Designing a Register File in Verilog and SystemVerilog

A register file is a collection of registers that can be read and written by a digital circuit. Register files are a fundamental building block in many digital designs, such as microprocessors and digital signal processors. In this tutorial, we will discuss how to design a register file in Verilog and SystemVerilog.

Overview of Register Files

A register file is typically implemented as an array of registers, where each register is a collection of storage elements. The register file has two major operations: read and write. During a read operation, the contents of a register are transferred to an output port. During a write operation, the contents of an input port are transferred to a register.

Handling Read/Write Conflicts

When the read index and write index are the same, the read data will be the current register data, not the updating data. This behavior is important to keep in mind when designing a register file, especially in applications where multiple instructions may attempt to read and write to the same register at the same time.

One potential solution to this problem is to build a bypassing version of the register file, which allows the processor to read the result of the current instruction before it has been written to the register file. However, this solution can be complex and may not be necessary if the control logic for the CPU is carefully designed.

Designing a Register File in Verilog and SystemVerilog

The following code shows an example implementation of a register file in Verilog and SystemVerilog:

module RegisterFile #(
  parameter DataWidth  = 8,
  parameter NumRegs    = 16,
  parameter IndexWidth = $clog2(NumRegs)
) (
  input  logic                  clk,
  input  logic                  writeEn,
  input  logic [IndexWidth-1:0] writeAddr,
  input  logic [ DataWidth-1:0] writeData,
  input  logic [IndexWidth-1:0] readAddr1,
  input  logic [IndexWidth-1:0] readAddr2,
  output logic [ DataWidth-1:0] readData1,
  output logic [ DataWidth-1:0] readData2
);

  logic [DataWidth-1:0] regs[NumRegs];

  always_ff @(posedge clk) begin
    if (writeEn) begin
      regs[writeAddr] <= writeData;
    end
  end

  assign readData1 = regs[readAddr1];
  assign readData2 = regs[readAddr2];

endmodule

The RegisterFile module has seven ports: clk, writeEn, readAddr1, readAddr2, writeAddr, writeData, readData1, and readData2. The clk port is the clock input, the writeEn port is the write enable input, the readAddr1 and readAddr2 ports are the read address inputs, the writeAddr port is the write address input, the writeData port is the write data input, and the readData1 and readData2 ports are the read data outputs.

The regs signal is an array of registers that stores the register file contents. The always_ff block is used to describe the behavior of the register file during a write operation. When the write enable signal writeEn is high, the contents of the register specified by the writeAddr input are updated with the data specified by the writeData input.

The assign statements are used to describe the behavior of the register file during a read operation. The contents of the registers specified by the readAddr1 and readAddr2 inputs are transferred to the readData1 and readData2 outputs, respectively. If the read address matches the write address, the read data is set to the current register data, rather than the updating data.

Conclusion

In this tutorial, we discussed how to design a register file in Verilog and SystemVerilog, including handling read/write conflicts. By understanding the principles behind register files and their implementation in Verilog and SystemVerilog, you can design custom register file circuits for your specific application. With the addition of proper control logic, the register file design presented here can handle most common read/write conflict scenarios.