MS108-2020

Repo Structure

|--docs/
|  |--YPU/              A demo-use simplified 5-stage pipelined CPU
|  |--vivadoDemo.pdf    A demo about how to create a project in Vivado
|--riscv/
|  |--ctrl/             Interface with FPGA
|  |--sim/              Testbench, add to Vivado project only in simulation
|  |--src/              Where your code should be
|  |  |--common/                Provided UART and RAM
|  |  |--Basys-3-Master.xdc     constraint file
|  |  |--cpu.v                  Fill it. 
|  |  |--hci.v                  A bus between UART/RAM and CPU
|  |  |--ram.v                  RAM
|  |  |--riscv_top.v            Top design
|  |--sys/              Help compile
|  |--testcase/         Testcases
|  |--autorun_fpga.sh   Autorun Testcase on FPGA
|  |--build_test.sh     Run it to build test.data from test.c
|  |--FPGA_test.py      Test correctness on FPGA
|  |--pd.tcl            Program device the bitstream onto FPGA
|  |--run_test.sh       Run test
|  |--run_test_fpga.sh  Run test on FPGA
|--serial/              A third-party library for interfacing with FPGA ports

Requirement

Basic Requirement

• Use Verilog to implement a CPU supporting part of RV32I Instruction set(2.1-2.6 in RISC-V user manual), with the provided code in this repository.
• The architecture should be pipelined or Tomasulo.

Grading Policy

• A 5-stage pipelined or Tomasulo CPU passing all tests in this repository with i-cache makes you get 75%.
• Implementing your design on FPGA makes you get 5%.
• Here are some bonus parts. Except (3), the sum of bonus will not exceed 20%:
  1. Implement branch prediction and d-cache. Based on your performance and complexity(e.g., 1-bit global branch prediction is obviously simpler than BTB), you will get 10%~20%.
  2. A Tomasulo architecture CPU that does not halt in every branch instruction. Send your design before code review. You will get 20% for this.
  3. Support the rest of RV32I Instruction set (2.7-2.9 in the manual above) and privileged architecture(section 2 and section 3 in RISC-V privileged architecture manual). Finally, you will be able to port FreeRTOS on FPGA with your design and some provided code. You will get 20% and an extra 5 pts for this.
  4. Whatever more, but discuss with TA first.
• A design meeting part of a requirement can get part of its corresponding points.
• The course project assignment is not mature yet. Please give practical suggestions or bug fixes for next year's project if you feel somewhere uncomfortable with current project. You should prepare a short note or presentation for your findings. You will get extra 2% for this. If you implement your suggestion and it's meaningful in both educational purpose and project perfection purpose, the extra credit will be raised up -- up to 10%. It will be a complement for your bonus part, or extra 1 point in the final grading if you get full mark in the project.

Details

RISCV-Toolchain

For prerequisities, go to see https://github.com/riscv/riscv-gnu-toolchain to install necessary packages. The configure is:

./configure --prefix=/opt/riscv --with-arch=rv32i --with-abi=ilp32
sudo make

DO NOT use sudo make linux which you may use in PPCA. If you have made it, just rerun sudo make without any deletion and everything will be ok. (BTW, you may use arch rv32gc for your compiler project, so keep the installation package)

The following are some common problems you may meet when make

make failed

Please first check whether you use sudo before make due to default permission setting of linux.

checking for sysdeps preconfigure fragments... aarch64 alpha arm csky hppa i386 m68k microblaze mips nios2 powerpc riscv glibc requires the A extension

Use configuration ./configure --prefix=/opt/riscv --with-arch=rv32ia --with-abi=ilp32

xxx-ld: cannot find -lgcc

Go to see riscv-collab/riscv-gnu-toolchain#522.

Custom

In this project, the size of memory(ram) is 128K, so only address lower than 0x20000 is available. However, reading and writing from 0x30000 and 0x30004 have special meaning, you can see riscv/src/cpu.v for more details.

Simulation using iverilog

cd ./riscv/src
iverilog *.v common/*/*.v
vvp a.out

Serial

Serial( wjwwood/serial) is a cross-platform serial port library to help your design working on FPGA when receiving from UART. Build it by:

cd serial
make
make install

Build test

Use the following command to build a test, it will be a test.data file in folder /riscv/test/:

cd riscv
./build_test.sh testname

You can see all tests in /riscv/testcase/ folder.

FPGA

We'll provide you with Basys3 FPGA board. Use Vivado to generate bitstream and program the FPGA device. Then:

In directory 'ctrl', build the controller by

./build.sh

Modify and run the script

./run_test_fpga.sh testname

One thing need to be modified is the USB port number of the script. For example in Windows you could find it in Devices and Printers -> Digilent USB Device -> Hardware. The number X that presented in the last line of Device Functions 'USB Serial Port (COMX)' is the port you need. The port format should be like:

on Linux: /dev/ttyUSBX
on WSL: /dev/ttySX
on Windows: COMX

Your Vivado may unable to discover your FPGA, this may be caused by the lack of corresponding driver, install it by(use your own version to replace 2018.2):

cd $PATH_TO_VIVADO/2018.2/data/xicom/cable_drivers/lin64
sudo cp -i -r install_script /opt
cd /opt/install_script/install_drivers
sudo ./install_drivers

Then restart Vivado.

To run your bitstream on FPGA, you can run:

cd riscv
python FPGA_test.py

You need to modify the path_of_bit in FPGA_test.py first.

Update Note

For some strong students that start project early based on last year's assignment, here are some changes we've made this year:

1. Fixed a bug in riscv_top.v that may cause you get wrong return value when two consecutive readings are from different data sources.

2. A new input wire io_buffer_full that will show the UART output buffer is full and you should stall -- otherwise some output will be missing when output requests are intensive. You can ignore the problem in the beginning stage.

   Note: you will receive io_buffer_full in the SECOND NEXT CYCLE from your write cycle since the FIFO module's limitation. To ensure FIFO is not full you have to stall one cycle when there are two consecutive writes to 0x30000 in two consecutive clock, especially when i-cache is on. This problem will be detected in the testcase uartboom.

   You're welcome to fix this problem by modifying preset code. Elegant implementation will be counted as bonus.

Q&A

1. rdy_in and rst_in

   The rst_in has higher priority with rdy_in, and you CANNOT DO ANYTING when rdy_in is zero. rdy_in does not affect the result of simulation, but has effect when running on FPGA.

2. Write twice in simulation

   This is often OK in simulation, because it uses $write() in a combinational circuit to simulate a write(you can find it in hci.v), and by the property of combinational circuit, this instruction may be executed twice.

   In FPGA if everything you write is correct this will not happen.

3. connect with FPGA

   Use the micro USB port on the FPGA, since we use RS232 to transmit data.

You may meet various problems, especially when start testing on FPGA. Feel free to contact any TA for help.

Known issues (2021.2.3)

1. Some will fail to run the second time on FPGA. One quick solution is to let rst = rst_in | ~rdy_in, however it's somehow incorrect. We hope the future TAs to investigate the phenomenom and give a correct solution.

2. There's not a quick simulation check test case. One possible way is to mix up some test cases that can check those CPUs failed to run on FPGA using tolerable time while not to be much easier than passing all test cases. We didn't write one yet. Also, it may be hard to design, since some failures occur with only a few strange and specific conditions.

3. Our updated version of hcl and top are not good enough, since it requires combinational circuits not to consider rst --- or timing loop will occur since rst is connected to program_finish driven by combinational circuits. Better design may be required.