SystemVerilog DPI-C: Calling C from SystemVerilog

The Direct Programming Interface (DPI-C) lets SystemVerilog call C/C++ functions and vice-versa. It is how testbenches reuse a C reference model, a cryptographic library, or a scoreboard written in software — without the overhead of the older PLI/VPI.

Importing a C function into SystemVerilog

You declare the C function with import "DPI-C" and call it like any SV function. The canonical form below imports a C crc8 reference and compares it against the RTL (this snippet needs a C toolchain at compile time, so it is shown for reference rather than run in the browser lab):

// ---- C side (crc.c) ----
#include <stdint.h>
unsigned char crc8_ref(unsigned char data) {
    unsigned char crc = data;
    for (int i = 0; i < 8; i++)
        crc = (crc & 0x80) ? (crc << 1) ^ 0x07 : (crc << 1);
    return crc;
}

// ---- SystemVerilog side ----
import "DPI-C" function byte unsigned crc8_ref(byte unsigned data);   // call C from SV
export "DPI-C" function sv_log;                                       // let C call SV
function void sv_log(int code); $display("C called back: %0d", code); endfunction

initial begin
    byte unsigned d = 8'hA5;
    $display("C reference CRC of %0h = %0h", d, crc8_ref(d));          // direct C call
end

Key direction rules: import "DPI-C" brings a C function into SV; export "DPI-C" exposes an SV function to C. SV and C types map by a defined table (byte unsigned↔unsigned char, int↔int, arrays via open arrays). Compile with the C source alongside the SV (vcs ... crc.c, xrun ... crc.c, or Verilator with the C file).

The idea, provable in pure SystemVerilog

The value of DPI is a trusted golden model checked against the DUT. Here is that pattern expressed entirely in SystemVerilog (a function standing in for the C reference) so you can run it in the browser and see the scoreboard concept:

module tb;
  // golden reference (in real projects this is the imported C via DPI-C)
  function automatic byte unsigned crc8_golden(byte unsigned data);
    byte unsigned crc = data;
    for (int i = 0; i < 8; i++)
      crc = crc[7] ? (crc << 1) ^ 8'h07 : (crc << 1);
    return crc;
  endfunction

  // "DUT" under test (same algorithm here; a bug would surface as a mismatch)
  function automatic byte unsigned crc8_dut(byte unsigned data);
    byte unsigned crc = data;
    for (int i = 0; i < 8; i++)
      crc = crc[7] ? (crc << 1) ^ 8'h07 : (crc << 1);
    return crc;
  endfunction

  initial begin
    int errors = 0;
    for (int v = 0; v < 256; v++) begin
      byte unsigned exp = crc8_golden(v[7:0]);
      byte unsigned got = crc8_dut(v[7:0]);
      if (exp !== got) begin errors++; $display("MISMATCH v=%0h exp=%0h got=%0h", v, exp, got); end
    end
    $display("checked 256 vectors against golden model, errors=%0d", errors);
  end
endmodule

checked 256 vectors against golden model, errors=0

Swap the golden function for an import "DPI-C" of a C model and you have the real DPI scoreboard — the C code becomes the reference of record while the RTL is the thing under test.

▶ Try it in the ChipXpert Lab

Run the 256-vector golden-vs-DUT check above in the cocotb verification lab. In cocotb the “golden model” is simply Python — the same reference-model idea DPI gives you in C.

Open the Verification Lab →

Check yourself

What is the difference between <code>import</code> and <code>export</code> in DPI-C?

import "DPI-C" declares a C function that SystemVerilog can call. export "DPI-C" exposes a SystemVerilog function so that C code can call back into the simulation. A testbench often uses both to pass control between the two languages.

Why reach for DPI-C instead of writing the reference model in SystemVerilog?

To reuse an existing, trusted C/C++ model — a codec, crypto core, instruction-set simulator or floating-point library — as the golden reference, and to run heavy computation faster than interpreted SV. The RTL is then checked against that independent implementation.

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