Flow overview

Figure 54 shows the simplified data flow within Yosys. Rectangles in the figure represent program modules and ellipses internal data structures that are used to exchange design data between the program modules.

Design data is read in using one of the frontend modules. The high-level HDL frontends for Verilog and VHDL code generate an abstract syntax tree (AST) that is then passed to the AST frontend. Note that both HDL frontends use the same AST representation that is powerful enough to cover the Verilog HDL and VHDL language.

The AST Frontend then compiles the AST to Yosys’s main internal data format, the RTL Intermediate Language (RTLIL). A more detailed description of this format is given in The RTL Intermediate Language (RTLIL).

There is also a text representation of the RTLIL data structure that can be parsed using the RTLIL Frontend which is described in RTLIL text representation.

The design data may then be transformed using a series of passes that all operate on the RTLIL representation of the design.

Finally the design in RTLIL representation is converted back to text by one of the backends, namely the Verilog Backend for generating Verilog netlists and the RTLIL Backend for writing the RTLIL data in the same format that is understood by the RTLIL Frontend.

With the exception of the AST Frontend, which is called by the high-level HDL frontends and can’t be called directly by the user, all program modules are called by the user (usually using a synthesis script that contains text commands for Yosys).

By combining passes in different ways and/or adding additional passes to Yosys it is possible to adapt Yosys to a wide range of applications. For this to be possible it is key that (1) all passes operate on the same data structure (RTLIL) and (2) that this data structure is powerful enough to represent the design in different stages of the synthesis.

Fig. 54 Yosys simplified data flow (ellipses: data structures, rectangles: program modules)