223cc6685e
git-svn-id: svn://svn.compuextreme.de/Viitor/V963/Viitor_cc65@5933 504e572c-2e33-0410-9681-be2bf7408885
126 lines
4.3 KiB
HTML
126 lines
4.3 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN">
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<HTML>
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<META NAME="GENERATOR" CONTENT="LinuxDoc-Tools 0.9.20">
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<TITLE>co65 Users Guide: Converting loadable drivers</TITLE>
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<LINK HREF="co65-5.html" REL=next>
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<LINK HREF="co65-3.html" REL=previous>
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<LINK HREF="co65.html#toc4" REL=contents>
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<A HREF="co65-5.html">Next</A>
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<A HREF="co65-3.html">Previous</A>
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<A HREF="co65.html#toc4">Contents</A>
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<HR>
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<H2><A NAME="s4">4.</A> <A HREF="co65.html#toc4">Converting loadable drivers</A></H2>
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<H2><A NAME="ss4.1">4.1</A> <A HREF="co65.html#toc4.1">Differences between static linking and runtime loading</A>
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</H2>
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<P>One main use of the utility is conversion of loadable drivers, so they may be
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linked statically to an application. Statically linking will cause a few
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things to be different from runtime loading:</P>
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<P>
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<UL>
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<LI> Without changing the segment names, all segments take the default
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names used by the standard linker configurations. This means that the
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driver code is no longer contingous in memory, instead the code
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segment is placed somewhere in between all other code segments, the
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data segment is placed with all other data segments and so on. If the
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driver doesn't do strange things this shouldn't be a problem.
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</LI>
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<LI> With statically linked code, data and bss segments will get intialized
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once (when the application is loaded), while a loadable driver will
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get its initialization each time the driver is loaded into memory
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(which may be more than once in the lifetime of a program). It depends
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on the driver if this is a problem. Currently, most drivers supplied
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with cc65 behave correctly when linked statically.
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</LI>
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</UL>
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</P>
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<H2><A NAME="ss4.2">4.2</A> <A HREF="co65.html#toc4.2">Additional requirements</A>
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</H2>
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<P>All loadable drivers used by cc65 have a header and a jump table at the start
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of the code segment. The header is needed to detect the driver (it may also
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contain some data that is necessary to access the driver). The jump table is
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used to access the functions in the driver code.</P>
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<P>When loading a driver at runtime, the load address of the driver is also the
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address of the code segment, so the locations of the header and jump table are
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known. However, when linking the driver statically, it is up to the programmer
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to provide this information to the driver API.</P>
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<P>For this purpose, it is necessary to define a code segment label that can be
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accessed from the outside later. Please note that the converter does currently
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<EM>not</EM> create such a label without being ordered to do so, even if the input
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file is a cc65 module.</P>
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<P>To create such a label, use the <CODE>--code-label</CODE> option when calling the
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converter. Be sure to begin the label with a leading underscore when accessing
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it from C code. In your code, define an arbitrary variable with this name. Use
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the address of this variable as the address of the code segment of the driver.
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Be sure to never modify the variable which is in reality the start of your
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driver!</P>
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<H2><A NAME="ss4.3">4.3</A> <A HREF="co65.html#toc4.3">Example: Convert and link a graphics driver</A>
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</H2>
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<P>As an example, here are some instructions to convert and use the c64-hi.tgi
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graphics driver:</P>
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<P>First, convert the driver, generating a label named "_c64_hi" for the code
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segment. Use the assembler to generate an object file from the assembler
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output.</P>
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<P>
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<BLOCKQUOTE><CODE>
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<PRE>
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co65 --code-label _c64_hi c64-hi.tgi
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ca65 c64-hi.s
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</PRE>
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</CODE></BLOCKQUOTE>
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</P>
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<P>Next, change your C code to declare a variable that is actually the first byte
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of the driver:</P>
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<P>
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<BLOCKQUOTE><CODE>
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<PRE>
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extern char c64_hi;
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</PRE>
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</CODE></BLOCKQUOTE>
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</P>
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<P>Instead of loading and unloading the driver, change the code to install and
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uninstall the driver, which will be already in memory after linking:</P>
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<P>
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<BLOCKQUOTE><CODE>
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<PRE>
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/* Install the driver */
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tgi_install (&c64_hi);
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...
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/* Uninstall the driver */
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tgi_uninstall ();
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</PRE>
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</CODE></BLOCKQUOTE>
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</P>
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<P>Don't forget to link the driver object file to your application, otherwise you
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will get an "undefined external" error for the _c64_hi symbol.</P>
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<HR>
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<A HREF="co65-5.html">Next</A>
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