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RANDOMIZATION CONTROLABILITY

Controlability

Additional to the controllability feauters supported by SystemVerilog, following are more points with which controlabiity can be achieved.

In the following example, MACROS MIN_D and MAX_D are defined. Set the MIN and MAX values in the pre_randomize as shown. As MIN_D and MAX_D are macros, they can be assigned from command line. Biggest disadvantage for the method shown below is dynamic controllability.

EXAMPLE:
`define MAX_D 100
`define MIN_D 50
class Base;
rand integer Var;
integer MIN,MAX;
constraint randge { Var < MAX ; Var > MIN ;}
function void pre_randomize ();
this.MIN = `MIN_D;
this.MAX = `MAX_D;
$display( " PRE_RANDOMIZE : MIN = %0d , MAX = %0d ",MIN,MAX);
endfunction
endclass

program inhe_42;
Base obj;
initial
begin
obj = new();
for(int i=0 ; i < 100 ; i++)
if(obj.randomize())
$display(" Randomization sucsessfull : Var = %0d ",obj.Var);
else
$display("Randomization failed");
end
endprogram

RESULTS:

# PRE_RANDOMIZE : MIN = 50 , MAX = 100
# Randomization sucsessfull : Var = 91
# PRE_RANDOMIZE : MIN = 50 , MAX = 100
# Randomization sucsessfull : Var = 93
# PRE_RANDOMIZE : MIN = 50 , MAX = 100
# Randomization sucsessfull : Var = 77
# PRE_RANDOMIZE : MIN = 50 , MAX = 100
# Randomization sucsessfull : Var = 68
# PRE_RANDOMIZE : MIN = 50 , MAX = 100
# Randomization sucsessfull : Var = 67
# PRE_RANDOMIZE : MIN = 50 , MAX = 100
# Randomization sucsessfull : Var = 52
# PRE_RANDOMIZE : MIN = 50 , MAX = 100
# Randomization sucsessfull : Var = 71
# PRE_RANDOMIZE : MIN = 50 , MAX = 100
# Randomization sucsessfull : Var = 98
...etc.

As in this example,a single object is created and randomized 100 times. Due to this,pre_reandomize is called 100 times, which may not be preferred.
By assigning the values while declaration itself this can be avoided. Simpler way to achieve the above logic.

EXAMPLE:
`define MAX_D 100
`define MIN_D 50
class Base;
rand integer Var;
integer MIN = `MIN_D;
integer MAX = `MAX_D;
constraint range { Var < MAX ; Var > MIN ;}
endclass

program inhe_43;
Base obj;
initial
begin
obj = new();
for(int i=0 ; i < 100 ; i++)
if(obj.randomize())
$display(" Randomization sucsessfull : Var = %0d ",obj.Var);
else
$display("Randomization failed");
end
endprogram

RESULTS:

# Randomization sucsessfull : Var = 91
# Randomization sucsessfull : Var = 93
# Randomization sucsessfull : Var = 77
# Randomization sucsessfull : Var = 68
# Randomization sucsessfull : Var = 67
# Randomization sucsessfull : Var = 52
# Randomization sucsessfull : Var = 71
# Randomization sucsessfull : Var = 98
# Randomization sucsessfull : Var = 69
# Randomization sucsessfull : Var = 70
...etc.

With the above approach also, dynamic controlability is lost. For dynamic controllability, define a task, pass this values as arguments when ever the changed is needed.

EXAMPLE:
class Base;
rand integer Var;
integer MIN = 10,MAX = 20; // Define default values,If function set is not called,with this it will work
constraint randge { Var < MAX ; Var > MIN ;}
task set (integer MIN,integer MAX);
this.MIN = MIN;
this.MAX = MAX;
$display( " SET : MIN = %0d , MAX = %0d ",MIN,MAX);
endtask
endclass

program inhe_44;
Base obj;

initial
begin
obj = new();
obj.set(0,100) ;
for(int i=0 ; i < 5 ; i++)
if(obj.randomize())
$display(" Randomization sucsessfull : Var = %0d ",obj.Var);
else
$display("Randomization failed");

obj.set(50,100) ;
for(int i=0 ; i < 5 ; i++)
if(obj.randomize())
$display(" Randomization sucsessfull : Var = %0d ",obj.Var);
else
$display("Randomization failed");
end
endprogram
RESULTS:

# SET : MIN = 0 , MAX = 100
# Randomization sucsessfull : Var = 24
# Randomization sucsessfull : Var = 68
# Randomization sucsessfull : Var = 43
# Randomization sucsessfull : Var = 11
# Randomization sucsessfull : Var = 4
# SET : MIN = 50 , MAX = 100
# Randomization sucsessfull : Var = 52
# Randomization sucsessfull : Var = 71
# Randomization sucsessfull : Var = 98
# Randomization sucsessfull : Var = 69
# Randomization sucsessfull : Var = 70

More simpler way to dynamically modifying the constraints is by modifying the data members of class via object reference.

EXAMPLE:
class Base;
rand integer Var;
integer MIN = 20,MAX =30;
constraint randge { Var < MAX ; Var > MIN ;}
endclass

program inhe_45;
Base obj;

initial
begin
obj = new();
obj.MIN = 0;
obj.MAX = 100;
for(int i=0 ; i < 5 ; i++)
if(obj.randomize())
$display(" Randomization sucsessfull : Var = %0d ",obj.Var);
else
$display("Randomization failed");
$display("MIN and MAX changed");

obj.MIN = 50;
obj.MAX = 100;
for(int i=0 ; i < 5 ; i++)
if(obj.randomize())
$display(" Randomization sucsessfull : Var = %0d ",obj.Var);
else
$display("Randomization failed");
end
endprogram

RESULTS:

# Randomization sucsessfull : Var = 24
# Randomization sucsessfull : Var = 68
# Randomization sucsessfull : Var = 43
# Randomization sucsessfull : Var = 11
# Randomization sucsessfull : Var = 4
# MIN and MAX changed
# Randomization sucsessfull : Var = 52
# Randomization sucsessfull : Var = 71
# Randomization sucsessfull : Var = 98
# Randomization sucsessfull : Var = 69
# Randomization sucsessfull : Var = 70

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