display vs strobe vs monitor in verilog? [closed]

Question

What is the difference between display vs strobe vs monitor in verilog?

Answer 1

I'll be nice and summarize the LRM (Language Reference Manual), but you should read it. Everything is in IEEE Std 1800-2012 § 21.2 Display system tasks (Technically SystemVerilog, but these functions are identical.)

• $display : print the immediate values
  • § 21.2.1 The display and write tasks
• $strobe : print the values at the end of the current timestep
  • § 21.2.2 Strobed monitoring
• $monitor : print the values at the end of the current timestep if any values changed. $monitor can only be called once; sequential call will override the previous.
  • § 21.2.3 Continuous monitoring
• $write : same as $display but doesn't terminate with a newline (\n)
  • § 21.2.1 The display and write tasks

Example:

reg [3:0] a,b;
integer i;
initial begin
  $monitor("monitor a:%h b:%h @ %0t", a, b, $time);
  for(i=0; i<4; i=i+1) begin
    $strobe("strobe  a:%h b:%h @ %0t", a, b, $time);
    $display("display a:%h b:%h @ %0t", a, b, $time);
    case(i)
      0 : a = 4;
      1 : b = 1;
      2 : begin end // do nothing
      3 : {a,b} = 9;
    endcase
    $display("display a:%h b:%h @ %0t", a, b, $time);
    #1;
  end
end

Outputs: (notice the print order and that monitor is not displayed at time 2)

display a:x b:x @ 0
display a:4 b:x @ 0
monitor a:4 b:x @ 0
strobe a:4 b:x @ 0
display a:4 b:x @ 1
display a:4 b:1 @ 1
monitor a:4 b:1 @ 1
strobe a:4 b:1 @ 1
display a:4 b:1 @ 2
display a:4 b:1 @ 2
strobe a:4 b:1 @ 2
display a:4 b:1 @ 3
display a:0 b:9 @ 3
monitor a:0 b:9 @ 3
strobe a:0 b:9 @ 3

Answer 2

Verilog/SystemVerilog contains a well organized event queue. All the statements in each and every time stamp executes according to this queue.

• $display executes in ACTIVE region, so if there is any non-blocking assignment(which executes in INACTIVE region), it won't be shown by $display.
• $write also executes in ACTIVE region, but an explicit call to newline character(\n) is required to insert another line. This system task is generally used when you want to display Multidimensional array using for loop.
• $strobe executes in MONITOR/POSTPONE region, that is, at the end of time stamp. Hence the updated value is shown by $strobe.
• $monitor displays every time one of its display parameters changes. Only one $monitor per Simulation is to be used.

Have a look at this image : VERILOG EVENT REGIONS

Sample code is available at : Display/Strobe/Monitor

Hope this code makes it clear.