how to nullify register in system verilog

Question

How do I nullify a register in SystemVerilog if I do not know the size? For example, if I have:

reg [DATA_WIDTH-1:0] data_stack;

with some parameter DATA_WIDTH given to the module. Is it possible to assign a zero to the register data_stack?

Note that it is sufficient to initialize a similar reg to zero as then I can simply perform a bitwise and between the two and nullify data_stack.

Answer 1

If I understand your question, you want to know how to generate a zero value with the same width as data_stack?

In that case, use the replication operator:

data_stack = {DATA_WIDTH{1'b0}};

This generates DATA_WIDTH number of zeros.

Answer 2

The IEEE Std 1800-2009 is the golden reference. The following sets all bits to 0:

data_stack = '0;

Refer to the section "Integer literal constants".

An unsized single-bit value can be specified by preceding the single-bit value with an apostrophe ( ' ), but without the base specifier. All bits of the unsized value shall be set to the value of the specified bit. In a self-determined context, an unsized single-bit value shall have a width of 1 bit, and the value shall be treated as unsigned.

Answer 3

I think your searches would yield better results if you searched for initialising registers, instead of nullify.

If this is a testbench variable then just :

initial begin
  data_stack = {DATA_WIDTH{1'b0}};
end

If it is to be used in a flip-flop you need to reset it:

always @(posedge clk or negedge reset_n) begin
  if (~reset_n) begin
    data_stack <= {DATA_WIDTH{1'b0}};
  end
  else begin
    //<=nextvalue;
  end
end