\item The wire name
\item A list of attributes
\item A width (buses are just wires with a width > 1)
+\item Bus direction (MSB to LSB or vice versa)
+\item Lowest valid bit index (LSB or MSB depending on bus direction)
\item If the wire is a port: port number and direction (input/output/inout)
\end{itemize}
coarse grain synthesis where the cells of the target architecture operate on
entire signal vectors instead of single bit wires.
+In Verilog and VHDL, busses may have arbitrary bounds, and LSB can have either
+the lowest or the highest bit index. In RTLIL, bit 0 always corresponds to LSB;
+however, information from the HDL frontend is preserved so that the bus will be
+correctly indexed in error messages, backend output, constraint files, etc.
+
An RTLIL::Cell object has the following properties:
\begin{itemize}
An RTLIL::Process is a container for zero or more RTLIL::SyncRule objects and
exactly one RTLIL::CaseRule object, which is called the {\it root case}.
-An RTLIL::SyncRule object contains an (optional) synchronization condition
-(signal and edge-type) and zero or more assignments (RTLIL::SigSig).
+An RTLIL::SyncRule object contains an (optional) synchronization condition (signal and edge-type) and zero or
+more assignments (RTLIL::SigSig). The {\tt always} synchronization condition is used to break combinatorial
+loops when a latch should be inferred instead.
An RTLIL::CaseRule is a container for zero or more assignments (RTLIL::SigSig)
and zero or more RTLIL::SwitchRule objects. An RTLIL::SwitchRule objects is a
sets {\tt \$0\textbackslash{}q[0:0]} to the value of {\tt \textbackslash{}d} if {\tt
\textbackslash{}enable} is active (lines $6 \dots 11$).
+A case can specify zero or more compare values that will determine whether it matches. Each of the compare values
+must be the exact same width as the control signal. When more than one compare value is specified, the case matches
+if any of them matches the control signal; when zero compare values are specified, the case always matches (i.e.
+it is the default case).
+
+A switch prioritizes cases from first to last: multiple cases can match, but only the first matched case becomes
+active. This normally synthesizes to a priority encoder. The {\tt parallel\_case} attribute allows passes to assume
+that no more than one case will match, and {\tt full\_case} attribute allows passes to assume that exactly one
+case will match; if these invariants are ever dynamically violated, the behavior is undefined. These attributes
+are useful when an invariant invisible to the synthesizer causes the control signal to never take certain
+bit patterns.
+
The lines $13 \dots 16$ then cause {\tt \textbackslash{}q} to be updated whenever there is
a positive clock edge on {\tt \textbackslash{}clock} or {\tt \textbackslash{}reset}.
All read accesses to the memory are transformed to {\tt \$memrd} cells and all write accesses to
{\tt \$memwr} cells by the language frontend. These cells consist of independent read- and write-ports
-to the memory. The \B{MEMID} parameter on these cells is used to link them together and to the
-RTLIL::Memory object they belong to.
+to the memory. Memory initialization is transformed to {\tt \$meminit} cells by the language frontend.
+The \B{MEMID} parameter on these cells is used to link them together and to the RTLIL::Memory object they belong to.
The rationale behind using separate cells for the individual ports versus
creating a large multiport memory cell right in the language frontend is that
\item {\tt passes/} \\
This directory contains a subdirectory for each pass or group of passes. For example as
of this writing the directory {\tt passes/opt/} contains the code for seven
-passes: {\tt opt}, {\tt opt\_const}, {\tt opt\_muxtree}, {\tt opt\_reduce},
-{\tt opt\_rmdff}, {\tt opt\_rmunused} and {\tt opt\_share}.
+passes: {\tt opt}, {\tt opt\_expr}, {\tt opt\_muxtree}, {\tt opt\_reduce},
+{\tt opt\_rmdff}, {\tt opt\_rmunused} and {\tt opt\_merge}.
\item {\tt techlibs/} \\
This directory contains simulation models and standard implementations for the
\end{sloppypar}
Good starting points for reading example source code to learn how to write passes
-are {\tt passes/opt/opt\_rmdff.cc} and {\tt passes/opt/opt\_share.cc}.
+are {\tt passes/opt/opt\_rmdff.cc} and {\tt passes/opt/opt\_merge.cc}.
See the top-level README file for a quick {\it Getting Started} guide and build
instructions. The Yosys build is based solely on Makefiles.
projects / yosys.git / blobdiff