Control and data flow
=====================

.. todo:: less academic

The data- and control-flow of a typical synthesis tool is very similar to the
data- and control-flow of a typical compiler: different subsystems are called in
a predetermined order, each consuming the data generated by the last subsystem
and generating the data for the next subsystem (see :numref:`Fig. %s
<fig:approach_flow>`).

.. figure:: ../../../images/approach_flow.*
	:class: width-helper
	:name: fig:approach_flow

	General data- and control-flow of a synthesis tool

The first subsystem to be called is usually called a frontend. It does not
process the data generated by another subsystem but instead reads the user
input—in the case of a HDL synthesis tool, the behavioural HDL code.

The subsystems that consume data from previous subsystems and produce data for
the next subsystems (usually in the same or a similar format) are called passes.

The last subsystem that is executed transforms the data generated by the last
pass into a suitable output format and writes it to a disk file. This subsystem
is usually called the backend.

In Yosys all frontends, passes and backends are directly available as commands
in the synthesis script. Thus the user can easily create a custom synthesis flow
just by calling passes in the right order in a synthesis script.