Parallel Process

Fork Join Basics

"Fork...Join" is an important concept in SystemVerilog. It is a key mechanism for implementing parallel processes and, therefore, is essential in constructing non-sequential code blocks.

Now let's briefly cover this topic.

Fork Join

In SystemVerilog, the fork...join construct is used to initiate concurrent processes. This means that all the statements enclosed within the fork...join block will start executing simultaneously (in parallel) when a fork statement is encountered during the execution.

Here is a basic example of its use:

initial 
begin 
  fork
    begin : process_1
      // code for process 1...
    end
    begin : process_2
      // code for process 2...
    end
  join
end

In this example, process_1 and process_2 will start executing concurrently when the fork statement is encountered. The initial block will not finish until both process_1 and process_2 have finished due to the join statement.

join, join_any, join_none

SystemVerilog provides three types of join: join, join_any and join_none.

join: The parent process will wait for all the child processes to finish.

join_any: The parent process will continue as soon as any one of the child processes finishes.

join_none: The parent process will continue immediately without waiting for any child processes to finish.

The animation below will help demonstrate the difference between the three.

<INSER ANIMATION>

The fork...join construct is very useful in verification for creating and controlling parallel threads, like running different test scenarios simultaneously or controlling multiple stimuli applied to a design-under-test.