204418"> </a>configuration of software at power on, for example, in set-top box applications,</font></p>
<p> <font size=2 face="Verdana, Arial, Helvetica, sans-serif"><li ><a name="1204419"> </a>loading time-critical functions into fast memory while the application is running,</font></p>
<p> <font size=2 face="Verdana, Arial, Helvetica, sans-serif"><li ><a name="1210640"> </a>freeing up the memory once the RCU has completed avoids having rarely-used code taking up space in fast memory.</font></p>
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<a name="1204420"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">RCUs may be loaded from an external source, for example an ST Micro Connect or satellite or copied from ROM.</font>
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<p>
<a name="1204421"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">This chapter describes how RCUs are built using the toolset, the mechanisms for interacting with them and special considerations when using them with OS20.</font>
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<a name="1204422"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">The toolset also provides support for calling functions from other programs using the <font size=2 face=Courier><strong>import</strong></font> and <font size=2 face=Courier><strong>export</strong></font> commands and the function prototypes in the <font size=2 face=Courier><strong>dl.h </strong></font>header file, which are implemented in <font size=2 face=Courier><strong>rcu.lib</strong></font>. This mechanism offers a simple way of handling RCUs and may be easier to use for new users of the toolset. An example implementation of it is provided in the <font size=2 face=Courier><strong>examples/dl</strong></font> directory. Another RCU example, which does not use this mechanism is provided in <font size=2 face=Courier><strong>examples/rcu</strong></font>.</font>
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<a name="1204426"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">The <font size=2 face=Courier><strong>import</strong></font> and <font size=2 face=Courier><strong>export</strong></font> commands are described in <a href="commands.htm#1212189"><font color="#0000ff" face="Verdana, Arial, Helvetica, sans-serif"><i>Chapter⌨️ 快捷键说明
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