R5P RISC-V processor family

Processors are named after pet rodents so that everybody can easily remember their names.

Instruction decoder

One of the main design aims was to create a reusable instruction decoder written in SystemVerilog. This means extensive use of SystemVerilog constructs:

• arrays, structures, unions, and enumerations,
• assignment patterns,
• don't care conditions and assignments in case statements.

Packed structures and enumerations are used to describe instructions formats and instruction encodings. The intention is to follow the ISA standard as literally as possible.

Long case statements and assignment patterns are used to implement the instruction decoder. Don't care values are used in both case statement conditions and assignment patterns

Pipeline

The first processor using the reusable instruction decoder has a 2-stage pipeline and all instructions are executed in a single clock cycle (IPC=1), without exceptions.

This is achieved without impractical solutions like asynchronous memories, except for GPR register file, which is rather common. Main memories are standard ASIC/FPGA block memories.

Staling during branches is avoided by placing both an address incrementer and a branch adder between the PC and the instruction memory address input.

As mentioned before GPR read is done asynchronously.

Staling during load instructions is avoided by matching the synchronous read delay with a write back delay for the remaining instruction, for example arithmetic/logic instructions. So all instructions write back into GPR with the same delay. The data hazard is avoided by providing a bypass in case the current instruction requires the result from the previous instruction.

Simulation

The code was developed with vanilla Verilator, my aim was not to cover the SystemVerilog standard. The main two issue I had with Verilator were:

1. lack of support for unpacked structures, so I just used packed structures everywhere, which is a bit ugly,
2. X propagation is not simulated well, so I had to run regression tests with mapping X separately to 0 and 1. The CPU is passing all I ISA tests, the C decoder is also in a good shape, but I avoided using it in FPGA synthesis.

Synthesis

I synthesized the code in Vivado 2021.2 (Artix), Quartus 21.1 (Cyclone V), and LatticeDiamond/SynplifyPro (EPC5), I did not test any of the FPGA builds yet, the aim was to get past tool errors. Vivado and Quartus produced apparently reasonable results. Synplify was able to parse the code without significant warnings, but synthesis optimized everything out due to constant propagation. I suspect there are some issues with don't care values (X propagation), so in the next step I plan to test with Vivado Simulator and QuestaSim/ModelSim, and probably try to run some netlist simulations.

My main aim for yosys synthesis would be Sky130 PDK. The CPU has a 2-stage pipeline and IPC=1 (all instructions execute in a single clock cycle, without exceptions). The register file requires asynchronous reads, on FPGA I used distributed memories, so I had to use FPGA families which support this memory type (see above), Cyclone 10 and iCE40HX families do not support asynchonous read distributed memories. For Sky130 I would use DFFRAM, the 32x32-bit words (2R1W) register file. Instruction and data closely coupled memories use standard ASIC/FPGA SRAM, so OpenRAM would be just fine.

TODO

Short

export VERILATOR_ROOT=/home/ijeras/VLSI/verilator
cd sim
make -f Makefile.verilator lint
make -f Makefile.verilator

Requirements

The default script is using ModelSim (version from Altera/Intel) for simulation.

Verilator

export SYSTEMC_INCLUDE=/opt/systemc-2.3.3/include/
export SYSTEMC_LIBDIR=/opt/systemc-2.3.3/lib-linux64/

Lattice diamond

Run license server

sudo /usr/local/diamond/3.12/ispfpga/bin/lin64/lmgrd -l /usr/local/diamond/3.12/license/license.log -c /usr/local/diamond/3.12/license/license.dat

Run Lattice Diamond

/usr/local/diamond/3.12/bin/lin64/diamond

Ubuntu 18.04, Quartus-lite-18.0.0.614-linux.tar

I had some problems with the installer crashing at the end of the install process, but apparently the tools were installed.

Some instructions: http://www.bitsnbites.eu/installing-intelaltera-quartus-in-ubuntu-17-10/

After installing ModelSim, a symbolink link must be created:

cd $HOME/intelFPGA_lite/18.0/modelsim_ase
ln -s linux linux_rh60

OVP simulator

www.OVPworld.org only provides a 32 bit version of OVPsim. So 32 bit support must be installed on 64 bit systems.

sudo dpkg --add-architecture i386
sudo apt-get update
sudo apt-get install libc6-i386
sudo apt-get install lib32stdc++6
sudo apt-get install lib32z1 lib32ncurses5 lib32bz2-1.0
sudo apt-get install gcc-multilib g++-multilib

To be able to download OVPsim, an account must be first created on OVPworld.org. Apparently all OVPsim download links point to the same installer file.

Follow instructions in Imperas_Installation_and_Getting_Started.pdf.

The downloaded OVPsim.*.exe executable must have the executable flag set, before beeing executed.

$ chmod +x OVPsim.20180716.0.Linux32.exe
$ sudo ./OVPsim.20180716.0.Linux32.exe

Type yes to accept the license, and then no to be able to install into an alternative location. This instructions are written for the the /opt/OVPsim-20181114/ install path.

Environment variables must be set. Add the following lines to settings.sh.

export OVP_HOME=$HOME/Workplace/Imperas.20181114
. $OVP_HOME/bin/setup.sh
setupImperas -m32 $OVP_HOME
. $OVP_HOME/bin/switchRuntime.sh
switchRuntimeImperas

https://wiki.ubuntu.com/DebootstrapChroot https://github.com/shoes/shoes3/wiki/Setup-an-schroot-build-environment http://logan.tw/posts/2018/02/24/manage-chroot-environments-with-schroot/

# apt install debootstrap schroot
# vi /etc/schroot/chroot.d/bionic32
$ mkdir $HOME/Workplace/bionic32
sudo debootstrap --variant=buildd --arch i386 bionic $HOME/Workplace/bionic32 http://archive.ubuntu.com/ubuntu/
schroot -c bionic32 -u root

cat <<EOT >> /etc/apt/sources.list
deb http://archive.ubuntu.com/ubuntu/ bionic main restricted
deb http://archive.ubuntu.com/ubuntu/ bionic-updates main restricted
deb http://archive.ubuntu.com/ubuntu/ bionic universe
deb http://archive.ubuntu.com/ubuntu/ bionic-updates universe
deb http://archive.ubuntu.com/ubuntu/ bionic multiverse
deb http://archive.ubuntu.com/ubuntu/ bionic-updates multiverse
deb http://security.ubuntu.com/ubuntu bionic-security main restricted
deb http://security.ubuntu.com/ubuntu bionic-security universe
deb http://security.ubuntu.com/ubuntu bionic-security multiverse
EOT
apt update
apt upgrade
apt install software-properties-common
add-apt-repository ppa:aelmahmoudy/ppa
add-apt-repository ppa:iztok.jeras/ppa
apt install gcc g++ verilator libsystemc-dev

[bionic32]
description=Ubuntu 18.04 (Bionic Beaver) 32-bit
directory=$HOME/Workplace/bionic32
personality=linux32
root-users=$USER
type=directory
users=$USER