\subsubsection{DReg}
\index{DReg@\te{DReg} (package,interface)}
\index{mkDReg@\te{mkDReg} (module)}
\index{mkDRegA@\te{mkDRegA} (module)}
\index{mkDRegU@\te{mkDRegU} (module)}
\label{ref-dreg}

{\bf Package}


\begin{verbatim}
import DReg :: * ;
\end{verbatim}

{\bf Description}


The \te{DReg} package allows a designer to create registers which
store a written value for only a single clock cycle. The value written
to a DReg is available to read one cycle after the write.  If more
than one cycle has passed since the register has been written however,
the value provided by the register is instead a default value (that is
specified during module instantiation). These registers are useful
when wanting to send pulse values that are only asserted for a single
clock cycle.  The DReg is the register equivalent of a DWire
\ref{lib-dwire}.

{\bf Modules}

The \te{DReg} package provides three modules; \te{mkDReg} creates a
register with a given reset/default value and synchronous reset logic,
\te{mkDRegU} creates a register without any reset (but which still takes 
a default value as an argument), and \te{mkDRegA} creates a register
with a given reset/default value and asynchronous reset logic.

\begin{tabular}{|p{1.2 in}|p{4.4 in}|}
\hline
&\\
\te{mkDReg}&Make a register with a given reset/default value.  Reset logic is synchronous\\
\cline{2-2}
&\begin{libverbatim}
module mkDReg#(a_type dflt_rst_val)(Reg#(a_type))
  provisos (Bits#(a_type, sizea));
\end{libverbatim}
\\
\hline
\end{tabular}

\begin{tabular}{|p{1.2 in}|p{4.4 in}|}
\hline
&\\
\te{mkDRegU}&Make a register without any reset but with a specified default; initial simulation value is alternating 01 bits.\\
\cline{2-2}
&\begin{libverbatim}
module mkDRegU#(a_type dflt_val)(Reg#(a_type))
  provisos (Bits#(a_type, sizea));
\end{libverbatim}
\\
\hline
\end{tabular}

\begin{tabular}{|p{1.2 in}|p{4.4 in}|}
\hline
&\\
\te{mkDRegA}&Make a register with a given reset/default value.  Reset logic is asynchronous.\\
\cline{2-2}
&\begin{libverbatim}
module mkDRegA#(a_type, dflt_rst_val)(Reg#(a_type))
  provisos (Bits#(a_type, sizea));
\end{libverbatim}
\\
\hline
\end{tabular}

\begin{center}
\begin{tabular}{|p{.75 in}|c|c|}
\hline
\multicolumn{3}{|c|}{Scheduling Annotations}\\
\multicolumn{3}{|c|}{mkDReg, mkDRegU, mkDRegA}\\
\hline
&{read}&{write}\\
\hline
\hline
{read}&CF&SB\\
\hline
{write}&SA& SBR\\
\hline
\hline
\end{tabular}
\end{center}