Scripting in Yosys

On the previous page we went through a synthesis script, running each command in the interactive Yosys shell. On this page, we will be introducing the script file format and how you can make your own synthesis scripts.

Yosys script files typically use the .ys extension and contain a set of commands for Yosys to run sequentially. These commands are the same ones we were using on the previous page like read_verilog and hierarchy.

Script parsing

As with the interactive shell, each command consists of the command name, and an optional whitespace separated list of arguments. Commands are terminated with the newline character, and anything after a hash sign # is a comment (i.e. it is ignored).

It is also possible to terminate commands with a semicolon ;. This is particularly useful in conjunction with the -p <command> command line option, where <command> can be a string with multiple commands separated by semicolon. In-line comments can also be made with the colon :, where the end of the comment is a semicolon ; or a new line.

Listing 23 Using the -p option
$ yosys -p "read_verilog fifo.v; :this is a comment; prep"


The space after the semicolon is required for correct parsing. log a;log b; for example will display a;log b instead of a and b as might be expected.

Another special character that can be used in Yosys scripts is the bang !. Anything after the bang will be executed as a shell command. This can only be terminated with a new line. Any semicolons, hashes, or other special characters will be passed to the shell. If an error code is returned from the shell it will be raised by Yosys. exec provides a much more flexible way of executing commands, allowing the output to be logged and more control over when to generate errors.

The synthesis starter script

All of the images and console output used in Synthesis starter were generated by Yosys, using Yosys script files found in docs/source/code_examples/fifo. If you haven’t already, let’s take a look at some of those script files now.

Listing 24 A section of fifo.ys, generating the images used for The addr_gen module
10echo on
11hierarchy -top addr_gen
12select -module addr_gen
13select -list
14select t:*
15select -list
16select -set new_cells %
17select -clear
18show -format dot -prefix addr_gen_show addr_gen
19show -format dot -prefix new_cells_show -notitle @new_cells
20show -color maroon3 @new_cells -color cornflowerblue p:* -notitle -format dot -prefix addr_gen_hier
22# ========================================================
23proc -noopt
24select -set new_cells t:$mux t:*dff
25show -color maroon3 @new_cells -notitle -format dot -prefix addr_gen_proc
27# ========================================================
28opt_expr; clean
29select -set new_cells t:$eq
30show -color cornflowerblue @new_cells -notitle -format dot -prefix addr_gen_clean
32# ========================================================

The first command there, echo on, uses echo to enable command echoes on. This is how we generated the code listing for hierarchy -top addr_gen output. Turning command echoes on prints the yosys> hierarchy -top addr_gen line, making the output look the same as if it were an interactive terminal. hierarchy -top addr_gen is of course the command we were demonstrating, including the output text and an image of the design schematic after running it.

We briefly touched on select when it came up in synth_ice40, but let’s look at it more now.

Selections intro

The select command is used to modify and view the list of selected objects:

yosys> select -module addr_gen

yosys [addr_gen]> select -list

yosys [addr_gen]> select t:*

yosys [addr_gen]*> select -list

yosys [addr_gen]*> select -set new_cells %

yosys [addr_gen]*> select -clear

When we call select -module addr_gen we are changing the currently active selection from the whole design, to just the addr_gen module. Notice how this changes the yosys at the start of each command to yosys [addr_gen]? This indicates that any commands we run at this point will only operate on the addr_gen module. When we then call select -list we get a list of all objects in the addr_gen module, including the module itself, as well as all of the wires, inputs, outputs, processes, and cells.

Next we perform another selection, select t:*. The t: part signifies we are matching on the cell type, and the * means to match anything. For this (very simple) selection, we are trying to find all of the cells, regardless of their type. The active selection is now shown as [addr_gen]*, indicating some sub-selection of the addr_gen module. This gives us the $add and $eq cells, which we want to highlight for the addr_gen module after hierarchy image.

We can assign a name to a selection with select -set. In our case we are using the name new_cells, and telling it to use the current selection, indicated by the % symbol. We can then use this named selection by referring to it as @new_cells, which we will see later. Then we clear the selection so that the following commands can operate on the full design. While we split that out for this document, we could have done the same thing in a single line by calling select -set new_cells addr_gen/t:*. If we know we only have the one module in our design, we can even skip the addr_gen/ part. Looking further down the fifo.ys code we can see this with select -set new_cells t:$mux t:*dff. We can also see in that command that selections don’t have to be limited to a single statement.

Many commands also support an optional [selection] argument which can be used to override the currently selected objects. We could, for example, call clean addr_gen to have clean operate on just the addr_gen module.

Detailed documentation of the select framework can be found under Selections or in the command reference at select - modify and view the list of selected objects.

Displaying schematics

While the select command is very useful, sometimes nothing beats being able to see a design for yourself. This is where show comes in. Note that this document is just an introduction to the show command, only covering the basics. For more information, including a guide on what the different symbols represent, see A look at the show command and the Interactive design investigation page.


Fig. 18 Calling show addr_gen after hierarchy


The show command requires a working installation of GraphViz and xdot for displaying the actual circuit diagrams.

This is the first show command we called in fifo.ys, as we saw above. If we look at the log output for this image we see the following:

yosys> show -format dot -prefix addr_gen_show addr_gen

4. Generating Graphviz representation of design.
Writing dot description to `'.
Dumping module addr_gen to page 1.

Calling show with -format dot tells it we want to output a .dot file rather than opening it for display. The -prefix addr_gen_show option indicates we want the file to be called addr_gen_show.*. Remember, we do this in fifo.ys because we need to store the image for displaying in the documentation you’re reading. But if you just want to display the images locally you can skip these two options. The -format option internally calls the dot command line program from GraphViz to convert to formats other than .dot. Check GraphViz output docs for more on available formats.


If you are using a POSIX based version of Yosys (such as for Mac or Linux), xdot will be opened in the background and Yosys can continue to be used. If it it still open, future calls to show will use the same xdot instance.

The addr_gen at the end tells it we only want the addr_gen module, just like when we called select -module addr_gen in Selections intro. That last parameter doesn’t have to be a module name, it can be any valid selection string. Remember when we assigned a name to a selection and called it new_cells? We saw in the select -list output that it contained two cells, an $add and an $eq. We can call show on that selection just as easily:


Fig. 19 Calling show -notitle @new_cells

We could have gotten the same output with show -notitle t:$add t:$eq if we didn’t have the named selection. By adding the -notitle flag there we can also get rid of the addr_gen title that would have been automatically added. The last two images were both added for this introduction. The next image is the first one we saw in Synthesis starter: showing the full addr_gen module while also highlighting @new_cells and the two PROC blocks. To achieve this highlight, we make use of the -color option:


Fig. 20 Calling show -color maroon3 @new_cells -color cornflowerblue p:* -notitle

As described in the the help output for show (or by clicking on the show link), colors are specified as -color <color> <object>. Color names for the <color> portion can be found on the GraphViz color docs. Unlike the final show parameter which can have be any selection string, the <object> part must be a single selection expression or named selection. That means while we can use @new_cells, we couldn’t use t:$eq t:$add. In general, if a command lists [selection] as its final parameter it can be any selection string. Any selections that are not the final parameter, such as those used in options, must be a single expression instead.

For all of the options available to show, check the command reference at show - generate schematics using graphviz.