Semaphore is just like a bucket with a fixed number of keys. Processes that use a semaphore must first get a key from the bucket before they can continue to execute. Other proceses must wait until keys are available in the bucket for them to use. In a sense, they are best used for mutual exclusion, access control to shared resources and basic synchronization.
Syntax
semaphore [identifier_name];
Note that semaphore is a built-in class and hence it should be used just like any other class object. It has a few methods with which we can allocate the number of keys for that semaphore object, get and put keys into the bucket.
Methods
| Name | Description |
|---|---|
| function new (int keyCount = 0); | Specifies number of keys initially allocated to the semaphore bucket |
| function void put (int keyCount = 1); | Specifies the number of keys being returned to the semaphore |
| task get (int keyCount = 1); | Specifies the number of keys to obtain from the semaphore |
| function int try_get (int keyCount = 1); | Specifies the required number of keys to obtain from the semaphore |
Example
module tb_top;
semaphore key;
initial begin
key = new (1);
fork
personA ();
personB ();
#25 personA ();
join_none
end
task getRoom (bit [1:0] id);
$display ("[%0t] Trying to get a room for id[%0d] ...", $time, id);
key.get (1);
$display ("[%0t] Room Key retrieved for id[%0d]", $time, id);
endtask
task putRoom (bit [1:0] id);
$display ("[%0t] Leaving room id[%0d] ...", $time, id);
key.put (1);
$display ("[%0t] Room Key put back id[%0d]", $time, id);
endtask
task personA ();
getRoom (1);
#20 putRoom (1);
endtask
task personB ();
#5 getRoom (2);
#10 putRoom (2);
endtask
endmodule
Simulation Log
[0] Trying to get a room for id[1] ... [0] Room Key retrieved for id[1] [5] Trying to get a room for id[2] ... [20] Leaving room id[1] ... [20] Room Key put back id[1] [20] Room Key retrieved for id[2] [25] Trying to get a room for id[1] ... [30] Leaving room id[2] ... [30] Room Key put back id[2] [30] Room Key retrieved for id[1] [50] Leaving room id[1] ... [50] Room Key put back id[1]
