SystemVerilog randomization also works on array data structures like static arrays, dynamic arrays and queues. The variable has to be declared with type rand or randc to enable randomization of the variable.
Static Arrays
Randomization of static arrays are straight-forward and can be done similar to any other type of SystemVerilog variable.
class Packet;
rand bit [3:0] s_array [7]; // Declare a static array with "rand"
endclass
module tb;
Packet pkt;
// Create a new packet, randomize it and display contents
initial begin
pkt = new();
pkt.randomize();
$display("queue = %p", pkt.s_array);
end
endmodule
ncsim> run
queue = '{'hf, 'hf, 'h2, 'h9, 'he, 'h4, 'ha}
ncsim: *W,RNQUIE: Simulation is complete.
Dynamic Arrays
Dynamic arrays are arrays where the size is not pre-determined during array declaration. These arrays can have variable size as new members can be added to the array at any time.
Consider the example below where we declare a dynamic array as indicated by the empty square brackets [] of type rand. A constraint is defined to limit the size of the dynamic array to be somewhere in between 5 and 8. Another constraint is defined to assign each element in the array with the value of its index.
class Packet;
rand bit [3:0] d_array []; // Declare a dynamic array with "rand"
// Constrain size of dynamic array between 5 and 10
constraint c_array { d_array.size() > 5; d_array.size() < 10; }
// Constrain value at each index to be equal to the index itself
constraint c_val { foreach (d_array[i])
d_array[i] == i;
}
// Utility function to display dynamic array contents
function void display();
foreach (d_array[i])
$display ("d_array[%0d] = 0x%0h", i, d_array[i]);
endfunction
endclass
module tb;
Packet pkt;
// Create a new packet, randomize it and display contents
initial begin
pkt = new();
pkt.randomize();
pkt.display();
end
endmodule
Randomization yields an empty array if the size is not constrainted -> applicable for dynamic arrays and queues
Note that the array size was randomized to 9 (from constraint c_array), and the element at each index has a value of the index itself (from constraint c_val.
ncsim> run d_array[0] = 0x0 d_array[1] = 0x1 d_array[2] = 0x2 d_array[3] = 0x3 d_array[4] = 0x4 d_array[5] = 0x5 d_array[6] = 0x6 d_array[7] = 0x7 d_array[8] = 0x8 ncsim: *W,RNQUIE: Simulation is complete.
Queue randomization
class Packet;
rand bit [3:0] queue [$]; // Declare a queue with "rand"
// Constrain size of queue between 5 and 10
constraint c_array { queue.size() == 4; }
endclass
module tb;
Packet pkt;
// Create a new packet, randomize it and display contents
initial begin
pkt = new();
pkt.randomize();
// Tip : Use %p to display arrays
$display("queue = %p", pkt.queue);
end
endmodule
ncsim> run
queue = '{'hf, 'hf, 'h2, 'h9}
ncsim: *W,RNQUIE: Simulation is complete.
