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+% Copyright 2018 by Till Tantau
+%
+% This file may be distributed and/or modified
+%
+% 1. under the LaTeX Project Public License and/or
+% 2. under the GNU Free Documentation License.
+%
+% See the file doc/generic/pgf/licenses/LICENSE for more details.
+
+
+\section{Design of the System Layer}
+
+\makeatletter
+
+\subsection{Driver Files}
+\label{section-pgfsys}
+
+The \pgfname\ system layer mainly consists of a large number of commands
+starting with |\pgfsys@|. These commands will be called \emph{system commands}
+in the following. The higher layers ``interface'' with the system layer by
+calling these commands. The higher layers should never use |\special| commands
+directly or even check whether |\pdfoutput| is defined. Instead, all drawing
+requests should be ``channeled'' through the system commands.
+
+The system layer is loaded and set up by the following package:
+
+\begin{package}{pgfsys}
+ This file provides ``default implementations'' of all system commands, but
+ most simply produce a warning that they are not implemented. The actual
+ implementations of the system commands for a particular driver like, say,
+ |pdftex| reside in files called |pgfsys-xxxx.sty|, where |xxxx| is the
+ driver name. These will be called \emph{driver files} in the following.
+
+ When |pgfsys.sty| is loaded, it will try to determine which driver is used
+ by loading |pgf.cfg|. This file should set up the macro |\pgfsysdriver|
+ appropriately. The |pgfsys.sty| will input the appropriate
+ |pgfsys-|\meta{drivername}|.sty|.
+\end{package}
+
+\begin{command}{\pgfsysdriver}
+ This macro should expand to the name of the driver to be used by |pgfsys|.
+ The default from |pgf.cfg| is |pgfsys-\Gin@driver|. This is very likely to
+ be correct if you are using \LaTeX. For plain \TeX, the macro will be set
+ to |pgfsys-pdftex.def| if |pdftex| is used and to |pgfsys-dvips.def|
+ otherwise.
+\end{command}
+
+\begin{filedescription}{pgf.cfg}
+ This file should set up the command |\pgfsysdriver| correctly. If
+ |\pgfsysdriver| is already set to some value, the driver normally should
+ not change it. Otherwise, it should make a ``good guess'' at which driver
+ will be appropriate.
+\end{filedescription}
+
+The currently supported backend drivers are discussed in
+Section~\ref{section-drivers}.
+
+
+\subsection{Common Definition Files}
+
+Some drivers share many |\pgfsys@| commands. For the reason, files defining
+these ``common'' commands are available. These files are \emph{not} usable
+alone.
+
+\begin{filedescription}{pgfsys-common-postscript}
+ This file defines some |\pgfsys@| commands so that they produce appropriate
+ PostScript code.
+\end{filedescription}
+
+\begin{filedescription}{pgfsys-common-pdf}
+ This file defines some |\pgfsys@| commands so that they produce appropriate
+ \textsc{pdf} code.
+\end{filedescription}
+
+
+%%% Local Variables:
+%%% mode: latex
+%%% TeX-master: "pgfmanual"
+%%% End: