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SEMAPHORE

Conceptually, a semaphore is a bucket. When a semaphore is allocated, a bucket that contains a fixed number of keys is created. Processes using semaphores must first procure a key from the bucket before they can continue to execute. If a specific process requires a key, only a fixed number of occurrences of that process can be in progress simultaneously. All others must wait until a sufficient number of keys is returned to the bucket. Semaphores are typically used for mutual exclusion, access control to shared resources, and basic synchronization.

Semaphore is a built-in class that provides the following methods:
  --  Create a semaphore with a specified number of keys: new()
  --  Obtain one or more keys from the bucket: get()
  --  Return one or more keys into the bucket: put()
  --  Try to obtain one or more keys without blocking: try_get()

EXAMPLE:semaphore
program main ;
semaphore sema = new(1);
initial begin
   repeat(3) begin
        fork
            ////////// PROCESS 1 ////////////////
             begin
                $display("1: Waiting for key");
                sema.get(1);
                $display("1: Got the Key");
                #(10);// Do some work
                sema.put(1);
                $display("1: Returning back key ");
                #(10);
             end
            ////////// PROCESS 2 ////////////////
             begin
                $display("2: Waiting for Key");
                 sema.get(1);
                 $display("2: Got the Key");
                 #(10);//Do some work
                 sema.put(1);
                $display("2: Returning back key ");
                 #(10);
             end
        join
    end
     #1000;
end
endprogram

RESULTS:

1: Waiting for key
1: Got the Key
2: Waiting for Key
1: Returning back key
2: Got the Key
2: Returning back key
1: Waiting for key
1: Got the Key
2: Waiting for Key
1: Returning back key
2: Got the Key
2: Returning back key
1: Waiting for key
1: Got the Key
2: Waiting for Key
1: Returning back key
2: Got the Key
2: Returning back key

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