Direct Programming Interface (DPI)

Definition

The Direct Programming Interface (DPI) is a SystemVerilog feature that allows SystemVerilog code to directly invoke functions implemented in C or C++ (and vice versa). In the reusable RISC-V vector accelerator verification environment, DPI is used as the inter-language bridge between the SystemVerilog/UVM testbench and the Spike Instruction Set Simulator (ISS), which is implemented in C++.

Role in the Verification Environment

Communication between the SystemVerilog UVM environment and the Spike ISS is accomplished via DPI. Because the ISS is written in C++, its routines are imported into SystemVerilog using DPI declarations of the form import "DPI-C" function .... The imported Spike functions are then called from the SystemVerilog class that extends the iss_wrapper, i.e., the concrete class that implements the pure virtual methods declared in the base ISS-wrapper abstraction. This pattern lets the verification environment treat any C/C++-based ISS through a common wrapper interface while delegating the actual ISS interaction to DPI-imported calls.

Imported Spike ISS Functions

The following functions are imported from the Spike C++ implementation into SystemVerilog using import "DPI-C":

1. spike_setup(input longint argc, input string argv) — initializes Spike with command-line arguments.
2. run_and_inject(input int instr, output iss_state_t iss_state) — runs Spike and injects an instruction.
3. exit_code() — returns the ISS exit code.
4. set_tohost_addr(input longint tohost_addr, input longint fromhost_addr) — configures the tohost/fromhost addresses used by Spike.
5. get_memory_data(output longint mem_element, input longint mem_addr) — reads a memory element from Spike's memory space.
6. start_execution() — begins instruction execution in Spike.
7. set_memory_data(input int unsigned data, input longint unsigned address, input int size) — writes data into Spike's memory.
8. do_step(input int unsigned n) — advances Spike by n instructions.
9. spike_set_external_interrupt(int mip_val) — injects an external interrupt into Spike.
10. spike_run_until_vector_ins(inout iss_state_t iss_state) — runs Spike until a vector instruction is encountered and returns its state.

Project-Specific Versioning

Because each project targets a different version of the RISC-V vector extension, a different version of Spike is used per project (e.g., version 0.7.1 of the vector extension required modifications to the original Spike repository). To support this in a seamless way, Spike's library file is kept project-dependent, and a dedicated branch of the ISS repository is maintained to generate the library accordingly.

Related Components

• iss_wrapper (CodeArtifact): the SystemVerilog wrapper class declaring the pure virtual methods that the SPIKE-specific class implements; this is where the DPI-imported functions are called from.
• Reusable Verification Environment for a RISC-V Vector Accelerator (Paper): the verification environment that employs DPI to integrate the Spike ISS as the reference model.