Verilog Interview Questions - v1.2

Q.1) Explain $display, $monitor and $strobe?

Answer) All these commands have the same syntax.
$display :- The $display displays once the value of parameters, when it is executed.
$strobe :- The $strobe displays the parameter at the very end of the current simulation time.
$monitor :- The $monitor displays once every time one of it's parameters changes.

The format string is same as that of C/C++.
Format characters include :
Decimal              :- %d
Hexadecimal      :- %h
Binary                :- %b
Character           :- %c
String                 :- %s
Time                   :- %t
Hierarchy Level  :- %m

Syntax :-
$display("format_string", val1,  val2,....)
$strobe("format_string", val1, val2, ...)
$monitor("format_string", val1, val2,.....)

Q.2) What is the difference between:
         y = (sel)? i1 : i2;       and                if(sel) y = i1;
                                                                  else    y = i2;

Answer) The ternary operator "?" , merges the answer.
              Example : sel = 2'bx1; i1 = 2'b00; i0 = 2'b10;
                              y = 2'bx0;

               The "if" treats "X" or "Z" as false values, so the answer always be in false condition, i.e. i2.

Q.3) What is the difference between equality operator (==) and case equality operator(===)?

Answer) The equality operator (==) can give the output 0,1 or X.
               Case equality operator (===) only gives the output TRUE (1) or FALSE (0).
               If I1 = 3'bx01 ; I2 = 3'b1x0;
               I1 == I2 gives 'X';
               I1 === I2 gives '0';

Q.4) Explain the difference between task and function.

Answer)

Q.5) Write a verilog code to swap contents of two registers with and without a temporary register.

Answer) With temporary register :

               always@(posedge clock)

                begin

                    temp = b;

                    b = a;

                    a = temp;

                end

                Without temporary register :

                 always @(posedge clock)

                 begin

                    a <= b;

                    b <= a;

                 end