🧵 What Is string?#
In SystemVerilog, the string type represents a dynamically sized sequence of characters. Unlike static byte or bit [7:0] arrays, it automatically adjusts size and manages memory.
string message;
message = "Hello, SystemVerilog!";
🧠 Key Features#
| Feature | Description |
|---|---|
| Dynamic length | Memory automatically adjusts to fit character count |
| Null-terminated | Each string ends with \0 |
| Built-in operators | Supports {}(concatination), ==, .len(), .toupper(), and more |
| Testbench-friendly | Ideal for logging, messages, and file I/O during simulation |
📦 Basic Usage#
string s1, s2, full;
s1 = "Axolot";
s2 = "Logic";
full = {s1 , " " , s2};
$display("✅ Full string: %s", full); // Output: Axolot Logic
🔧 Common String Methods#
| Method | Description | Example Use |
|---|---|---|
.len() | Returns string length | s.len() → 12 |
.toupper() | Converts to uppercase | s.toupper() |
.tolower() | Converts to lowercase | s.tolower() |
.compare() | Compares two strings | s1.compare(s2) |
.substr(i, n) | Extracts substring | s.substr(2, 4) |
.getc(i) | Returns character at index i | s.getc(0) → "H" |
.putc(i) | Replace character at index i | "H" → s.putc(0) |
🧪 Using Strings in Testbenches#
In verification environments, string is perfect for naming, logging, error messages, or file operations:
string filename, test_id = "test.txt";
int file;
filename = $sformatf("results/run_%0d.log", test_id);
file = $fopen(filename, "w");
$fwrite(file, "Simulation started for ID: %0d\n", test_id);
🚫 Is string Synthesizable?#
No. SystemVerilog strings are for simulation only. They are not synthesizable and cannot be used in actual RTL implementations.
🧠 For RTL, use static arrays like
byte[]orlogic [7:0]instead.
module string_demo_tb;
// Declare string variables
string s1, s2, full;
int length;
string upper_str, lower_str, sub_str;
byte first_char;
// File I/O
string filename;
int file;
int test_id = 42;
initial begin
// Basic assignment and concatenation
s1 = "Axolot";
s2 = "Logic";
full = {s1 , " " , s2};
$display("✅ Full string: %s", full); // Output: Axolot Logic
// String methods
length = full.len();
upper_str = full.toupper();
lower_str = full.tolower();
sub_str = full.substr(2, 4);
first_char = full.getc(0);
$display("🔍 Length : %0d", length);
$display("🔠 Uppercase : %s", upper_str);
$display("🔡 Lowercase : %s", lower_str);
$display("🔖 Substring : %s", sub_str);
$display("🆎 First char : %s", first_char);
// Comparing strings
if (s1.compare("Axolot") == 0)
$display("✔️ Strings match: %s", s1);
else
$display("❌ Strings differ!");
// File handling with string
filename = $sformatf("results/run_%0d.log", test_id);
file = $fopen(filename, "w");
if (file) begin
$fwrite(file, "📂 Simulation log for test ID: %0d\n", test_id);
$fclose(file);
$display("📁 File written: %s", filename);
end else begin
$display("⚠️ Failed to open file: %s", filename);
end
end
endmodule
Simulation Log#
# ? Full string: Axolot Logic
# ? Length : 12
# ? Uppercase : AXOLOT LOGIC
# ? Lowercase : axolot logic
# ? Substring : olo
# ? First char : A
# ?? Strings match: Axolot
📌 Summary#
stringis ideal for simulation and verification environments.- It’s more readable and flexible than fixed-length character arrays.
- It cannot be synthesized — use only in testbenches.
✨ Use
stringfor logs, messages, and filenames — not for hardware!
🔖 Note#
The
string type involves dynamic memory and runtime features.
It should only be used in testbenches — not in synthesizable RTL.
For hardware implementations, prefer logic [7:0] arrays or fixed-size buffers.