Explain the difference between initial and always blocks.

In Verilog, initial and always blocks are both procedural constructs used to describe the behavior of digital circuits. However, they have key differences in their execution:

initial Block

Executes only once: The statements within an initial block are executed only at the beginning of the simulation, and then the block terminates.
Typically used for:
- Initialization: Setting initial values for registers and variables.
- Testbench stimulus generation: Generating test vectors and applying them to the design under test.
Not synthesizable (generally): initial blocks are primarily used in testbenches and are not typically synthesizable into hardware.

always Block

Executes repeatedly: The statements within an always block are executed repeatedly, based on the specified sensitivity list or timing control.
Used for:
- Modeling sequential logic: Implementing registers, flip-flops, and other sequential circuits.
- Modeling combinational logic: Describing combinational circuits with appropriate sensitivity lists.
Synthesizable: always blocks are essential for synthesizing hardware designs.

// Initial block to set initial value of register
initial begin
  reg_a = 1'b0; 
end

// Always block for a D flip-flop
always @ (posedge clk) begin
  if (reset) 
    reg_b <= 1'b0; 
  else
    reg_b <= data; 
end

In this example :

The initial block sets the initial value of reg_a to 0 at the start of the simulation.
The always block describes the behavior of a D flip-flop, where the value of reg_b is updated on the rising edge of the clock signal (posedge clk).

By understanding the differences between initial and always blocks, you can effectively model and simulate digital circuits in Verilog.