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yosys -- Yosys Open SYnthesis Suite
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Copyright (C) 2012 - 2016 Clifford Wolf <clifford@clifford.at>
Permission to use, copy, modify, and/or distribute this software for any
purpose with or without fee is hereby granted, provided that the above
copyright notice and this permission notice appear in all copies.
yosys Yosys Open SYnthesis Suite
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This is a framework for RTL synthesis tools. It currently has
extensive Verilog-2005 support and provides a basic set of
synthesis algorithms for various application domains.
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Yosys can be adapted to perform any synthesis job by combining
the existing passes (algorithms) using synthesis scripts and
adding additional passes as needed by extending the yosys C++
code base.
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Yosys is free software licensed under the ISC license (a GPL
compatible license that is similar in terms to the MIT license
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or the 2-clause BSD license).
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Web Site
More information and documentation can be found on the Yosys web site:
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You need a C++ compiler with C++11 support (up-to-date CLANG or GCC is
recommended) and some standard tools such as GNU Flex, GNU Bison, and GNU Make.
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TCL, readline and libffi are optional (see ENABLE_* settings in Makefile).
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Xdot (graphviz) is used by the ``show`` command in yosys to display schematics.
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For example on Ubuntu Linux 16.04 LTS the following commands will install all
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prerequisites for building yosys:
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$ sudo apt-get install build-essential clang bison flex \
libreadline-dev gawk tcl-dev libffi-dev git mercurial \
graphviz xdot pkg-config python3
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Similarily, on Mac OS X MacPorts or Homebrew can be used to install dependencies:
$ brew install bison flex gawk libffi \
git mercurial graphviz pkg-config python3
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$ sudo port install bison flex readline gawk libffi \
git mercurial graphviz pkgconfig python36
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There are also pre-compiled Yosys binary packages for Ubuntu and Win32 as well
as a source distribution for Visual Studio. Visit the Yosys download page for
more information: http://www.clifford.at/yosys/download.html
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To configure the build system to use a specific compiler, use one of
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$ make config-clang
$ make config-gcc
For other compilers and build configurations it might be
necessary to make some changes to the config section of the
$ vi Makefile # ..or..
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$ vi Makefile.conf
To build Yosys simply type 'make' in this directory.
$ make
$ make test
$ sudo make install
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Note that this also downloads, builds and installs ABC (using yosys-abc
as executable name).
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Getting Started
Yosys can be used with the interactive command shell, with
synthesis scripts or with command line arguments. Let's perform
a simple synthesis job using the interactive command shell:
$ ./yosys
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the command ``help`` can be used to print a list of all available
commands and ``help <command>`` to print details on the specified command:
yosys> help help
reading the design using the Verilog frontend:
yosys> read_verilog tests/simple/fiedler-cooley.v
writing the design to the console in yosys's internal format:
yosys> write_ilang
elaborate design hierarchy:
yosys> hierarchy
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convert processes (``always`` blocks) to netlist elements and perform
some simple optimizations:
yosys> proc; opt
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display design netlist using ``xdot``:
yosys> show
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the same thing using ``gv`` as postscript viewer:
yosys> show -format ps -viewer gv
translating netlist to gate logic and perform some simple optimizations:
yosys> techmap; opt
write design netlist to a new Verilog file:
yosys> write_verilog synth.v
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a similar synthesis can be performed using yosys command line options only:
$ ./yosys -o synth.v -p hierarchy -p proc -p opt \
-p techmap -p opt tests/simple/fiedler-cooley.v
or using a simple synthesis script:
$ cat synth.ys
read_verilog tests/simple/fiedler-cooley.v
hierarchy; proc; opt; techmap; opt
write_verilog synth.v
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$ ./yosys synth.ys
It is also possible to only have the synthesis commands but not the read/write
commands in the synthesis script:
$ cat synth.ys
hierarchy; proc; opt; techmap; opt
$ ./yosys -o synth.v tests/simple/fiedler-cooley.v synth.ys
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The following very basic synthesis script should work well with all designs:
# check design hierarchy
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# translate processes (always blocks)
proc; opt
# detect and optimize FSM encodings
fsm; opt
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# implement memories (arrays)
memory; opt
# convert to gate logic
techmap; opt
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If ABC is enabled in the Yosys build configuration and a cell library is given
in the liberty file ``mycells.lib``, the following synthesis script will
synthesize for the given cell library:
# the high-level stuff
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hierarchy; proc; fsm; opt; memory; opt
# mapping to internal cell library
techmap; opt
# mapping flip-flops to mycells.lib
dfflibmap -liberty mycells.lib
# mapping logic to mycells.lib
abc -liberty mycells.lib
# cleanup
If you do not have a liberty file but want to test this synthesis script,
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you can use the file ``examples/cmos/cmos_cells.lib`` from the yosys sources.
Liberty file downloads for and information about free and open ASIC standard
cell libraries can be found here:
- http://www.vlsitechnology.org/html/libraries.html
- http://www.vlsitechnology.org/synopsys/vsclib013.lib
The command ``synth`` provides a good default synthesis script (see
``help synth``). If possible a synthesis script should borrow from ``synth``.
For example:
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# the high-level stuff
synth -run coarse
# mapping to internal cells
techmap; opt -fast
dfflibmap -liberty mycells.lib
abc -liberty mycells.lib
Yosys is under construction. A more detailed documentation will follow.
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Unsupported Verilog-2005 Features
The following Verilog-2005 features are not supported by
yosys and there are currently no plans to add support
for them:
- Non-synthesizable language features as defined in
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IEC 62142(E):2005 / IEEE Std. 1364.1(E):2002
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- The ``tri``, ``triand``, ``trior``, ``wand`` and ``wor`` net types
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- The ``config`` keyword and library map files
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- The ``disable``, ``primitive`` and ``specify`` statements
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- Latched logic (is synthesized as logic with feedback loops)
Verilog Attributes and non-standard features
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- The ``full_case`` attribute on case statements is supported
(also the non-standard ``// synopsys full_case`` directive)
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- The ``parallel_case`` attribute on case statements is supported
(also the non-standard ``// synopsys parallel_case`` directive)
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- The ``// synopsys translate_off`` and ``// synopsys translate_on``
directives are also supported (but the use of ``` `ifdef .. `endif ```
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is strongly recommended instead).
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- The ``nomem2reg`` attribute on modules or arrays prohibits the
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automatic early conversion of arrays to separate registers. This
is potentially dangerous. Usually the front-end has good reasons
for converting an array to a list of registers. Prohibiting this
step will likely result in incorrect synthesis results.
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- The ``mem2reg`` attribute on modules or arrays forces the early
conversion of arrays to separate registers.
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- The ``nomeminit`` attribute on modules or arrays prohibits the
creation of initialized memories. This effectively puts ``mem2reg``
on all memories that are written to in an ``initial`` block and
are not ROMs.
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- The ``nolatches`` attribute on modules or always-blocks
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prohibits the generation of logic-loops for latches. Instead
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all not explicitly assigned values default to x-bits. This does
not affect clocked storage elements such as flip-flops.
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- The ``nosync`` attribute on registers prohibits the generation of a
storage element. The register itself will always have all bits set
to 'x' (undefined). The variable may only be used as blocking assigned
temporary variable within an always block. This is mostly used internally
by yosys to synthesize Verilog functions and access arrays.
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- The ``onehot`` attribute on wires mark them as onehot state register. This
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is used for example for memory port sharing and set by the fsm_map pass.
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- The ``blackbox`` attribute on modules is used to mark empty stub modules
that have the same ports as the real thing but do not contain information
on the internal configuration. This modules are only used by the synthesis
passes to identify input and output ports of cells. The Verilog backend
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also does not output blackbox modules on default.
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- The ``keep`` attribute on cells and wires is used to mark objects that should
never be removed by the optimizer. This is used for example for cells that
have hidden connections that are not part of the netlist, such as IO pads.
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Setting the ``keep`` attribute on a module has the same effect as setting it
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on all instances of the module.
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- The ``keep_hierarchy`` attribute on cells and modules keeps the ``flatten``
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command from flattening the indicated cells and modules.
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- The ``init`` attribute on wires is set by the frontend when a register is
initialized "FPGA-style" with ``reg foo = val``. It can be used during
synthesis to add the necessary reset logic.
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- The ``top`` attribute on a module marks this module as the top of the
design hierarchy. The ``hierarchy`` command sets this attribute when called
with ``-top``. Other commands, such as ``flatten`` and various backends
use this attribute to determine the top module.
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- The ``src`` attribute is set on cells and wires created by to the string
``<hdl-file-name>:<line-number>`` by the HDL front-end and is then carried
through the synthesis. When entities are combined, a new |-separated
string is created that contains all the string from the original entities.
- In addition to the ``(* ... *)`` attribute syntax, yosys supports
the non-standard ``{* ... *}`` attribute syntax to set default attributes
for everything that comes after the ``{* ... *}`` statement. (Reset
by adding an empty ``{* *}`` statement.)
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- In module parameter and port declarations, and cell port and parameter
lists, a trailing comma is ignored. This simplifies writing verilog code
generators a bit in some cases.
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- Modules can be declared with ``module mod_name(...);`` (with three dots
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instead of a list of module ports). With this syntax it is sufficient
to simply declare a module port as 'input' or 'output' in the module
- When defining a macro with `define, all text between triple double quotes
is interpreted as macro body, even if it contains unescaped newlines. The
tipple double quotes are removed from the macro body. For example:
`define MY_MACRO(a, b) """
assign a = 23;
assign b = 42;
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- The attribute ``via_celltype`` can be used to implement a Verilog task or
function by instantiating the specified cell type. The value is the name
of the cell type to use. For functions the name of the output port can
be specified by appending it to the cell type separated by a whitespace.
The body of the task or function is unused in this case and can be used
to specify a behavioral model of the cell type for simulation. For example:
module my_add3(A, B, C, Y);
parameter WIDTH = 8;
input [WIDTH-1:0] A, B, C;
output [WIDTH-1:0] Y;
module top;
(* via_celltype = "my_add3 Y" *)
(* via_celltype_defparam_WIDTH = 32 *)
function [31:0] add3;
input [31:0] A, B, C;
add3 = A + B + C;
- A limited subset of DPI-C functions is supported. The plugin mechanism
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(see ``help plugin``) can be used to load .so files with implementations
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of DPI-C routines. As a non-standard extension it is possible to specify
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a plugin alias using the ``<alias>:`` syntax. For example:
module dpitest;
import "DPI-C" function foo:round = real my_round (real);
parameter real r = my_round(12.345);
$ yosys -p 'plugin -a foo -i /lib/libm.so; read_verilog dpitest.v'
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- Sized constants (the syntax ``<size>'s?[bodh]<value>``) support constant
expressions as <size>. If the expression is not a simple identifier, it
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must be put in parentheses. Examples: ``WIDTH'd42``, ``(4+2)'b101010``
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- The system tasks ``$finish`` and ``$display`` are supported in initial blocks
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in an unconditional context (only if/case statements on parameters
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and constant values). The intended use for this is synthesis-time DRC.
Non-standard or SystemVerilog features for formal verification
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- Support for ``assert``, ``assume``, and ``restrict`` is enabled when
``read_verilog`` is called with ``-formal``.
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- The system task ``$initstate`` evaluates to 1 in the initial state and
to 0 otherwise.
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- The system task ``$anyconst`` evaluates to any constant value.
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- The system task ``$anyseq`` evaluates to any value, possibly a different
value in each cycle.
- The SystemVerilog tasks ``$past``, ``$stable``, ``$rose`` and ``$fell`` are
supported in any clocked block.
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- The syntax ``@($global_clock)`` can be used to create FFs that have no
explicit clock input ($ff cells).
Supported features from SystemVerilog
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When ``read_verilog`` is called with ``-sv``, it accepts some language features
from SystemVerilog:
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- The ``assert`` statement from SystemVerilog is supported in its most basic
form. In module context: ``assert property (<expression>);`` and within an
always block: ``assert(<expression>);``. It is transformed to a $assert cell.
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- The ``assume`` and ``restrict`` statements from SystemVerilog are also
supported. The same limitations as with the ``assert`` statement apply.
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- The keywords ``always_comb``, ``always_ff`` and ``always_latch``, ``logic``
and ``bit`` are supported.
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- SystemVerilog packages are supported. Once a SystemVerilog file is read
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into a design with ``read_verilog``, all its packages are available to
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SystemVerilog files being read into the same design afterwards.
Building the documentation
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Note that there is no need to build the manual if you just want to read it.
Simply download the PDF from http://www.clifford.at/yosys/documentation.html
On Ubuntu, texlive needs these packages to be able to build the manual:
sudo apt-get install texlive-binaries
sudo apt-get install texlive-science # install algorithm2e.sty
sudo apt-get install texlive-bibtex-extra # gets multibib.sty
sudo apt-get install texlive-fonts-extra # gets skull.sty and dsfont.sty
sudo apt-get install texlive-publishers # IEEEtran.cls
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Also the non-free font luximono should be installed, there is unfortunately
no Ubuntu package for this so it should be installed separately using
wget https://tug.org/fonts/getnonfreefonts/install-getnonfreefonts
sudo texlua install-getnonfreefonts # will install to /usr/local by default, can be changed by editing BINDIR at MANDIR at the top of the script
getnonfreefonts luximono # installs to /home/user/texmf
Then execute, from the root of the repository:
make manual
- To run `make manual` you need to have installed yosys with `make install`,
otherwise it will fail on finding `kernel/yosys.h` while building