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EECS 151/251A ASIC Project Specification: Checkpoint 2

Prof. Bora Nikolic

TAs: Daniel Grubb, Nayiri Krzysztofowicz, Zhaokai Liu

Department of Electrical Engineering and Computer Science

College of Engineering, University of California, Berkeley

Fully functioning core

1. Additional Instructions

1.1 Control and Status Register (CSR)

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 Chapter 9 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 2^12 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).

2. Details

Your job is to implement the core of the 3-stage RISC-V CPU.

2.1 Reset

Your CPU will have an input reset signal that the testbench toggles. Once out of reset, your CPU should start at PC address 0x2000 (defined as PC_RESET in src/const.vh) and begin executing instructions.

2.2 Misaligned Addresses

According to the RISC-V ISA spec, reads and writes to memory addresses not aligned to a 32-bit word boundary (or 16-bit for halfword) should cause an exception. In this project, for the purpose of simplicity, we ignore the misaligned bits (i.e. set them to zero), which is done in Memory151.v by only using address bits 31:2.

3. File Structure

Implement the datapath and control logic for your RISC-V processor in the file Riscv151.v. 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. Target this testbench in your sim-rtl.yml file by changing the tb_name key to rocketTestHarness.

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. You should only be running this test suite after you have eliminated some of the bugs using single instruction tests, as described below.

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 a single asm test with the following command:

make sim-rtl test_asm=simple.out

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

make sim-rtl test_asm=simple.vpd

’simple’ may be 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 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 addi.hex

6. Checkpoint 2 Deliverables

Checkoff due: Nov 10 (Wednesday), Nov 12 (Friday), Nov 15 (Monday), 2021

Congratulations! You’ve started the design of your datapath by implementing your pipeline diagram, 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.

  3. Push your implementation and updated pipeline diagram to your repo.

Acknowledgement

This project is the result of the work of many EECS151/251 GSIs over the years including: Written By:

  • Nathan Narevsky (2014, 2017)
  • Brian Zimmer (2014) Modified By:
  • John Wright (2015,2016)
  • Ali Moin (2018)
  • Arya Reais-Parsi (2019)
  • Cem Yalcin (2019)
  • Tan Nguyen (2020)
  • Harrison Liew (2020)
  • Sean Huang (2021)
  • Daniel Grubb, Nayiri Krzysztofowicz, Zhaokai Liu (2021)