Arrays

Part of the Fundamentals section of Coddy's Verilog journey — lesson 10 of 90.

An array lets you store multiple values in one variable. Each element in the array can be accessed by its index.

Note: An array is not a separate data type. It is a collection of wire, reg, integer, or real types.

Declaring Arrays

Syntax: <data_type> <name> [<size>];

reg [7:0] memory [0:255];   // 256 bytes of memory (each 8 bits)
reg data [0:7];              // 8 single-bit registers
wire [3:0] bus [0:3];       // 4 buses, each 4 bits wide
integer counters [0:9];      // 10 integers

The number in brackets [ ] is the array size, not the bit width.

Accessing Array Elements

reg [7:0] memory [0:3];

memory[0] = 165;        // Decimal 165
memory[1] = 90;         // Decimal 90
memory[2] = memory[0] + memory[1];

$display("%d", memory[2]);  // Prints: 255

Arrays are very useful in testbenches for storing test data.

Multi-Dimensional Arrays

You can create arrays with multiple dimensions:

reg [7:0] matrix [0:3][0:3];   // 4x4 array of 8-bit values

matrix[0][0] = 255;            // 8'hFF = 255 decimal
matrix[2][1] = 85;             // 8'h55 = 85 decimal

Array vs Vector

	Vector	Array
What it is	Multi-bit wire or reg	Collection of multiple values
Syntax	`[MSB:LSB]`	`[size]`
Example	`reg [7:0] data;`	`reg [7:0] mem [0:255];`
Access	`data[3]` (bit 3)	`mem[3]` (element 3)

A vector is one value with multiple bits.
An array is multiple values, each with its own bits.

Important Notes

Arrays are not synthesizable in many tools when used with large sizes
Arrays are mostly used in testbenches
For hardware memory, use special memory primitives

Challenge

Complete the code below to create an array that stores 4 test values.

What to do:

Declare an array called test_data
1. Use the reg data type (because it stores values in a testbench)
2. Each element should be 8 bits wide ([7:0])
3. The array should have 4 elements ([0:3])

Cheat sheet

Arrays store multiple values in one variable, accessed by index. An array is a collection of wire, reg, integer, or real types.

Declaration syntax: <data_type> <name> [<size>];

reg [7:0] memory [0:255];  // 256 elements, each 8 bits wide
wire [3:0] bus [0:3];      // 4 elements, each 4 bits wide
integer counters [0:9];    // 10 integers

Accessing elements:

memory[0] = 165;
memory[2] = memory[0] + memory[1];
$display("%d", memory[2]);

Multi-dimensional arrays:

reg [7:0] matrix [0:3][0:3];  // 4x4 array of 8-bit values
matrix[0][0] = 255;

Array vs Vector:

Vector — one value with multiple bits: reg [7:0] data; — data[3] accesses bit 3
Array — multiple values, each with its own bits: reg [7:0] mem [0:255]; — mem[3] accesses element 3

Arrays are mostly used in testbenches; for hardware memory, use special memory primitives.

Try it yourself

module arrays;
  
  // Declare an array called test_data
  // It should have 4 elements, each 8 bits wide
  // Use the reg data type (because it stores values in a testbench)

  
  integer i;
  
  initial begin
    test_data[0] = 170;   
    test_data[1] = 240;    
    test_data[2] = 204;    
    test_data[3] = 15;   
    
    for (i = 0; i < 4; i = i + 1) begin
      $display("test_data[%0d] = %b", i, test_data[i]);
    end
    $finish;
  end
  
endmodule