Randcase in SystemVerilog

randcase is a case statement that randomly selects one of its branches.

Randcase can be used in class or modules.

The randcase item expressions are non-negative integral values that constitute the branch weights.

An item weight divided by the sum of all weights gives the probability of taking that branch.

For example:

	randcase
	3 : x = 1;
	1 : x = 2;
	4 : x = 3;
	endcase

view raw randcase1.sv hosted with ❤ by GitHub

The sum of all weights is 8; therefore, the probability of taking the first branch is (3/8)0.375, the probability

of taking the second is (1/8)0.125, and the probability of taking the third is (4/8)0.5.

If a branch specifies a zero weight, then that branch is not taken.

If all randcase_items specify zero weights, then no branch is taken and a warning can be issued.

The randcase weights can be arbitrary expressions, not just constants.

For example:

	byte a, b;
	randcase
	a + b : x = 1;
	a - b : x = 2;
	a ^ ~b : x = 3;
	12'b800 : x = 4;
	endcase

view raw randcase2.sv hosted with ❤ by GitHub

In the preceding example, the first three weight expressions are computed using 8-bit precision, and the fourth

expression is computed using 12-bit precision.

The resulting weights are added as unsigned values using 12-bit precision. The weight selection then uses unsigned

12-bit comparison.

Each call to randcae statement will return a random number in the range from 0 to SUM.

$urandom_range(0,SUM) is used to generate a random number.

	module rand_case;
	integer x;
	integer cnt_1, cnt_2, cnt_3;
	initial begin
	cnt_1 = 0;
	cnt_2 = 0;
	cnt_3 = 0;
	repeat(100000) begin
	randcase
	3 : x = 1;
	1 : x = 2;
	4 : x = 3;
	endcase
	if(x == 1) begin
	cnt_1++;
	end
	else if(x == 2) begin
	cnt_2++;
	end
	else if(x ==3) begin
	cnt_3++;
	end
	end
	$display("count_1 = %0d, count_2 = %0d, count_3 = %0d", cnt_1, cnt_2, cnt_3);
	$display("Probability of count_1 = %0f, count_2 = %0f, count_3 = %0f", (cnt_1/100000.0), (cnt_2/100000.0), (cnt_3/100000.0));
	end
	endmodule : rand_case
	//Output:
	// count_1 = 37378, count_2 = 12480, count_3 = 50142
	// Probability of count_1 = 0.373780, count_2 = 0.124800, count_3 = 0.501420

view raw randcase3.sv hosted with ❤ by GitHub