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VERILOG SEMAPHORE

Semaphore In Verilog

A semaphore is a type of Interposes communication resource used for synchronization and mutual exclusion between any two asynchronous processes. A semaphore object is a synchronization object that maintains a count between zero and a specified maximum value. The count is decremented each time a thread completes a wait for the semaphore object and incremented each time a thread releases the semaphore. When the count reaches zero, no more threads can successfully wait for the semaphore object state to become signaled. The state of a semaphore is set to signaled when its count is greater than zero, and non-signaled when its count is zero. The semaphore object is useful in controlling a shared resource that can support a limited number of users. It acts as a gate that limits the number of threads sharing the resource to a specified maximum number.

Take an example, Many components in the testbench wants to access the dut memory. But memory has only one interface. So only one can do write and read operation at a time. Using semaphore, we can make sure that only one operation is done at a time.

Imagine a home with six persons living. They have only one car. Everyone wants to drive the car. But others plan to make a trip, when some other has gone out with car. The eldest person in home made a rule. Key will be with  him. Whoever wants, come to me and get the key. After finishing the job, return the key to him. This way, only one can plan for the trip.

EXAMPLE:
module sema();

    integer keys;

    initial
       keys = 1;

    task get_key();
    input integer i;
    begin
        if ( keys == 0)
        begin
        $display(" KEY IS NOT AVAILABLE : WAITING FOR KEYS : process %d",i);
        wait(keys == 1);
        end
        $display(" GOT THE KEY : GET SET GO :process %d",i);
        keys = 0;
    end
    endtask

    task put_keys();
    input integer i;
    begin
        keys = 1 ;
        $display(" PROCESS %d gave the key back ",i);
    end
    endtask

    initial
    begin
       # 10 ;
       get_key(1);
       repeat(4)
       # 10 $display(" PROCESS 1 GOT KEYS : OTHERS CANT WRITE MESSAGE TO LOG ");
       put_keys(1);
    end

    initial
    begin
        # 10 ;
        get_key(2);
        repeat(4)
        # 10 $display(" PROCESS 2 GOT KEYS : OTHERS CANT WRITE MESSAGE TO LOG ");
        put_keys(2);
    end

endmodule

RESULTS:

GOT THE KEY : GET SET GO :process 1
KEY IS NOT AVAILABLE : WAITING FOR KEYS : process 2
PROCESS 1 GOT KEYS : OTHERS CANT WRITE MESSAGE TO LOG
PROCESS 1 GOT KEYS : OTHERS CANT WRITE MESSAGE TO LOG
PROCESS 1 GOT KEYS : OTHERS CANT WRITE MESSAGE TO LOG
PROCESS 1 GOT KEYS : OTHERS CANT WRITE MESSAGE TO LOG
PROCESS 1 gave the key back
GOT THE KEY : GET SET GO :process 2
PROCESS 2 GOT KEYS : OTHERS CANT WRITE MESSAGE TO LOG
PROCESS 2 GOT KEYS : OTHERS CANT WRITE MESSAGE TO LOG
PROCESS 2 GOT KEYS : OTHERS CANT WRITE MESSAGE TO LOG
PROCESS 2 GOT KEYS : OTHERS CANT WRITE MESSAGE TO LOG
PROCESS 2 gave the key back

In this home, some of them are not interested to wait until they got the key. So they want tp progress to other works without waiting for keys.
The following example shows, if keys are not available, the process don't wait.

EXAMPLE:
module sema();

integer keys;

initial
keys = 1;

task get_key();
input integer i;
begin
if ( keys == 0)
begin
$display(" KEY IS NOT AVAILABLE : WAITING FOR KEYS : process %d",i);
wait(keys == 1);
end
$display(" GOT THE KEY : GET SET GO :process %d",i);
keys = 0;
end
endtask

function get_key_dont_wait();
input integer i;
reg got;
begin
got =0;
if ( keys == 0)
$display(" KEY IS NOT AVAILABLE : LEAVING WITHOUT WAITING FOR KEYS : process %d",i);
else
begin
$display(" GOT THE KEY : GET SET GO :process %d",i);
keys = 0;
got = 1;
end
get_key_dont_wait = got;
end
endfunction

task put_keys();
input integer i;
begin
keys = 1 ;
$display(" PROCESS %d gave the key back ",i);
end
endtask

initial
begin
# 10 ;
get_key(1);
repeat(4)
# 10 $display(" PROCESS 1 GOT KEYS : OTHERS CANT WRITE MESSAGE TO LOG ");
put_keys(1);
end

initial
begin
# 10 ;
if(get_key_dont_wait(2))
begin
repeat(4)
# 10 $display(" PROCESS 2 GOT KEYS : OTHERS CANT WRITE MESSAGE TO LOG ");
put_keys(2);
end
else
$display(" IM not interested to wait ");
end

endmodule

RESULTS:

GOT THE KEY : GET SET GO :process 1
KEY IS NOT AVAILABLE : LEAVING WITHOUT WAITING FOR KEYS : process 2
IM not interested to wait
PROCESS 1 GOT KEYS : OTHERS CANT WRITE MESSAGE TO LOG
PROCESS 1 GOT KEYS : OTHERS CANT WRITE MESSAGE TO LOG
PROCESS 1 GOT KEYS : OTHERS CANT WRITE MESSAGE TO LOG
PROCESS 1 GOT KEYS : OTHERS CANT WRITE MESSAGE TO LOG
PROCESS 1 gave the key back

After some days, they got new car to home. Now they have two cars, at once 2 members can go on drive. Looking at the following code. The keys are initialized to 2. Two processes are running at once.

EXAMPLE:
module sema();

integer keys;

initial
keys = 2;

task get_key();
input integer i;
begin
if ( keys == 0)
begin
$display(" KEY IS NOT AVAILABLE : WAITING FOR KEYS : process %d",i);
wait(keys > 0);
end
$display(" GOT THE KEY : GET SET GO :process %d",i);
keys = keys - 1;
end
endtask

function get_key_dont_wait();
input integer i;
reg got;
begin
got =0;
if ( keys == 0)
$display(" KEY IS NOT AVAILABLE : LEAVING WITHOUT WAITING FOR KEYS : process %d",i);
else
begin
$display(" GOT THE KEY : GET SET GO :process %d",i);
keys = keys - 1;
got = 1;
end
get_key_dont_wait = got;
end
endfunction

task put_keys();
input integer i;
begin
keys = keys + 1 ;
$display(" PROCESS %d gave the key back ",i);
end
endtask

initial
begin
# 10 ;
get_key(1);
repeat(4)
# 10 $display(" PROCESS 1 GOT KEYS : IM ALOS RUNNING ");
put_keys(1);
end

initial
begin
# 10 ;
if(get_key_dont_wait(2))
begin
repeat(4)
# 10 $display(" PROCESS 2 GOT KEYS : IM ALSO RUNNING ");
put_keys(2);
end
else
$display(" IM not interested to wait ");
end

endmodule

RESULTS:

GOT THE KEY : GET SET GO :process 1
GOT THE KEY : GET SET GO :process 2
PROCESS 1 GOT KEYS : IM ALOS RUNNING
PROCESS 2 GOT KEYS : IM ALSO RUNNING
PROCESS 1 GOT KEYS : IM ALOS RUNNING
PROCESS 2 GOT KEYS : IM ALSO RUNNING
PROCESS 1 GOT KEYS : IM ALOS RUNNING
PROCESS 2 GOT KEYS : IM ALSO RUNNING
PROCESS 1 GOT KEYS : IM ALOS RUNNING
PROCESS 1 gave the key back
PROCESS 2 GOT KEYS : IM ALSO RUNNING
PROCESS 2 gave the key back

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