jg-fossh / mdu

M-extension for SERV RISC-V cores.

Multiplication and Division Unit -MDU

This multiplication and division unit is in compliance with RISC-V standard M-extension, which can be integrated with any existing RISC-V processor.

Two operands, function3, and a valid signal are fetched from the target processor and fed to the MDU. The function3 signal which is named as i_mdu_op, it decides that which operation MDU has to perform on the arrival of a valid signal as shown in the table below.

Instruction	i_mdu_op
MUL	000
MULH	001
MULHSU	010
MULHU	011
DIV	100
DIVU	101
REM	110
REMU	111

---

Instantiation template

mdu_top
#(
  .WIDTH(32)
)i_mdu_top(
  .i_clk,  
  .i_rst,
  .i_mdu_rs1,
  .i_mdu_rs2,
  .i_mdu_op,
  .i_mdu_valid,
  .o_mdu_ready,
  .o_mdu_rd
);

---

Getting Started

MDU uses FuseSoC, which is a famous package manager and build abstraction tool for FPGA/ASIC development.

If the target core supports FuseSoC then you can easily add mdu as a dependency in .core file and making changes in RTL.

Demo: MDU integration with SERV

SERV is the world smallest bit-serial RISC-V core. It is famous because of its size and lumbering pace. The changes made for this integration can be tracked in PR #60.
SERV uses FuseSoC so it makes our work easier, you can run the SERV with MDU by simply following the steps below.

Create a directory to keep all the different parts of the project together. We will refer to this directory as $WORKSPACE from now on. All commands will be run from this directory unless otherwise stated.

Install FuseSoC

pip install fusesoc 

Add SERV as a separate library into the workspace

fusesoc library add serv https://github.com/olofk/serv

Now add MDU

fusesoc library add mdu https://github.com/zeeshanrafique23/mdu

If Verilator is installed, we can use that as a linter to check the SERV source code

fusesoc run --target=lint serv

If everything worked, the output should look like

INFO: Preparing ::serv:1.1.0
INFO: Setting up project

INFO: Building simulation model
INFO: Running

Now it's time to run SERV with MDU

fusesoc run --target=verilator_tb --flag=mdu servant

Note: All the FuseSoC commands should run from $WORKSPACE.

Acknowledgment

The integration of MDU with SERV was the part of Google Summer of Code 2021 project under the banner of FOSSI Foundation, it is completed by Zeeshan Rafique under the mentorship of Olof Kindgren. Special thanks to Olof Kindgren for helping me throughout the project.