Verilog versions and portability

Verilog revisions add language features without making every simulator and synthesizer support the same set. Specify a language version in the tool command and document the required constructs. Keep Verilog-2001/2005 RTL separate from SystemVerilog features such as logic, always_comb and interfaces when teaching or maintaining a portable Verilog codebase.

Worked example

A Verilog variable driven in a clocked process is written as output reg. The example intentionally avoids SystemVerilog-only declarations. Its portability still depends on whether the target synthesis flow supports the chosen reset style.

module portable_reg(input clk,reset, input [7:0] d, output reg [7:0] q);
  always @(posedge clk) begin
    if (reset) q <= 8'h00;
    else q <= d;
  end
endmodule

What to watch for

A .sv extension or permissive compiler switch can hide accidental use of newer features. Passing one tool does not establish compatibility with all others.

Try it yourself

What does Icarus -g2012 request?

Answer: SystemVerilog-2012 language mode. Use an appropriate Verilog mode when deliberately checking Verilog-only source portability.

How to study this topic

Save the example in a .v file and identify its top module before compiling. Separate stimulus and observation from synthesizable RTL. Change one input, width or boundary condition at a time, predict the result, and compare that prediction with simulation. A syntax check does not establish synthesis support or timing closure.