SystemVerilog Strings

What is a SystemVerilog string ?

The string data-type is an ordered collection of characters. The length of a string variable is the number of characters in the collection which can have dynamic length and vary during the course of a simulation. A string variable does not represent a string in the same way as a string literal. No truncation occurs when using the string variable.

Syntax

	string  variable_name [= initial_value];

variable_name is a valid identifier and the optional initial_value can be a string literal, the value "" for an empty string, or a string data type expression. If an initial value is not specified at the time of declaration, then the variable defaults to "", an empty string literal.

SystemVerilog String Example

module tb;	
  // Declare a string variable called "dialog" to store string literals
  // Initialize the variable to "Hello!"
  string   dialog = "Hello!";            
	
  initial begin
    // Display the string using %s string format
    $display ("%s", dialog);
    
    // Iterate through the string variable to identify individual characters and print
    foreach (dialog[i]) begin
      $display ("%s", dialog[i]);
    end
  end
endmodule

ncsim> run
Hello!
H
e
l
l
o
!
ncsim: *W,RNQUIE: Simulation is complete.

How are strings represented in Verilog ?

A single ASCII character requires 8-bits (1 byte) and to store a string we would need as many bytes as there are number of characters in the string.

	reg  [16*8-1:0] 	my_string;           // Can store 16 characters
	
	my_string = "How are you";              // 5 zeros are padded from MSB, and 11 char are stored
	my_string = "How are you doing?";       // 19 characters; my_string will get "w are you doing?"

String Operators

Example

module tb;
  string firstname = "Joey";
  string lastname  = "Tribbiani";
  
  initial begin
    // String Equality : Check if firstname equals or not equals lastname
    if (firstname == lastname)
      $display ("firstname=%s is EQUAL to lastname=%s", firstname, lastname);
    
    if (firstname != lastname)
      $display ("firstname=%s is NOT EQUAL to lastname=%s", firstname, lastname);
    
    // String comparison : Check if length of firstname < length of lastname
    if (firstname < lastname)
      $display ("firstname=%s is LESS THAN lastname=%s", firstname, lastname);
    
    // String comparison : Check if length of firstname > length of lastname
    if (firstname > lastname)
      $display ("firstname=%s is GREATER THAN lastname=%s", firstname, lastname);
    
    // String concatenation : Join first and last names into a single string
    $display ("Full Name = %s", {firstname, " ", lastname});
    
    // String Replication
    $display ("%s", {3{firstname}});
    
    // String Indexing : Get the ASCII character at index number 2 of both first and last names
    $display ("firstname[2]=%s lastname[2]=%s", firstname[2], lastname[2]);
    
  end
endmodule

ncsim> run
firstname=Joey is NOT EQUAL to lastname=Tribbiani
firstname=Joey is LESS THAN lastname=Tribbiani
Full Name = Joey Tribbiani
JoeyJoeyJoey
firstname[2]=e lastname[2]=i
ncsim: *W,RNQUIE: Simulation is complete.

Basic String Methods

SystemVerilog also includes a number of special methods to work with strings, which use built-in method notation.

Example

module tb;
   string str = "Hello World!";
   
   initial begin
   	  string tmp;

	  // Print length of string "str"
      $display ("str.len() = %0d", str.len());

      // Assign to tmp variable and put char "d" at index 3
      tmp = str;
      tmp.putc (3,"d");
      $display ("str.putc(3, d) = %s", tmp);

      // Get the character at index 2
      $display ("str.getc(2) = %s (%0d)", str.getc(2), str.getc(2));

      // Convert all characters to lower case
      $display ("str.tolower() = %s", str.tolower());

      // Comparison
      tmp = "Hello World!";
      $display ("[tmp,str are same] str.compare(tmp) = %0d", str.compare(tmp));
      tmp = "How are you ?";
      $display ("[tmp,str are diff] str.compare(tmp) = %0d", str.compare(tmp));

      // Ignore case comparison
      tmp = "hello world!";
      $display ("[tmp is in lowercase] str.compare(tmp) = %0d", str.compare(tmp));
      tmp = "Hello World!";
      $display ("[tmp,str are same] str.compare(tmp) = %0d", str.compare(tmp));

      // Extract new string from i to j
      $display ("str.substr(4,8) = %s", str.substr (4,8));

   end
endmodule

str.len() = 12                               
str.putc(3, d) = Heldo World!
str.getc(2) = l (108)
str.tolower() = hello world!
[tmp,str are same] str.compare(tmp) = 0
[tmp,str are diff] str.compare(tmp) = -1
[tmp is in lowercase] str.compare(tmp) = -1
[tmp,str are same] str.compare(tmp) = 0
str.substr(4,8) = o Wor

String Conversion Methods