Yosys Open SYnthesis Suite
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yosys -- Yosys Open SYnthesis Suite

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

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.

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.

Yosys is free software licensed under the ISC license (a GPL compatible license that is similar in terms to the MIT license or the 2-clause BSD license).

Web Site

More information and documentation can be found on the Yosys web site: http://www.clifford.at/yosys/


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. TCL, readline and libffi are optional (see ENABLE_* settings in Makefile). Xdot (graphviz) is used by the show command in yosys to display schematics.

For example on Ubuntu Linux 16.04 LTS the following commands will install all prerequisites for building yosys:

$ sudo apt-get install build-essential clang bison flex \
    libreadline-dev gawk tcl-dev libffi-dev git mercurial \
    graphviz xdot pkg-config python3

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
$ sudo port install bison flex readline gawk libffi \
    git mercurial graphviz pkgconfig python36

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

To configure the build system to use a specific compiler, use one of

$ 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 Makefile.

$ vi Makefile            # ..or..
$ vi Makefile.conf

To build Yosys simply type 'make' in this directory.

$ make
$ make test
$ sudo make install

Note that this also downloads, builds and installs ABC (using yosys-abc as executable name).

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

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

convert processes (always blocks) to netlist elements and perform some simple optimizations:

yosys> proc; opt

display design netlist using xdot:

yosys> show

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

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

$ ./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

The following very basic synthesis script should work well with all designs:

# check design hierarchy

# translate processes (always blocks)
proc; opt

# detect and optimize FSM encodings
fsm; opt

# implement memories (arrays)
memory; opt

# convert to gate logic
techmap; opt

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
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, 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:

The command synth provides a good default synthesis script (see help synth). If possible a synthesis script should borrow from synth. For example:

# 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.

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 IEC 62142(E):2005 / IEEE Std. 1364.1(E):2002

  • The tri, triand, trior, wand and wor net types

  • The config keyword and library map files

  • The disable, primitive and specify statements

  • Latched logic (is synthesized as logic with feedback loops)

Verilog Attributes and non-standard features

  • The full_case attribute on case statements is supported (also the non-standard // synopsys full_case directive)

  • The parallel_case attribute on case statements is supported (also the non-standard // synopsys parallel_case directive)

  • The // synopsys translate_off and // synopsys translate_on directives are also supported (but the use of `ifdef .. `endif is strongly recommended instead).

  • The nomem2reg attribute on modules or arrays prohibits the 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.

  • The mem2reg attribute on modules or arrays forces the early conversion of arrays to separate registers.

  • 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.

  • The nolatches attribute on modules or always-blocks prohibits the generation of logic-loops for latches. Instead all not explicitly assigned values default to x-bits. This does not affect clocked storage elements such as flip-flops.

  • 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.

  • The onehot attribute on wires mark them as onehot state register. This is used for example for memory port sharing and set by the fsm_map pass.

  • 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 also does not output blackbox modules on default.

  • 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. Setting the keep attribute on a module has the same effect as setting it on all instances of the module.

  • The keep_hierarchy attribute on cells and modules keeps the flatten command from flattening the indicated cells and modules.

  • 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.

  • 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.

  • 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.)

  • 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.

  • Modules can be declared with module mod_name(...); (with three dots 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 body.

  • 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;
  • 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 (see help plugin) can be used to load .so files with implementations of DPI-C routines. As a non-standard extension it is possible to specify 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'
  • Sized constants (the syntax <size>'s?[bodh]<value>) support constant expressions as . If the expression is not a simple identifier, it must be put in parentheses. Examples: WIDTH'd42, (4+2)'b101010

  • The system tasks $finish and $display are supported in initial blocks in an unconditional context (only if/case statements on parameters and constant values). The intended use for this is synthesis-time DRC.

Non-standard or SystemVerilog features for formal verification

  • Support for assert, assume, restrict, and cover'' is enabled whenread_verilogis called with-formal``.

  • The system task $initstate evaluates to 1 in the initial state and to 0 otherwise.

  • The system task $anyconst evaluates to any constant value. This is equivalent to declaring a reg as rand const, but also works outside of checkers. (Yosys also supports rand const outside checkers.)

  • The system task $anyseq evaluates to any value, possibly a different value in each cycle. This is equivalent to declaring a reg as rand, but also works outside of checkers. (Yosys also supports rand variables outside checkers.)

  • The SystemVerilog tasks $past, $stable, $rose and $fell are supported in any clocked block.

  • The syntax @($global_clock) can be used to create FFs that have no explicit clock input ($ff cells).

Supported features from SystemVerilog

When read_verilog is called with -sv, it accepts some language features from SystemVerilog:

  • 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.

  • The assume, restrict, and cover statements from SystemVerilog are also supported. The same limitations as with the assert statement apply.

  • The keywords always_comb, always_ff and always_latch, logic and bit are supported.

  • Declaring free variables with rand and rand const is supported.

  • Checkers without a port list that do not need to be instantiated (but instead behave like a named block) are supported.

  • SystemVerilog packages are supported. Once a SystemVerilog file is read into a design with read_verilog, all its packages are available to SystemVerilog files being read into the same design afterwards.

Building the documentation

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 instead.

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

Also the non-free font luximono should be installed, there is unfortunately no Ubuntu package for this so it should be installed separately using getnonfreefonts:

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 PRESENTATION_Prog.