Process Statement

A process statement is a concurrent command that consists of a label, sensitivity list, declaration area, begin–end (body) area, and sequential statements. An example of a process statement description in VHDL is:

process (<Sensitivity List>)
  -- Variable declaration area
begin
  -- VHDL statement block here
end process;

The difference between a concurrent signal assignment statement and a process statement lies in the sequential statements. The syntax or statements inside the begin–end (body) section are executed sequentially, line by line, just like in general programming. The process label itself is simply a self-descriptive naming form to help us recognize which process is being executed in that section, so the naming can be changed or even omitted.

In a concurrent statement, every time a change occurs in the input, the output is re-evaluated. In a behavioral style model using a process statement, whenever a change occurs in a signal listed in the sensitivity list of the process, all the sequential statements within the process body are re-evaluated.

Since a process statement is itself a concurrent statement, if there are two processes in the architecture body, the execution of both processes will be carried out concurrently.