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STATIC AND AUTOMATIC LIFETIME

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Static: For a variable static lifetime is , its memory never de-allocated until simulation ends.

Automatic: For a variable Automatic lifetime is , it is stack storage of variable (for multiple entry to a task, function or block, it will have stack storage) 

and its memory will be de-allocated once execution of that method or block is over.

Default Lifetime of variables:

1. Class variable : Automatic

2. Method variable : Automatic

3. Local variable of loop : Automatic

4. Module or Program block variable : Static

5. Variable declared in initial of always block : Static

Default lifetime of Methods/subroutines:

1. Method in module or program block : Static

2. Method in class : Automatic

Let’s understand through below example.

In below mentioned example factorial function is recursive. But as default it is static it will not create stack entry for it.

module tryfact;  

  function  integer factorial (input [31:0] operand);

  begin

    if (operand >= 2)

    factorial = factorial (operand - 1) * operand;

    else

    factorial = 1;

  end

  endfunction: factorial

  integer result;

  initial begin

    for (int n = 0; n <= 7; n++) begin

      result = factorial(n);

      $display("%0d factorial=%0d", n, result);

    end

  end

endmodule: tryfact

life time of method in program block and module is static. This is to keep it same with existing verilog reference. So for module function lifetime is static. Its variable and return type is static. So if we call recursive function, it will not create multiple stack entry for call of function and stack memory for internal variable.

See in below example where function is automatic.

module tryfact;

  

  function automatic integer factorial (input [31:0] operand);

  begin

    if (operand >= 2)

    factorial = factorial (operand - 1) * operand;

    else

    factorial = 1;

  end

  endfunction: factorial

  

  integer result;

  initial begin

    for (int n = 0; n <= 7; n++) begin

      result = factorial(n);

      $display("%0d factorial=%0d", n, result);

    end

  end

endmodule: tryfact

In above example we explicitly mentioned function as an automatic. So, it will create multiple call entry and variable(related to method) memory entry. As you can see that in output of the given example. 

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Lần cập nhật cuối ( Thứ ba, 14 Tháng 9 2021 19:06 )  

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