A register

A D flip-flop can store one bit. We can imagine a register as a row of D flip-flops, each storing one bit, with the possibility to load new values into all D flip-flops simultaneously.

A register implementation in Verilog is shown in Figure 6.

module n_bit_register(clk, data_in, data_out);

   parameter N = 8;

   input clk;
   input[N-1:0] data_in;
   output[N-1:0] data_out;

   wire 	 clk;
   wire [N-1:0]  data_in;
   reg [N-1:0] 	 reg_value;

   always @(posedge clk)
     reg_value <= data_in;
   assign data_out = reg_value;


Figure 6. A register in Verilog.

This the Verilog view - other views are VHDL - SystemC-TLM

The code in Figure 6 defines a module. The module has two inputs, called clk and data_in, and one output, called data_out.

The module defines a register variable called reg_value, that will contain the actual value stored in the register.

An always block ensures that the state variable reg_value is updated at every rising edge of the clock.

An assignment of the variable data_out is done, outside of the always block. This assignment ensures that the output data_out has the same value as the current value of the state variable reg_value.