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]);
endmodule
ncsim> run
Hello!
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.
Click here to learn about Verilog Arrays
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
| Operator | Semantics | |
|---|---|---|
| Equality | Str1 == Str2 | Returns 1 if the two strings are equal and 0 if they are not |
| Inequality | Str1 != Str2 | Returns 1 if the two strings are not equal and 0 if they are |
| Comparison |
Str1 < Str2
Str1 <= Str2 Str1 > Str2 Str1 >= Str2 |
Returns 1 if the correspondig condition is true and 0 if false |
| Concatenation | {Str1, Str2, ..., StrN} | All strings will be concatenated into one resultant string |
| Replication | {multiplier{Str}} | Replicates the string N number of times, where N is specified by the multiplier |
| Indexing | Str[index] | Returns a byte, the ASCII code at the given index. If given index is out of range, it returns 0 |
| Methods | Str.method([args]) | The dot(.) operator is used to call string functions |
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]);
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.
| Usage | Definition | Comments |
|---|---|---|
| str.len() | function int len() | Returns the number of characters in the string |
| str.putc() | function void putc (int i, byte c); | Replaces the i th character in the string with the given character |
| str.getc() | function byte getc (int i); | Returns the ASCII code of the i th character in str |
| str.tolower() | function string tolower(); | Returns a string with characters in str converted to lowercase |
| str.compare(s) | function int compare (string s); | Compares str and s, as in the ANSI C strcmp function |
| str.icompare(s) | function int icompare (string s); | Compares str and s, like the ANSI C strcmp function |
| str.substr (i, j) | function string substr (int i, int j); | Returns a new string that is a substring formed by characters in position i through j of str |
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));
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