Display And Monitor

Part of the Fundamentals section of Coddy's Verilog journey — lesson 75 of 90.

$display and $monitor are system tasks used to print information from your simulation. They help you see what is happening inside your design.

$display

$display prints a message once at the moment it is executed.

Syntax:

$display("message", variables);

Example:

initial begin
  $display("Simulation started");
  #10;
  $display("Time 10");
  #10;
  $display("Time 20");
end

Output:

Simulation started
Time 10
Time 20

$monitor

$monitor prints a message automatically whenever any of its variables change.

Syntax:

$monitor("message", variables);

Example:

initial begin
  a = 0; b = 0;
  $monitor("Time %0t: a=%b, b=%b", $time, a, b);
  #10 a = 1;
  #10 b = 1;
  #10 a = 0;
end

Output:

Time 0: a=0, b=0
Time 10: a=1, b=0
Time 20: a=1, b=1
Time 30: a=0, b=1

$display vs $monitor

	$display	$monitor
When it prints	Once when executed	Every time a variable changes
How many times	As many times as you call it	Continuously (until changed)
Use for	Headers, test messages	Tracking changing signals

Common Format Specifiers

Specifier	Meaning	Example
`%b`	Binary	`$display("%b", a);`
`%d`	Decimal	`$display("%d", count);`
`%h`	Hexadecimal	`$display("%h", data);`
`%t`	Time	`$display("%t", $time);`
`%0t`	Time (no spaces)	`$display("%0t", $time);`
`%s`	String	`$display("%s", "Hello");`

Important Rules

Rule	Explanation
`$display` prints once	Good for headers and final results
`$monitor` prints on change	Good for watching signals
Only one `$monitor` active	Last one overrides previous
Use `$finish` to stop	Otherwise simulation may run forever

Challenge

Add the missing $display and $monitor statements to this testbench.

What to do:

Add $display to print a header: "Testing OR Gate"
Add $monitor to print time, x, y, and z whenever any signal changes. Format: "Time %0t: x=%b, y=%b, z=%b"
Add $display at the end to print "Test complete"

Cheat sheet

$display prints once when executed; $monitor prints automatically whenever any of its variables change.

$display("message", variables);
$monitor("message", variables);

Example:

initial begin
  $display("Simulation started");
  $monitor("Time %0t: a=%b, b=%b", $time, a, b);
  #10 a = 1;
  #10 b = 1;
end

Common format specifiers:

%b — Binary
%d — Decimal
%h — Hexadecimal
%t / %0t — Time (with/without padding)
%s — String

Key rules:

Only one $monitor is active at a time — the last one overrides previous ones.
Use $finish to stop the simulation.

Try it yourself

module or_gate (
  input x,
  input y,
  output z
);
  assign z = x | y;
endmodule

module testbench;
  reg x, y;
  wire z;
  
  or_gate dut (
    .x(x),
    .y(y),
    .z(z)
  );

  initial begin
    // TODO: Add $display header "Testing OR Gate"
    
    
    // TODO: Add $monitor to track time, x, y, z
    // Format: "Time %0t: x=%b, y=%b, z=%b"
    
    
    // Apply stimulus
    x = 0; y = 0; #10;
    x = 0; y = 1; #10;
    x = 1; y = 0; #10;
    x = 1; y = 1; #10;
    
    // TODO: Add $display "Test complete"
    
    
    $finish;
  end
endmodule