Added documentation for t_fiu_records and fault model bits

docs/technical_reference_manual/content/04-hardware.tex

@@ -80,9 +80,28 @@ selection. If, however, area or timing is critical, fault injection units
 can be configured to implement only one fault model each.
 Fault injection units are daisy-chained in \texttt{\texttt{fault\_injection\_top}} as described in \Cref{sec:hw_fi_top}.
 
-\fixme{Add this as footnote to the specification of \texttt{t\_fiu\_records}?
-Due to the encoding of the forwarding/fault type in 3 bits, another 2 fault types can be added easily.
-More than the total of 8 forwarding types would require some hardware and software changes.}
+The number of \acp{FIU} and their configuration are determined by the constant
+\texttt{c\_fiu\_config} in the VHDL package \texttt{public\_config\_pkg}.
+This constant is of the type \texttt{t\_fiu\_records}, which is an array
+of \texttt{t\_single\_fiu\_record}s. This record consists of the following
+fields:
+\begin{itemize}
+    \item \texttt{fault\_mode}: specifies which fault model is available at runtime.
+                               This is an instance of the type \texttt{t\_select\_fault\_models}
+                               from \texttt{fault\_selection\_type\_pkg}.
+    \item \texttt{lfsr\_mask}: specifies an AND mask for the global \ac{LFSR}.
+                              The result of the AND operation is in turn
+                              AND-reduced to a single bit which is used
+                              in the stuck-open fault model.
+\end{itemize}
+
+The fault model of \acp{FIU} configured as \texttt{RUNTIME} is configured
+at runtime using a 3-bit-wide configuration register for each fault configuration
+as seen in Figure~\ref{fig:hardwareblockd}. Six of the possible values
+are already mapped to fault models as can be seen in the VHDL package
+\texttt{fault\_injection\_unit\_pkg} (via the constants \texttt{c\_fault\_select\_*}).
+Two more fault models can be added without changing the width of these
+registers.
 
 \begin{table}
     \caption{Design Constants}
@@ -101,4 +120,4 @@ More than the total of 8 forwarding types would require some hardware and softwa
     \caption{Toplevel VHDL Outputs}
     \input{content/tab_vhdl_outputs.tex}
     \label{tab:toplevel_outputs}
-\end{table}
+\end{table}
\ No newline at end of file