Merge pull request #2285 from chipsalliance/dev/seldridge/spec-zero-width-select-mux

Relax spec on 0-bit mux select, use SFC behavior

2 files changed, 8 insertions, 2 deletions

diff --git a/spec/spec.pdf b/spec/spec.pdf
index fcd690e6..d961a28d 100644
--- a/spec/spec.pdf
+++ b/spec/spec.pdf
diff --git a/spec/spec.tex b/spec/spec.tex
index df3422e2..d41d0562 100644
--- a/spec/spec.tex
+++ b/spec/spec.tex
@@ -1290,7 +1290,7 @@ module MyModule :
 
 \subsection{Multiplexers}\label{multiplexers}
 
-A multiplexer outputs one of two input expressions depending on the value of an unsigned single bit selection signal.
+A multiplexer outputs one of two input expressions depending on the value of an unsigned selection signal.
 
 The following example connects to the \verb|c| port the result of selecting between the \verb|a| and \verb|b| ports. The \verb|a| port is selected when the \verb|sel| signal is high, otherwise the \verb|b| port is selected.
 \begin{lstlisting}
@@ -1304,7 +1304,13 @@ module MyModule :
 
 A multiplexer expression is legal only if the following holds.
 \begin{enumerate}
-\item The type of the selection signal is a single bit unsigned integer.
+\item The type of the selection signal is an unsigned integer.
+\item The width of the selection signal is any of:
+  \begin{enumerate}
+  \item Unspecified
+  \item Zero-bit\footnote{A zero-bit width selection signal will be treated as a one-bit, zero-valued selection signal.  \emph{This means that a multiplexer expression with a zero-bit selection signal will always take the false path and will likely be optimized away by a FIRRTL compiler.}}
+  \item One-bit
+  \end{enumerate}
 \item The types of the two input expressions are equivalent.
 \item The types of the two input expressions are passive (see section \ref{passive_types}).
 \end{enumerate}