cse141L Lab 2: TrivialScalar Datapath


April 9

Made a couple changes to the IO module. Please take a look if you've begun work on it.

April 10

Removed the IO module all together, updated the Datapath PDF, updated Verilog interfaces to reflect new Datapath PDF, and updated Register File diagram. I would just be sure that everything that you've completed thus far is consistent with what's written here.

Due: April 15


In this lab, you will implement a datapath for a really simple processor that implements a very simple instruction set. It is so simple, in fact, that it does not even have a branch instruction, so it cannot execute most programs. The processor does, however, have all the parts that a real processor has: A fetch unit, decode logic, functional units, a register file, IO support, and access to memory.

This lab is mostly about learning the skills you will need in the labs 4-6, and we structured it so that you can learn from each other. All of you will be implementing the same datapath, so you will all run into similar problems. Learn from your classmates, and then go apply what you learn to your own design.

Once you have mastered the skills here, you will go and apply them creatively to implementing your processor in labs 4 and 5.

NOTE: This lab can be performed in groups of two or three, but your groups must remain consistent throughout the duration of Labs 2 and 3. On that same note, we encourage groups to help each other out (especially with tool problems).

Please remember that you must conform to the class coding standards. They are available here. If you find a bug in them (e.g., they are causing you to do something horribly ugly), please let the professor or one of the TAs know.

The greatest common divisor example is available here. Create a new project and import all the files. If you simulate using gdb_tbw.tbw, it will run a short test. This example demonstrates the coding standards for the datapath and the modules therein.

Getting Started

General Notes

Datapath Implementation

We'll start by implementing the top-level schematic for this lab: datapath.v.


module datapath
	// All of these signals originate from the control section of the processor
	// Controls the operation of the processor, determining if it should be stalled, reset or running
	input [1:0] run_stall_reset,
	// Tells the register file that we want to write this cycle
	input write_en_regfile,
	// Tells the dmem wrapper if we're going to read or write this cycle
	input read_write_req_dmem,
	// Selects from where the register file should derive its write input
	input [1:0] reg_sel,
	// Signals DRAM that it should write at the end of this cycle
	input write_en_dmem,
	// Tells the ALU which operation (ADD, SUB, MULT) it should perform this cycle
	input [1:0] op_code,

	// This signal allows the control section of the processor access to the current instruction
	output [15:0] inst,
	// Global clock
	input clk,
	// Global reset
	input reset,
	// All of these signals come from or go to a module external to the processor
	// These signals are described in the IO section
	input [7:0] in_data, // From IO Device
	output [7:0] out_data // To IO Device

This is the top-level interface for your datapath. You should use this to connect to the control portion of your processor and to the outer processor interface. You should just instantiate and connect all of the inner pieces that you'll be designing below in this module.

Question 1: Using the definitions of the different modules below, go ahead and construct our top-level verilog file (datapath.v).


// Parameters:
//	INST_WIDTH -> Width of an instruction
//	LOG_NUM_REGS -> Log-base-2 of the number of registers (number of bits needed to identify a register)
//	IMM_WIDTH -> Width of an immediate
module decode#(parameter INST_WIDTH = 16, LOG_NUM_REGS = 2, IMM_WIDTH = 8)
	// Incoming instruction from the Imem
	input [INST_WIDTH - 1 : 0] inst,

	// The encoded register bits (going to the register file)
	output [LOG_NUM_REGS - 1 : 0] r1,
	output [LOG_NUM_REGS - 1 : 0] r2,

	// The immediate (possibly ending up at the register file)
	output [IMM_WIDTH - 1 : 0] imm

The general ISA format is as follows: <4 bit opcode><2 bit R1><2 bit R2><8 bit Immediate>
With this in mind, there will be 9 different instructions and 4 different registers to choose from:

Opcode Instruction Format Instruction Definition
1 ADD R1 R2 R1 = R1 + R2
2 SUB R1 R2 R1 = R1 - R2
3 MULT R1 R2 R1 = R1 * R2
4 LD R1 R2 R1 = MEM[R2]
5 ST R1 R2 MEM[R2] = R1
6 LI R1 <Immediate> R1 = <Immediate>
7 READ R1 R1 = Input from IO
8 WRITE R1 Output to IO = R1
9 HALT End of execution

The decode module should just correctly parse the operands from each instruction and feed them into their specific modules.

Question 2: Design the decode.v file. It should handle each case listed in the table above.


// Parameters:
//	WIDTH -> Width of the incoming data (via the register file)
module alu#(parameter WIDTH = 8)
	// Incoming inputs from the register file
	input [WIDTH - 1 : 0] v1,
	input [WIDTH - 1 : 0] v2,

	// Opcode of the operation to perform (derived from the ISA)
	input [1 : 0] op_code,

	// Result of the given operation
	output reg [WIDTH - 1 : 0] result

The ALU for this processor is fairly straightforward, as it only needs to implement three separate instructions: ADD, SUB, and MULT. Using the opcodes and other ISA information listed in the Decode section, go ahead and design this module.

Question 3: Implement alu.v using the table in the Decode section and the information above.

Instruction and Data Memories

There are two memory modules to be generated for this processor. One will be the instruction memory(imem), which will be a ROM, and the other will be a data memory (dmem), which will be a RAM. You should use the provided verilog wrapper to access the dmem. For those of you unfamiliar to wrappers, it will just act as a method for us to access the dmem. The wrapper is located here. The wrapper is what you should be using throughout your code, and not the RAM module you are about to generate. Also, please do not change the contents of the wrapper. If you find it not working properly, please notify one of the TAs through email or the webboard.

Here's the interface for instantiating and communicating with a dmem module:

module dmem#(parameter A_WIDTH = 8, D_WIDTH = 8)
	// Global reset
	input  reset,
	// Global clock
	input  clk,

	// Used to specify either a read or a write
	// request.
	// 1 -> read/write request
	// 0 -> no requests
	input  read_write_req,
	// Assert for writing (write enable)
	input  write_en,
	// Address for data we'd like to retrieve
	input  [A_WIDTH - 1 : 0] addr,
	// Data input for writing
	input  [D_WIDTH - 1 : 0] din,
	// Data output for reading
	output [D_WIDTH - 1 : 0] dout,
	// Don't worry about this output
	output refused

Interfacing with the dmem should be fairly straightforward. All signals are edge-triggered. Here's a basic overview of how each operation works:

Question 4: Go through the following tutorial to generate both the imem and dmem modules. Be on the lookout for separate instructions regarding each module. In your report, please generate a diagram showing the interface you'll be using for the two modules (imem and dmem).

Memory Module Generation Tutorial

NOTE: The Xilinx Core Generator may not function correctly in Linux. Our experiences have yielded errors late in the generation verison. You can either brave it in Linux, or you can use the Windows version (in the lab, if need be). If you've had success with the Core Generator in Linux, please let one of the TAs know.

  1. In the ISE viewer go to "Project"->"New Source". Select "IP (CORE Generator & Architecture Wizard)" on the left. Decide on a file name for the particular component you're designing and type it into the "File name" field. The naming scheme in the screenshot below demonstrates a <memory type>_mem_<width>_<size> scheme. Please use the name written in the screenshot below for the dmem module, as the wrapper depends on it being consistent. Click next at this point.
  2. In the "Memories & Storage Elements"->"RAMs & ROMs" folder, choose "Block Memory Generator", click next, and the finish.
  3. All of your memory modules should now be imported into the project. For usage instructions, please consult the *.veo file (one is generated for each module), which should now be located in your project directory.

Register File

// Parameters:
// 	WIDTH -> Width of the data stored in each register
//	LOG_NUM_REGS -> Log-base-2 of the number of registers (number of bits needed to specify a register)
module reg_file#(parameter WIDTH = 8, LOG_NUM_REGS = 2)
	// Global clock
	input clk,
	// Global reset
	input reset,
	// Encoded register inputs
	input [LOG_NUM_REGS - 1 : 0] r1,
	input [LOG_NUM_REGS - 1 : 0] r2,
	// Data to be written on the next clock cycle
	input [WIDTH - 1 : 0] r_data,
	// Should we write this cycle?
	input write_en,

	// The asynchronous outputs for register reads (indexed via the inputs)
	output [WIDTH - 1 : 0] v1,
	output [WIDTH - 1 : 0] v2

The above figure shows the basic structure of our register file. From this figure, it can be inferred that a write is edge-triggered, while a read is level-triggered. The implication of this is that if write_en is asserted before or on a given clock edge, the data at r_data will not be stored into the register specified by r1 until the next clock edge. However, if either r1 or r2 is asserted between clock edges, we would expect the output to appear at v1 or v2 within the same clock cycle.

The following waveform demonstrates this concept:

Question 5: Implement reg_file.v noting the information above.

Useful Resources


  • Submit your report for the questions above to both TAs (to reduce the risk of a lost or bounced email) via e-mail by the due date before the beginning of the class.
    • Answer all of the questions (5) found in the lab description.
    • The report should be in a single PDF file (including answers to questions, verilog source code, graphs, screenshots, etc). There are many tools out there capable of integrating text and graphics and producing PDF files (OpenOffice does a pretty good job).
    • Name your PDF file cse141L-lab1-LastName1-FirstName1-LastName2-FirstName2-LastName3-FirstName3.pdf with your group members' last names and first names substituted for LastName1-3 and FirstName1-3, respectively.
    • The subject line of your email should read "[CSE141L] Lab 2 Submission - LastName1, FirstName1 - LastName2, FirstName2 - LastName3, FirstName3".

Due: April 15