Disable Statement

Part of the Fundamentals section of Coddy's Verilog journey — lesson 62 of 90.

The disable statement stops the execution of a named block (like a forever loop, begin-end block, or task). It is useful for breaking out of loops that would otherwise run forever.

Syntax:

disable block_name;

The block must be labeled with a name.

Disable with Forever Loop

begin : counter // counter gives the block the name counter.
  forever begin
    #10 count = count + 1;
    if (count == 5) begin
      disable counter;   // disable counter stops that entire named block.
    end
  end
end

The forever loop is inside a named block counter. The disable counter statement is placed inside the loop. When count reaches 5, disable counter executes and stops the entire block counter — which immediately stops the forever loop as well.

Disable in a Separate Block

begin : counter
  forever begin
    #10 count = count + 1;
  end
end

initial begin
  #50;
  disable counter;   // Stops the counter block from outside
end

Here, the forever loop runs continuously inside the named block counter. The disable counter statement is placed in a separate initial block. After 50 time units, disable counter executes and stops the counter block — which terminates the forever loop from the outside.

Important Rules

Rule	Explanation
Block must be named	`begin : block_name`
Disable stops execution immediately	Code stops at that moment
Can be used from anywhere	From any block in the module
Useful for stopping `forever` loops	Otherwise they never stop

Challenge

You are given a module with a counter that runs forever.

What to do:

Complete the empty block by replacing each comment with the correct code:

Add #50; to wait 50 time units
Add disable counter; to stop the counter block
Add $display("Counter stopped"); to print the message
Add $finish; to end the simulation

Cheat sheet

The disable statement stops execution of a named block (e.g., forever loop). The block must be labeled using begin : block_name.

Disable from inside the block:

begin : counter
  forever begin
    #10 count = count + 1;
    if (count == 5) disable counter;
  end
end

Disable from a separate block:

begin : counter
  forever begin
    #10 count = count + 1;
  end
end

initial begin
  #50;
  disable counter; // stops the named block from outside
end

Try it yourself

module disable_challenge;
  integer count;
  
  initial begin
    count = 0;
    fork
      begin : counter
        forever begin
          #10 count = count + 1;
          $display("count = %d", count);
        end
      end
      
      begin
        // Step 1: Wait 50 time units
        
        // Step 2: Disable the counter block
        
        // Step 3: Print "Counter stopped"
        
        // Step 4: End the simulation
        
      end
    join
  end
endmodule