The purpose of this tutorial was to introduce the computer architecture research community to the features and capabilities of the new version of PyMTL, a Python-based hardware generation, simulation, and verification framework. This half-day tutorial was held on Saturday, June 22nd, 2019, co-located with ISCA-46 in Phoenix, AZ. Over fourty researchers participated in the workshop, which included a mix of presentations and hands-on activities.

Why PyMTL? — Computer architecture researchers are increasingly exploring the hardware and software aspects of accelerator-centric architectures, and this has resulted in a trend towards implementing accelerators at the register-transfer level and even fabricating accelerator-centric test chips. However, the conventional wisdom is that designing, implementing, testing, and evaluating RTL accelerators is a complex, time-consuming, and frustrating process. These challenges in the computer architecture research community mirror the challenges faced by commercial, government, and hobbyist hardware designers. These challenges have motivated some design teams to augment or even replace traditional domain-specific hardware description languages (HDLs) with a mix of different high-level hardware generation, simulation, and verification frameworks. PyMTL is a next-generation Python-based framework that unifies hardware generation, simulation, and verification into a single environment. The Python language provides a flexible dynamic type system, object-oriented programming paradigms, powerful reflection and introspection, lightweight syntax, and rich standard libraries. PyMTL builds upon these productivity features to enable a designer to write more succinct descriptions, to avoid crossing any language boundaries for development, testing, and evaluation, and to use the complete expressive power of the host language for verification, debugging, instrumentation, and profiling. The hope is that PyMTL can reduce time-to-paper (or time-to-solution) by improving the productivity of design, implementation, verification, and evaluation.

The PyMTL Workflow — A typical workflow using PyMTL is shown above. The designer starts from developing a functional-level (FL) design-under-test (DUT) and test bench completely in Python. Then the DUT is iteratively refined to the cycle level (CL) and register-transfer level (RTL), along with verification and evaluation using Python-based simulation and the same test bench. The designer can then translate a PyMTL RTL model to Verilog and use the same test bench for co-simulation. Note that designers can also co-simulate existing SystemVerilog source code with a PyMTL test bench. The ability to simulate/co-simulate the design in the Python runtime environment drastically reduces the iterative development cycle, eliminates any semantic gap, and makes it feasible to adopt verification methodologies emerging in the open-source software community. Finally, the designer can push the translated DUT through an FPGA/ASIC toolflow and can even reuse the same PyMTL test bench during prototype bringup.

The New Version of PyMTL — This hands-on tutorial introduced participants to the new version of PyMTL. This version of PyMTL maintains some of the best features of the current version including: support for highly paramterized chip generators; a unified framework for functional-, cycle-, and register-transfer level modeling; pure-Python-based simulation; elegant translation of PyMTL RTL to Verilog RTL; and first-class support for co-simulation of PyMTL and Verilog models through Python/Verilator integration. The new version of PyMTL will additionally include: a completely new execution model based on statically scheduled concurrent sequential update blocks; improved simulation performance; first-class support for method-based interfaces; PyMTL passes for analyzing, instrumenting, and transforming PyMTL models; and improved verification methodologies.

Tutorial Objectives

Our objective was to provide attendees with answers to the following questions:

Tutorial Code

The VirtualBox VM used for the tutorial included a complete PyMTL development environment, a set of open-source EDA tools (e.g., Yosys, Graywolf, Qrouter, KLayout), the FreePDK45 physical design kit, and Nangate standard cells. The VirtualBox VM is available for download so that anyone can complete the hands-on activities on their own.

We recommend using VirtualBox v6 or later. You should be able to simply download the OVA file and double click on it to decompress and import the VirtualBox VM. The VirtualBox VM will use about 10GB on your local hard drive. Once the import is complete, you can start the VirtualBox VM by clicking the Start button in VirtualBox. Once fully booted, you will be automatically logged in. If the font is too small, click on Machine > Settings, and change the scale Factor in the Display tab. The activities will require you to work at the command line. Double click the Terminal icon on the desktop, to launch a terminal you can use.

The following links enabled participants to browse the previous version of PyMTL, the new version of PyMTL, and the PyMTL designs used in the tutorial:

Tutorial Schedule

12:301:45pm Lunch
1:452:00pm Virtual Machine Installation and Setup
While attendees are welcome to simply listen during the tutorial, we strongly encourage attendees to engage in the hands-on activities throughout the tutorial. Attendees should arrive at 1:45pm so that there is time to install and setup the virtual machine for the hands-on activities.
2:002:15pm Presentation: PyMTL Overview slides
2:152:55pm Part 1: PyMTL Basics
In this part, we will focus on the PyMTL basics which serve as the foundation for productive multi-level modeling and VLSI design. We will start by learning the basics of modeling fixed-bitwidth types in Python and writing simple unit tests using py.test. We will then learn about update blocks, wires, value ports, method ports, and interfaces.
2:553:30pm Part 2: Introduction to Multi-Level Modeling with PyMTL
In this part, we will explore a hardware implementation of a variant of Fletcher's checksum algorithm. This part will enable attendees to build off of the fundamentals from the previous part to understand basic PyMTL syntax, constructs, and semantics including PyMTL's emphasis on concurrent structural modeling. We will implement a functional-level (FL) model of the algorithm and use directed testing to verify its functionality. We will then implement both a cycle-level (CL) and register-transfer-level (RTL) model of a checksum hardware unit. We will then translate our RTL model into Verilog and push it through an open-source EDA toolflow to generate layout using the open FreePDK45 technology and Nangate standard cells. We will end by exploring techniques for multi-level verification with an emphasis on property-based random testing.
3:304:00pm Coffee Break
4:004:45pm Part 3: Modeling Processors in PyMTL
In this part, we will explore FL, CL, and RTL models for a basic RISC-V processor with support for 10 instructions. This part will enable attendees to gain additional understanding of multi-level modeling in the context of a more complicated model. We will add a new AND instruction at each level of abstraction, and we will write a new assembly test generated and assembled with Python for verifying this AND instruction. We will end by using a PyMTL simulator to evaluate the performance of an assembly implementation of Fletcher's checksum algorithm.
4:455:30pm Part 4: Multi-Level Composition in PyMTL
In this part, we will explore composing the work done in the previous two parts to create a simple processor+accelerator system. This part will enable attendees to understand multi-level composition where models at different levels of abstraction can be arbitrarily mixed. We will start by exploring how to wrap the hardware unit from part 1 to provide an register-mapped accelerator interface. We will then compose this accelerator with the processor from part 2 to create a multi-level simulator. Both the processor and accelerator have FL, CL, and RTL implementations, so we will experiment with all nine possible compositions and we will compare the performance on a Fletcher checksum benchmark both with and without the accelerator. We will end by translating an RTL model of the processor+accelerator system into Verilog and pushing it through an open-source EDA toolflow to generate layout using the open FreePDK45 technology and Nangate standard cells.

Tutorial Prerequisites

We expect attendees that wish to engage in the hands-on activities throughout the tutorial to have some experience with Linux-based software and/or hardware development (e.g., working at the Linux command line, using a Linux text editor such as emacs, vim, or gedit). Attendees should bring a laptop and charger for the tutorial. Each attendee will receive a USB flash drive containing: VirtualBox installers for Windows, Mac OS X, Linux; a CentOS-based virtual machine containing all of the code and tools for the tutorial; and PDFs for all of the tutorial slides and handouts. While it is certainly possible to install PyMTL on your own UNIX-like laptops, we encourage attendees to use our virtual machine to simplify the setup. Attendees will need approximately 5GB of free space in order to install and use the VirtualBox virtual machine.

Tutorial Organizers

PyMTL Publications

Tutorial Sponsors

Research and development for the PyMTL project is funded in part by NSF CRI Award #1512937, NSF SHF Award #1527065, the Defense Advanced Research Projects Agency through a DARPA POSH Award #FA8650-18-2-7852, and by the Center for Applications Driving Architectures (ADA), one of six centers of JUMP, a Semiconductor Research Corporation program co-sponsored by DARPA.