In SystemVerilog, initial blocks and always blocks are used to specify the behavior of a module or a hardware system. Both initial and always blocks are procedural blocks that are executed sequentially, but there are some key differences between the two.
Initial blocks are used to specify the initial values of signals and variables when a simulation starts. An initial block is executed only once, at the beginning of the simulation, and is used to initialize the state of the system. Here is an example of an initial block:
initial begin
a = 1'b0;
b = 1'b1;
end
In this example, the initial block sets the initial values of the signals “a” and “b” to 0 and 1, respectively.
Always blocks, on the other hand, are used to specify the behavior of a system over time. An always block is executed repeatedly, whenever a signal or signals in its sensitivity list change. An always block can be used to model combinational logic or sequential logic, depending on the type of statements inside the block. Here is an example of an always block:
always @(posedge clk) begin
if (reset) begin
counter <= 0;
end else begin
counter <= counter + 1;
end
end
In this example, the always block is triggered by the positive edge of the "clk" signal, and the behavior of the block depends on the value of the "reset" signal. If the "reset" signal is high, the "counter" variable is set to 0. Otherwise, the "counter" variable is incremented by 1.
In summary, initial blocks are used to set the initial values of signals and variables, while always blocks are used to specify the behavior of a system over time. Initial blocks are executed only once, at the beginning of the simulation, while always blocks are executed repeatedly, whenever the signals in their sensitivity list change.