wait-fork and disable-fork

SystemVerilog provides advanced constructs to manage concurrency and synchronization in testbenches and hardware design. Two powerful constructs in this context are wait-fork and disable-fork. These constructs are especially useful for managing processes running within fork...join blocks. Let’s explore their functionality with detailed explanations and examples.

The Need for wait-fork and disable-fork

In some scenarios, you might want to:

  1. Wait for all child threads to complete at a specific point in your code (wait-fork).
  2. Terminate all currently running threads (disable-fork).

wait-fork

The wait-fork construct suspends the parent process until all child threads spawned by fork...join have completed. It’s particularly useful when you use fork...join_none or fork...join_any, where the parent process doesn’t inherently wait for all child processes to finish.

Example:

initial begin
    fork
        #5 $display("Thread A");
        #10 $display("Thread B");
    join_none  // Parent process continues without waiting
    $display("Parent process continues.");
    wait fork; // Wait for all child threads to finish
    $display("All threads completed.");
end

Output:

Parent process continues.
Thread A
Thread B
All threads completed.

In this example:

  • The parent process doesn’t initially wait for the child threads.
  • wait fork suspends the parent until all child threads (Thread A and Thread B) are done.

disable-fork

The disable-fork construct terminates all active child threads immediately. This is particularly useful for interrupting ongoing processes when a specific condition is met or an error occurs.

Example:

initial begin
    fork
        begin
            #10 $display("Thread X");
        end
        begin
            #20 $display("Thread Y");
        end
    join_none  // Parent process continues without waiting
    #5 disable fork; // Terminate all threads after 5 time units
    $display("All threads terminated.");
end

Output:

All threads terminated.

Here:

  • The parent process continues after spawning threads.
  • After 5 time units, disable fork terminates Thread X and Thread Y, so their $display statements are never executed.

Combining wait-fork and disable-fork

Sometimes, you might want to ensure all threads either complete or get terminated after a specific timeout. This can be achieved by combining wait-fork with a timeout condition.

Example:

initial begin
    fork
        #15 $display("Thread 1 finished.");
        #30 $display("Thread 2 finished.");
    join_none
    #20;
    if (!$time) begin
        disable fork; // Timeout condition met
        $display("Timeout! Terminating threads.");
    end else begin
        wait fork; // Wait for all threads to complete
        $display("All threads completed within the timeout.");
    end
end

Output:

Thread 1 finished.
Timeout! Terminating threads.

Here:

  • The parent process waits for 20 time units.
  • If threads are still running, disable fork terminates them.

Practical Use Cases

  1. Testbench Synchronization: Use wait-fork to ensure that all concurrently running testbench processes are complete before ending a simulation phase.
  2. Timeout Mechanism: Use disable-fork to implement a timeout mechanism where testbench processes are halted after exceeding a specified time.
  3. Error Handling: Combine disable-fork with error-detection logic to stop testbench processes when an error is encountered.
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