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SNAP Framework Hardware and Software

1. Overview

The SNAP Framework enables programmers and computer engineers to quickly create FPGA-based acceleration actions that work on server host data, as well as data from storage, flash, Ethernet, or other connected resources. SNAP, therefore, is an acronym for “Storage, Network, and Analytics Programming”. The SNAP framework makes it easy to create accelerated actions utilizing the IBM Coherent Accelerator Processor Interface (CAPI).

Note that SNAP addresses only CAPI1 and CAPI2 attached Cards. For CAPI3 (also named OpenCAPI) please refere to oc-accel at : snap_concept_diagram The framework hardware consists of a AXI-to-CAPI bridge unit, memory-mapped register I/O, host DMA, and a job management unit. It interfaces with a user-written action (a.k.a. kernel) through an AXI-lite control interface, and gives coherent access to host memory through AXI. Optionally, it also provides access to the on-card DRAM via AXI. A NVMe host controller-AXI bridge complements the framework for storage or database applications as an independent unit. Software gets access to the action through the libsnap library, allowing applications to call a "function" instead of programming an accelerator. The framework supports multi-process applications and can be extended to support multiple instantiated hardware actions in parallel.
Note: The current 1.x releases support a single action per FPGA.

This project is an initiative of the OpenPOWER Foundation Accelerator Workgroup. Please see here for more details:

What is CAPI, education materials and more information


Currently the SNAP Framework supports CAPI1.0 on POWER8 based hosts and CAPI2.0 on POWER9 based hosts. A similar OpenCAPI SNAP framework is going to be added in a new repository. Users working on SNAP today can easily transfer their CAPI1.0 work to CAPI2.0 or OpenCAPI as the interface for "Software Program" and "Hardware Action" (shown in the yellow areas of the above figure) will stay the same.

2. A 3 steps process

Developing an FPGA accelerated application on SNAP can be done following the steps listed below, but this sequence is not mandatory.

  • Preparation: Decide the software function to be moved to FPGA. This function, usually computation intensive, is named as "action" in the following description.

  • Step1. Put the action code into a separate function in the main software code, and determine the function parameters required. Add the few libsnap API functions that required to set up CAPI to the main software. The best way is to start from an example (See in actions) and read the code within the "sw" directory.

  • Step2. Write the "hardware action" in a supported programming language, such as Vivado HLS or Verilog/VHDL. For HLS, developers can write their algorithms in C/C++ syntax within an function wrapper "hls_action()". Developers who prefer HDL(Verilog/VHDL), can use their adapted version of "action_wrapper.vhd" as top-level. It includes several AXI master interfaces and one AXI-lite slave interface. Refer to the "hw" directory in "hls_*" or "hdl_*" for action examples.
    For simulation of the hardware action, the PSLSE (Power Service Layer Simulation Engine) provides a software emulation of the whole path from libcxl library to the Power Service Layer (see the blue boxes in the picture above). This allows for simulating the action without access to an FPGA card or a POWER system. When the simulation is successful, you are ready to generate the FPGA bitstream. Note : there is no need to build a specific testbench to test your application in FPGA. This is a key advantage as your code is the testbench. Please read the hardware/ for more details.

  • Step3. Program the bitstream to a real FPGA card plugged into a POWER or OpenPOWER machine and run your calling software from it. This step is also called Deployment. Please see for instructions on how to program the FPGA bitstream.

For a step-by-step help, please refer to the SNAP Workbooks in the doc directory. For example, make sure you read the QuickStart Guide if you're a first time user. Some other user application notes are also there.

Please also have a look at actions to see several examples which may help you get started with coding. Each example has a detailed description in its own "doc" directory.

3. Dependencies

3.1 FPGA Card selection

As of now, the following FPGA cards can be used with SNAP if they contain CAPI logic (see cards ressources details and instructions to program FPGA card to be CAPI enabled):

3.2 Development (Step1 & Step2)

Development is usually done on a Linux (x86) computer since as of now, Xilinx Vivado Design Suite is supported only on this platform. See examples of supported development configurations. The required tools and packages are listed below. Web access to github is recommended to follow the build instructions. A real FPGA card is not required for the plain hardware development.

(a) Install Xilinx Vivado Design Suite: the tool to build and program the FPGA.

SNAP currently supports Xilinx FPGA devices, exclusively. For synthesis, simulation model and FPGA/image build, the Xilinx Vivado HL Design Edition 2018.1 tool suite is recommended. Different licenses are available. Some licenses are limiting the components that can be used; check the FPGA you target. (CAPI1.0 cards use UltraScale family components and CAPI2.0 cards use UltraScale+ family components). This Design Suite includes a C synthesizer (Vivado HLS), a simulator (xsim), a synthesizer and FPGA/Image build tools (Vivado).

(b) Download CAPI1.0 PSL or CAPI2.0 BSP: the "connection box" between the POWER server and the FPGA.

Access to CAPI from the FPGA card requires the Power Service Layer (PSL) for CAPI1.0 cards or the Board Support Package (BSP) for CAPI2.0 cards. After accepting the terms of conditions, user will need to download a file to build the FPGA code. Detailed information is available in hardware/

(c) Install the basics for the Build process

First clone snap (git clone Then use the usual development tools: gcc, make, sed, awk to build the code and to run the make environment. If not installed already, the installer package build-essential will set up the most important tools.

Configuring the SNAP framework via make snap_config will call a standalone tool that is based on the Linux kernel kconfig tool. This tool gets automatically cloned from

The ncurses library must be installed to use the menu-driven user interface for kconfig.

Please see Image and model build for more information on the build process.

(d) Download the PSL Engine for Simulation: the "POWER + FPGA" emulation box

For simulation, SNAP relies on the xterm program and on the PSL Simulation Environment (PSLSE) which is free and available on github

Please see PSLSE Setup for more information.

Simulating the NVMe host controller including flash storage devices requires licenses for the Cadence Incisive Simulator (IES) and DENALI Verification IP (PCIe and NVMe). However, building images is possible without these licenses. For more information, see the Simulation README.

3.3 Deployment (Step3)

Deployment is on a Power or OpenPower server with a CAPI programmed FPGA card plugged. See instructions to program any FPGA card to be recognized as a CAPI card. See examples of supported deployment configurations.

(a) Install CAPI accelerator library

This code uses libcxl to access the CAPI hardware. Install it with the package manager of your Linux distribution, e.g. sudo apt-get install libcxl-dev for Ubuntu, or sudo yum install libcxl-devel for RHEL.
For more information, please see

(b) Install CAPI programmation tool

SNAP uses the generic program capi-flash-script to upload FPGA code/bitstreams into the CAPI FPGA cards. This can be downloaded from This tool can be used ONLY if a CAPI image has already been put once in the FPGA. If not, please follow instructions to program any FPGA card to be recognized as a CAPI card or ask help from CAPI support.

4. Contributing

This is an open-source project. We greatly appreciate your contributions and collaboration. Before contributing to this project, please read and agree to the rules in

To simplify the sign-off, you may want to create a ".gitconfig" file in you home by executing:

$ git config --global "John Doe"
$ git config --global

Then, for every commit, use git commit -s to add the "Signed-off by ..." message.

By default the git repository is read-only. Users can fork the snap repository, make the changes there and issue a pull request. Even members with write access to this repository can't commit directly into the protected master branch. To contribute changes, please create a branch, make the changes there and issue a pull request.

Pull requests to merge into the master branch must be reviewed before they will be merged.

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