Version 3.

3 March 14, 2020 1

University of California at Berkeley

College of Engineering
Department of Electrical Engineering and Computer Science

EECS 151/251A, Spring 2020

Brian Zimmer, Nathan Narevsky, and John Wright
Modified by Arya Reais-Parsi and Cem Yalcin (2019), Tan Nguyen (2020)

Project Specification
EECS 151/251A RISC-V Processor Design

Contents
1 Introduction 2

2 Front-end design (Phase 1) 4

3 Checkpoint #1: ALU design and pipeline diagram 5

4 Checkpoint #2: Fully functioning core 12

5 Checkpoint #3: CPU + Cache 14

6 Back-end Design (Phase 2) 14

7 Checkpoint #4: Synthesis & PAR 14

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1 Introduction
The primary goal of this project is to familiarize students with the methods and tools of digital design.
In order to make the project both interesting and useful, we will guide you through the implementation
of a CPU that is intended to be integrated on a modern SOC. Working alone or in teams of 2, you will
be designing a simple 3-stage CPU that implements the RISC-V ISA, developed here at UC Berkeley.
If you work in a team, you both must work on the project together (i.e. you are not allowed to divide up
the work), and you will both receive the same grade.
Your first and most important goal is to write a functional implementation of your processor. To
better expose you to real design decisions, you will also be tasked with improving the performance of
your processor. You will be required to meet a minimum performance to be specified later in the project.
You will use Verilog HDL to implement this system. You will be provided with some testbenches to
verify your design, but you will be responsible for creating additional testbenches to exercise your entire
design. Your target implementation technology will be the ASAP7 7nm Educational PDK, a predictive
model technology used for instruction. The project will give you experience designing synthesizeable
RTL (Register Transfer Level) code, resolving hazards in a simple pipeline, building interfaces, and
approaching system-level optimization.
Your first step will be to map our high level specification to a design which can be translated into
a hardware implementation. You will then generate and debug that implementation in Verilog. These
steps may take significant time if you do not put effort into your system architecture before attempting
implementation. After you have built a working design, you will be optimizing it for speed in the 7nm
technology that we have been using this semester.

1.1 RISC-V
The final project for this class will be a VLSI implementation of a RISC-V (pronounced risk-five) CPU.
RISC-V is a new instruction set architecture (ISA) developed here at UC Berkeley. It was originally
developed for computer architecture research and education purposes, but recently there has been a
push towards commercialization and industry adoption. For the purposes of this lab, you don’t need to
delve too deeply into the details of RISC-V. However, it may be good to familiarize yourself with it, as
this will be at the core of your final project. Check out the official Instruction Set Manual and explore
http://riscv.org for more information.

• Read through sections 2.2 and 2.3 starting on page 11 in the RISC-V Instruction Set Manual to
understand how the different types of instructions are encoded.
• Read through sections 2.4, 2.5, and 2.6 starting on page 13 in the Instruction Set Manual and
think about how each of the instructions will use the ALU.

You do not need to read 2.7 or 2.8, as you will not be implementing those instructions in the project.

1.2 Project phases

Your project will consist of two different phases: front-end and back-end. Within each phase, you will
have multiple checkpoints that will ensure you are making consistent progress. These checkpoints will
contribute (although not significantly) to your final grade. You are free to make design changes after
they have been checked off if they will help subsequent phases or improve QOR.
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In the first phase (front-end), you will design and implement a 3-stage RISC-V processor in Verilog,
and run simulations to test for functionality. At this point, you will only have a functional description
of your processor that is independent of technology (there are no standard cells yet). You have about
5 weeks to complete the first phase, but you are highly encouraged to try to finish each checkpoint
early, as each checkpoint will be released before the due date of the ongoing one. Everything will take
much longer than you expect, and finishing early gives you more time to improve your QOR (Quality
of Results, e.g. clock period).
In the second phase (back-end), you will implement your front-end design in the ASAP7 7nm kit
using the VLSI tools you used in lab. When you have finished phase 2, you will have a design that could
actually be fabricated if this were a real process. You will have about 2 weeks to complete the second
phase after its release.

1.3 Philosophy
This document is meant to describe a high-level specification for the project and its associated support
hardware. You can also use it to help lay out a plan for completing the project. As with any design
you will encounter in the professional world, we are merely providing a framework within which your
project must fit.
You should consider the GSI(s) a source of direction and clarification, but it is up to you to produce
a fully functional design, as well as a physical implementation. We will attempt to help, when possible,
but ultimately the burden of designing and debugging your solution lies on you.

1.4 General Project Tips

Be sure to use top-down design methodologies in this project. We began by taking the problem of
designing a basic computer system, modularizing it into distinct parts, and then refining those parts
into manageable checkpoints. You should take this scheme one step further; we have given you each
checkpoint, so break each into smaller, manageable pieces.
As with many engineering disciplines, digital design has a normal development cycle. In the norm,
after modularizing your design, your strategy should roughly resemble the following steps:
Design your modules well, make sure you understand what you want before you begin to code.
Code exactly what you designed; do not try to add features without redesigning.
Simulate thoroughly; writing a good testbench is as much a part of creating a module as actually
coding it.
Debug completely; anything which can go wrong with your implementation will.
Document your project thoroughly as you go. Your design review documents will help, but you
should never forget to comment your Verilog and to keep your diagrams up to date. Aside from the final
project report (you will need to turn in a report documenting your project), you can use your design
documents to help the debugging process. Finish the required features first. Attempt extra features after
everything works well.
This project is divided into checkpoints. Each checkpoint will be due 1 to 2 weeks after its release,
but the next checkpoint will be released early. Use this to your advantage- try to get ahead so that you
have additional time to debug. Your TA will clarify the specific timeline for your semester.
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The most important goal is to design a functional processor- this alone is 50-60% of the final grade,
and you must have it working completely to receive any credit for performance.

2 Front-end design (Phase 1)

The first phase in this project is designed to guide the development of a three-stage pipelined RISC-V
CPU that will be used as a base system for your back-end implementation.
Phase 1 will last for 5 weeks and has weekly checkpoints.

• Checkpoint 1: ALU design and pipeline diagram (due End of 03/16-03/22, 2020)

• Checkpoint 2: Core implementation (due End of 04/06-04/12, 2020)

• Checkpoint 3: Core + memory system implementation (due End of 04/20-04/26, 2020)

2.1 Project Setup

The skeleton files for the project will be delivered as a git repository provided by the staff. You should
clone this repository as follows. It is highly recommended to familiarize yourself with git and use it to
manage your development.

% git clone /home/ff/eecs151/labs/project_skeleton /path/to/my/project

As soon as you start your project, you must post your group information as a private note on Piazza.
Please provide each group member’s name, student ID number, and instructional account name for the
group members (e.g. eecs151-aa). Please do this even if you are working alone, as these git repos will
be used for part of the final checkoff. Once it is setup you will be given a team number, and you will
be given a repo hosted on the servers for version control for the project. You should be able to add the
remote host of “geecs151:teamXX” where “XX” is the team number that you are assigned. An example
working flow to be able to pull from the skeleton as well as push/pull with your team repository is
shown below:

% git clone /home/ff/eecs151/labs/project_skeleton /path/to/my/project

% git remote add myOrigin geecs151:teamXX

Then to pull changes from the skeleton, you would need to type:

% git pull origin master

To pull changes from your team repository you would type:

% git pull myOrigin master

And to push changes to your team repository, you would usually want to pull first (above) and then
type:

% git push myOrigin master

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3 Checkpoint #1: ALU design and pipeline diagram

The ALU that we will implement in this lab is for a RISC-V instruction set architecture. Pay close
attention to the design patterns and how the ALU is intended to function in the context of the RISC-V
processor. In particular it is important to note the separation of the datapath and control used in this
system which we will explore more here.
The specific instructions that your ALU must support are shown in the tables below. The branch
condition should not be calculated in the ALU. Depending on your CPU implementation, your ALU
may or may not need to do anything for branch, jump, load, and store instructions (i.e., it can just output
0).

3.1 Making a pipeline diagram

The first step in this project is to make a pipeline diagram of your processor, as described in lecture. You
only need to make a diagram of the datapath (not the control). Each stage should be clearly separated
with a vertical line, and flip-flops will form the boundary between stages. It is a good idea to name
signals depend on what stage they are in (eg. s1 killf, s2 rd0). Also, it is a good idea to separately name
the input/output (D/Q) of a flip flop (eg. s0 next pc, s1 pc). Draw your diagram in a drawing program,
because you will need to keep it up-to-date as you build your processor. It helps to print out scratch
copies while you are debugging your processor and to keep your drawings revision-controlled with git.
Once you have finished your initial datapath design, you will implement the main building block in the
datapath—the ALU.

3.2 ALU functional specification

Given specifications about what the ALU should do, you will create an ALU in Verilog and write a test
harness to test the ALU.
The encoding of each instruction is shown in the table below. There is a detailed functional descrip-
tion of each of the instructions in Section 2.4 (starting on page 13) of the Instruction Set Manual. Pay
close attention to the functional description of each instruction as there are some subtleties.
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31 27 26 25 24 20 19 15 14 12 11 7 6 0
funct7 rs2 rs1 funct3 rd opcode R-type
imm[11:0] rs1 funct3 rd opcode I-type
imm[11:5] rs2 rs1 funct3 imm[4:0] opcode S-type
imm[12|10:5] rs2 rs1 funct3 imm[4:1|11] opcode SB-type
imm[31:12] rd opcode U-type
imm[20|10:1|11|19:12] rd opcode UJ-type

RV32I Base Instruction Set

imm[31:12] rd 0110111 LUI rd,imm
imm[31:12] rd 0010111 AUIPC rd,imm
imm[20|10:1|11|19:12] rd 1101111 JAL rd,imm
imm[11:0] rs1 000 rd 1100111 JALR rd,rs1,imm
imm[12|10:5] rs2 rs1 000 imm[4:1|11] 1100011 BEQ rs1,rs2,imm
imm[12|10:5] rs2 rs1 001 imm[4:1|11] 1100011 BNE rs1,rs2,imm
imm[12|10:5] rs2 rs1 100 imm[4:1|11] 1100011 BLT rs1,rs2,imm
imm[12|10:5] rs2 rs1 101 imm[4:1|11] 1100011 BGE rs1,rs2,imm
imm[12|10:5] rs2 rs1 110 imm[4:1|11] 1100011 BLTU rs1,rs2,imm
imm[12|10:5] rs2 rs1 111 imm[4:1|11] 1100011 BGEU rs1,rs2,imm
imm[11:0] rs1 000 rd 0000011 LB rd,rs1,imm
imm[11:0] rs1 001 rd 0000011 LH rd,rs1,imm
imm[11:0] rs1 010 rd 0000011 LW rd,rs1,imm
imm[11:0] rs1 100 rd 0000011 LBU rd,rs1,imm
imm[11:0] rs1 101 rd 0000011 LHU rd,rs1,imm
imm[11:5] rs2 rs1 000 imm[4:0] 0100011 SB rs1,rs2,imm
imm[11:5] rs2 rs1 001 imm[4:0] 0100011 SH rs1,rs2,imm
imm[11:5] rs2 rs1 010 imm[4:0] 0100011 SW rs1,rs2,imm
imm[11:0] rs1 000 rd 0010011 ADDI rd,rs1,imm
imm[11:0] rs1 010 rd 0010011 SLTI rd,rs1,imm
imm[11:0] rs1 011 rd 0010011 SLTIU rd,rs1,imm
imm[11:0] rs1 100 rd 0010011 XORI rd,rs1,imm
imm[11:0] rs1 110 rd 0010011 ORI rd,rs1,imm
imm[11:0] rs1 111 rd 0010011 ANDI rd,rs1,imm
0000000 shamt rs1 001 rd 0010011 SLLI rd,rs1,shamt
0000000 shamt rs1 101 rd 0010011 SRLI rd,rs1,shamt
0100000 shamt rs1 101 rd 0010011 SRAI rd,rs1,shamt
0000000 rs2 rs1 000 rd 0110011 ADD rd,rs1,rs2
0100000 rs2 rs1 000 rd 0110011 SUB rd,rs1,rs2
0000000 rs2 rs1 001 rd 0110011 SLL rd,rs1,rs2
0000000 rs2 rs1 010 rd 0110011 SLT rd,rs1,rs2
0000000 rs2 rs1 011 rd 0110011 SLTU rd,rs1,rs2
0000000 rs2 rs1 100 rd 0110011 XOR rd,rs1,rs2
0000000 rs2 rs1 101 rd 0110011 SRL rd,rs1,rs2
0100000 rs2 rs1 101 rd 0110011 SRA rd,rs1,rs2
0000000 rs2 rs1 110 rd 0110011 OR rd,rs1,rs2
0000000 rs2 rs1 111 rd 0110011 AND rd,rs1,rs2
imm[11:0] rs1 001 rd 1110011 CSRRW rd,rs1,imm
imm[11:0] rs1 101 rd 1110011 CSRRWI rd,rs1,imm
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3.3 Lab Files

We have provided a skeleton directory structure to help you get started.
Inside, you should see a src folder, as well as a tests folder. Similar to the setting of previous
labs, there are also some YAML files for using HAMMER to run simulation, synthesis, and place-and-
route. The src folder contains all of the verilog modules for this phase, and the tests folder contains
some RISC-V test binaries for your processor.

3.4 Testing the Design

Before writing any of modules, you will first write the tests so that once you’ve written the modules
you’ll be able to test them immediately. This is effectively Test-driven Development (TDD). Writing
tests first is good practice- it forces you to write thorough tests, and ensures that tests will exist when
you need to rapidly iterate through module design tweaks. Thorough understanding of the expected
functionality is key to writing good tests (or RTL). You will be expected to write unit tests for any mod-
ules that you design and implement and write integration tests. Unit tests will verify the functionality
of individual modules against your specification. Integration tests verify that all the modules work as a
system once you connect them together.

3.4.1 Verilog Testbench

One way of testing Verilog code is with testbench Verilog files. The outline of a test bench file has been
provided for you in ALUTestbench.v. There are several key components to this file:

• `timescale 1ns / 1ps - This specifies, in order, the reference time unit and the precision.
This example sets the unit delay in the simulation to 1ns (i.e. #1 = 1ns) and the precision to 1ps
(i.e. the finest delay you can set is #0.001 = 1ps).

• The clock is generated by the code below. Since the ALU is only combinational logic, this is not
necessary, but it will be a helpful reference once you have sequential elements.

– The initial block sets the clock to 0 at the beginning of the simulation. You should be
sure to only change your stimulus when the clock is falling, since the data is captured on
the rising edge. Otherwise, it will not only be difficult to debug your design, but it will also
cause hold time violations when you run gate level simulation.
– You must use an always block without a sensitivity list (the @ part of an always statement)
to cause the clock to run automatically.
parameter Halfcycle = 5; //half period is 5ns
localparam Cycle = 2*Halfcycle;
reg Clock;
// Clock Signal generation:
initial Clock = 0;
always #(Halfcycle) Clock = ˜Clock;

• task checkOutput; - this task contains Verilog code that you would otherwise have to copy
paste many times. Note that it is not the same thing as a function (as Verilog also has functions).
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• {$random} & 31'h7FFFFFFF - $random generates a pseudorandom 32-bit integer. A

bitwise AND will mask the result for smaller bit widths.

For these two modules, the inputs and outputs that you care about are opcode, funct, add_rshift_type,
A, B and Out. To test your design thoroughly, you should work through every possible opcode,
funct, and add_rshift_type that you care about, and verify that the correct Out is generated
from the A and B that you pass in.
The test bench generates random values for A and B and computes REFout = A + B. It also
contains calls to checkOutput for load and store instructions, for which the ALU should perform
addition. It will be up to you to write tests for the remaining combinations of opcode, funct, and
add_rshift_type to test your other instructions.
Remember to restrict A and B to reasonable values (e.g. masking them, or making sure that they are
not zero) if necessary to guarantee that a function is sufficiently tested. Please also write tests where
the inputs and the output are hard-coded. These should be corner cases that you want to be certain are
stressed during testing.

3.4.2 Test Vector Testbench

An alternative way of testing is to use a test vector, which is a series of bit arrays that map to the inputs
and outputs of your module. The inputs can be all applied at once if you are testing a combinational
logic block or applied over time for a sequential logic block (e.g. an FSM).
You will write a Verilog testbench that takes the parts of the bit array that correspond to the inputs
of the module, feeds those to the module, and compares the output of the module with the output bits
of the bit array. The bit vector should be formatted as follows:

[106:100] = opcode
[99:97] = funct
[96] = add_rshift_type
[95:64] = A
[63:32] = B
[31:0] = REFout

Open up the skeleton provided to you in ALUTestVectorTestbench.v. You need to complete the
module by making use of $readmemb to read in the test vector file (named testvectors.input),
writing some assign statements to assign the parts of the test vectors to registers, and writing a for loop
to iterate over the test vectors.
The syntax for a for loop can be found in ALUTestbench.v. $readmemb takes as its arguments
a filename and a reg vector, e.g.:

reg [5:0] bar [0:20];

$readmemb(foo.input, bar);

3.4.3 Writing Test Vectors

Additionally, you will also have to generate actual test vectors to use in your test bench. A test vector
can either be generated in Verilog (like how we generated A, B using the random number generator and
iterated over the possible opcodes and functs), or using a scripting language like python. Since we have
Version 3.3 March 14, 2020 9

already written a Verilog test bench for our ALU and decoder, we will tackle writing a few test vectors
by hand, then use a script to generate test vectors more quickly.
Test vectors are of the format specified above, with the 7 opcode bits occupying the left-most bits.
Open up the file tests/testvectors.input and add test vectors for the following instructions
to the end (i.e. manually type the 107 zeros and ones required for each test vector): SLT, SLTU, SRA,
and SRL.
In the same directory, we’ve also provided a test vector generator written in Python, which is a
popular language used for scripting. We used this generator to generate the test vectors provided to you.
If you’re curious, you can read the next paragraph and poke around in the file. If not, feel free to skip
ahead to the next section.
The script ALUTestGen.py is located in tests. Run it so that it generates a test vector file in
the tests folder. Keep in mind that this script makes a couple assumptions that aren’t necessary and
may differ from your implementation:
• Jump, branch, load and store instructions will use the ALU to compute the target address.

• For all shift instructions, A is shifted by B. In other words, B is the shift amount.

• For the LUI instruction, the value to load into the register is fed in through the B input.
You can either match these assumptions or modify the script to fit with your implementation. All the
methods to generate test vectors are located in the two Python dictionaries opcodes and functs.
The lambda methods contained (separated by commas) are respectively: the function that the operation
should perform, a function to restrict the A input to a particular range, and a function to restrict the B
input to a particular range.
If you modify the Python script, run the generator to make new test vectors. This will overwrite
the file, so if you want to save your handwritten test vectors, rename the file before running the script,
then append them once the file has been generated.
% python3 ALUTestGen.py
This will write the test vector into the file testvectors.input. Use this file as the target test vector
file when loading the test vectors with $readmemb.

3.5 Writing Verilog Modules

For this exercise, we’ve provided the module interfaces for you. They are logically divided into a
control (ALUdec.v) and a datapath (ALU.v). The datapath contains the functional units while control
contains the necessary logic to drive the datapath. You will be responsible for implementing these two
modules. Descriptions of the inputs and outputs of the modules can be found in the first few lines of each
file. The ALU should take an ALUop and its two inputs A and B, and provide an output dependent on the
ALUop. The operations that it needs to support are outlined in the Functional Specification. Don’t worry
about sign extensions–they should take place outside of the ALU. The ALU decoder uses the opcode,
funct, and add_rshift_type to determine the ALUop that the ALU should execute. The funct
input corresponds to the funct3 field from the ISA encoding table. The add_rshift_type input
is used to distinguish between ADD/SUB, SRA/SRL, and SRAI/SRLI; you will notice that each of
these pairs has the same opcode and funct3, but differ in the funct7 field.
You will find the case statement useful, which has the following syntax:
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always@(*) begin
case(foo)
3'b000: // something happens here
3'b001: // something else happens here
3'b010, 3'b011: // you can have more than
// one case do the same thing
default: // everything else
endcase
end
To make your job easier, we have provided two Verilog header files: Opcode.vh and ALUop.vh.
They provide, respectively, macros for the opcodes and functs in the ISA and macros for the different
ALU operations. You should feel free to change ALUop.vh to optimize the ALUop encoding, but if
you change Opcode.vh, you will break the test bench skeleton provided to you. You can use these
macros by placing a backtick in front of the macro name, e.g.:
case(opcode)
`OPC_STORE:
is the equivalent of:
case(opcode)
7'b0100011:

3.6 Running the Simulation

Open the file sim-rtl.yml, set the testbench’s name to be ALUTestbench

tb_name: &TB_NAME "ALUTestbench"

By typing make sim-rtl you will run the ALU simulation.

You may change the testbench’s name to ALUTestVectorTestbench to use the test vector testbench.
Once you have a working design, you should see the following output when you run either of the
given testbenches:

# ALL TESTS PASSED!

3.7 Viewing Waveforms

As in the previous labs, you should use DVE to view waveforms.
1. List of the modules involved in the test bench. You can select one of these to have its signals
show up in the object window.
2. Object window - this lists all the wires and regs in your module. You can add signals to the
waveform view by selecting them, right-clicking, and doing Add → To Wave → Selected Signals.
3. Waveform viewer - The signals that you add from the object window show up here. You can
navigate the waves by searching for specific values, or going forward or backward one transition
at a time.
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As an example of how to use the waveform viewer, suppose you get the following output when you run
ALUTestbench:

# FAIL: Incorrect result for opcode 0110011, funct: 101:, add_rshift_type: 1

# A: 0x92153524, B: 0xffffde81, DUTout: 0x490a9a92, REFout: 0xc90a9a92

The $display() statement actually already tells you everything you need to know to fix your bug,
but you’ll find that this is not always the case. For example, if you have an FSM and you need to look
at multiple time steps, the waveform viewer presents the data in a much neater format. If your design
had more than one clock domain, it would also be nearly impossible to tell what was going on with only
$display() statements.
Add all the signals from ALUTestbench to the waveform viewer and you see the following win-
dow: The two highlighted boxes contain the tools for navigation and zoom. You can hover over the
icons to find out more about what each of them do. You can find the location (time) in the waveform
viewer where the test bench failed by searching for the value of DUTout output by the $display()
statement above (in this case, 0x490a9a92:

1. Selecting DUTout

2. Clicking Edit > Wave Signal Search > Search for Signal Value > 0x490a9a92

Now you can examine all the other signal values at this time. Compare the DUTout and REFout
values at this time, and you should see that they are similar but not quite the same. From the opcode,
funct, and add_rshift_type, you know that this is supposed to be an SRA instruction, but it
looks like your ALU performed a SRL instead. However, you wrote

Out = A >>> B[4:0];

That looks like it should work, but it doesn’t! It turns out you need to tell Verilog to treat B as a signed
number for SRA to work as you wish. You change the line to say:

Out = $signed(A) >>> B[4:0];

After making this change, you run the tests again and cross your fingers. Hopefully, you will see the
line:

# ALL TESTS PASSED!

If not, you will need to debug your module until all test from the test vector file and the hard-coded test
cases pass.
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3.8 Checkpoint #1: Simple test program

Checkoff due: End of 03/16-03/22, 2020
Congratulations! You’ve started the design of your datapath by drawing a pipeline diagram, and
written and thoroughly tested a key component in your processor. You should now be well-versed in
testing Verilog modules. Please answer the following questions to be checked off by a TA:

1. Show your pipeline diagram, and explain when writes and reads occur in the register file and
memory relative to the pipeline stages.

2. Show your working ALU test bench files to your TA and explain your hard-coded cases. You
should also be able to show that the tests for the test vectors generated by the python script and
your hard-coded test vectors both work.

3. In ALUTestbench, the inputs to the ALU were generated randomly. When would it be preferable
to perform an exhaustive test rather than a random test?

4. What bugs, if any, did your test bench help you catch?

5. For one of your bugs, come up with a short assembly program that would have failed had you not
caught the bug. In the event that you had no bugs and wrote perfect code the first time, come up
with an assembly program to stress the SRA bug mentioned in the above section.

4 Checkpoint #2: Fully functioning core

4.1 Additional Instructions
In order to run the testbenches, there are a few new instructions that need to be added for help in
debugging/creating testbenches. Read through section 6.2 in the RISC-V specification. A CSR (or
control status register) is some state that is stored independent of the register file and the memory.
While there are 212 possible CSR addresses, you will only use one of them (tohost = 0x51E). The
tohost register is monitored by the test harness, and simulation ends when a value is written to this
register. A value of 1 indicates success, a value greater than 1 gives clues as to the location of the failure.
There are 2 CSR related instructions that you will need to implement:

1. csrw tohost,t2 (short for csrrw x0,csr,rs1 where csr = 0x51E)

2. csrwi tohost,1 (short for csrrwi x0,csr,zimm where csr = 0x51E)

csrw will write the value from register in rs1. csrwi will write the immediate (stored in rs1) to
the addressed csr. Note that you do not need to write to rd (writing to x0 does nothing).

31 20 19 15 14 12 11 76 0
csr rs1 funct3 rd opcode
12 5 3 5 7
source/dest source CSRRW dest SYSTEM
source/dest zimm[4:0] CSRRWI dest SYSTEM
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4.2 Details
Your job is to implement the core of the 3-stage RISC-V CPU.

4.3 File Structure

Implement the datapath and control logic for your RISC-V processor in the file Riscv151.v that is
provided. Make sure that the inputs and outputs remain the same, since this module connects to the
memory system for system-level testing. If you look at riscv_test_harness.v you can see a
testbench that is provided.

4.4 Running the Test

This testbench will load a program into the instruction memory, and will then run until the exit code
register has been set. There is also a timeout to make sure that the simulation does not run forever. This
will also tell you whether or not your testbench is passing the test. You should only be running this test
suite after you have eliminated some of the bugs using single instruction tests described below.

4.5 Running assembly tests

We have provided a suite of assembly tests to help you debug all of the instructions you need to estimate.
To run all of them:

make sim-rtl test_asm=all

This will generate .out files in the asm output/ directory, and summarize which tests passed and
failed. You can also run single asm test with the following command:

make sim-rtl test_asm=rv32ui-p-simple.out

If you would like to generate waveforms for a single test:

make sim-rtl test_asm=rv32ui-p-simple.vpd

, where ’simple’ gets replaced with any of the available tests defined in the Makefile.
You can read the assembly code of the programs by looking at the dump file. Comments in the code
will help you understand what is happening.

cd tests/asm/
vim rv32ui-p-addi.dump

Last, you can see the hex code that is loaded directly into the memory by looking at the hex file.

cd tests/asm/
vim rv32ui-p-addi.hex
Version 3.3 March 14, 2020 14

4.6 Checkpoint #2 Deliverables

Checkoff due: End of 04/06-04/12, 2020

Congratulations! You’ve started the design of your datapath by implementing your pipeline dia-
gram, and written and thoroughly tested a key component in your processor and should now be well-
versed in testing Verilog modules. Please answer the following questions to be checked off by a TA.

1. Show that all of the assembly tests pass

2. Show your final pipeline diagram, updated to match the code.

5 Checkpoint #3: CPU + Cache

Available Wednesday, April 1, 2020. Checkoff due: End of 04/20-04/26, 2020.

6 Back-end Design (Phase 2)

7 Checkpoint #4: Synthesis & PAR

Available Wednesday, April 8, 2020. Checkoff due: Friday, May 8, 2020.