SystemVerilog -- 2.3 Data Types ~ SystemVerilog Strings

SystemVerilog Strings

What is a SystemVerilog string ?

数据类型是有序的字符集合。变量的长度是集合中的字符数，这些字符数可以具有动态长度，并且在模拟过程中会发生变化。字符串变量表示字符串的方式与字符串文本不同。使用变量时不会发生截断。string

Syntax

string variable_name [=initial_value];

variable_name是有效的标识符，initial_value可以是字符串文本、空字符串的值“”或字符串数据类型表达式。如果在声明时未指定初始值，则变量默认为“”，即空字符串文本。

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

模拟日志

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

How are strings represented in Verilog ?

单个 ASCII 字符需要 8 位（1字节），要存储字符串，我们需要与字符串中的字符数一样多的字节。

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
Eqquality	Str1 == Str2	Returns 1 if the two strings are equal and 0 if they are not
Inequality	Str != 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		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 fistname < length of lastname
    if (firstname < lastname)
      $display ("firstname=%s is LESS THAN to lastname=%s", firstname, lastname);

    // String comparison :  Check if length of fistname > length of lastname
    if (firstname > lastname)
      $display ("firstname=%s is GREATER THAN to 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 ("firstnamep[2]=%s lastname[2]=%s", firstnamep[2], lastname[2]);
  end
endmodule

模拟日志

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

Basic String Methods

Usage	Defination	Comments
str.len()	function int len();	Returns the number of characters in the string
str.putc()	function void putc (int i, byte c);	Returns 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.en() = %0d", str.len());

    // Assign to tmp variable and put char "s" at index 3
    tmp = str;
    tmp.pubc (3, "d");
    $display ("str.getc(2) = %s (%0d)", str.getc(2), str.getc(2));

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

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

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

    // Ignore case comparison
    tmp = "hello world!";
    $display ("[tmp is in lowercase] str.compare(tmp) = %d", str.compare(tmp));
    tmp = "Hello World!";
    $display (("[tmp, str are same] str.compare(tmp) = %d", str.compare(tmp));
    // Extract new string from i to j
    $display ("str.substr(4,8)=%s", str.substr (4,8));
  end
endmodule

模拟日志

str.len() = 12
str.putc(3, d) = Hello World!
str.getc(2) = 1 （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.sbstr(4,8) = o Wor

String Conversion Methods

/	/	/
str.atoi()	function integer atoi();	Returns the integer corresponding to the ASCII decimal representation in str
str.atohex()	function integer atohex();	Interprets the string as hexadecimal
str.atooct()	function integer atooct();	Interprets the string as octal
str.atobin()	function integer atobin();	Interprets the string as binary
str.atoreal()	function real atoreal();	Returns the real number corresponding to the ASCII decimal representation in str
str.itoa(i)	function void itoa(integer i);	Stores the ASCII decimal representation of i into str
str.hextoa(i)	function void hextoa(integer i);	Stores the ASCII hexadecimal representation of i into str
str.octtoa(i)	function void octtoa(integer i);	Stores the ASCII octal representation of i into str
str.bintoa(i)	function void bintoa(integer i);	Stores the ASCII binary representation of i into str
str.realtoa(r)	function void realtoa(real r);	Stores the ASCII real representation of i into str