Mutexes in Verilog

In the previous example, we saw how to manage simple counter state using atomic operations. For more complex state management in Verilog, we can use a semaphore to safely access data across multiple processes.

module Mutex;
  // Container holds a map of counters
  reg [31:0] counters [1:0];
  reg mutex;

  // Initialize the counters and mutex
  initial begin
    counters[0] = 0; // counter "a"
    counters[1] = 0; // counter "b"
    mutex = 0;       // mutex is initially unlocked
  end

  // Function to increment a named counter
  task inc;
    input [0:0] name;
    begin
      // Lock the mutex before accessing counters
      while (mutex == 1) #1;
      mutex = 1;

      // Increment the counter
      counters[name] = counters[name] + 1;

      // Unlock the mutex
      mutex = 0;
    end
  endtask

  // Main process
  initial begin
    fork
      // Start three concurrent processes
      process_a1();
      process_a2();
      process_b();
    join

    // Print the final counter values
    $display("counters = {a: %0d, b: %0d}", counters[0], counters[1]);
  end

  // Process to increment counter "a" 10000 times
  task process_a1;
    integer i;
    begin
      for (i = 0; i < 10000; i = i + 1) begin
        inc(0);
      end
    end
  endtask

  // Another process to increment counter "a" 10000 times
  task process_a2;
    integer i;
    begin
      for (i = 0; i < 10000; i = i + 1) begin
        inc(0);
      end
    end
  endtask

  // Process to increment counter "b" 10000 times
  task process_b;
    integer i;
    begin
      for (i = 0; i < 10000; i = i + 1) begin
        inc(1);
      end
    end
  endtask

endmodule

In this Verilog implementation:

1. We define a module called Mutex that contains our counters and mutex.

2. The counters array represents our map of counters. In Verilog, we use a fixed-size array instead of a dynamic map.

3. The mutex reg acts as our mutual exclusion mechanism.

4. The inc task is equivalent to the inc method in the original code. It uses a simple busy-wait mechanism to implement the mutex lock.

5. We use fork-join to start three concurrent processes, similar to goroutines in the original code.

6. Each process (process_a1, process_a2, and process_b) increments its respective counter 10000 times.

7. At the end, we display the final counter values.

To run this Verilog code, you would typically use a Verilog simulator. The output should show that the counters were updated as expected:

counters = {a: 20000, b: 10000}

This example demonstrates how to implement mutex-like behavior in Verilog to safely manage shared state across multiple concurrent processes. While Verilog doesn’t have built-in mutex primitives like some high-level programming languages, we can implement similar functionality using basic synchronization techniques.

Next, we’ll look at implementing this same state management task using only concurrent processes and inter-process communication mechanisms in Verilog.