1. Non-Determinism In C and RTL Models ICCAD 2006 – Ira Chayut, Verification Architect

2. © 2006 NVIDIA Corporation Slide 2 “C Models” in a Variety of Languages “C Model” == “Reference Model” Reference Model could be in C, C++, Perl, Python, Java, Verilog, SystemVerilog, SystemC, emulation, silicon rendition of the Device Under Test (DUT), etc. The DUT can be any implementation, this presentation assumes a Verilog RTL DUT

3. © 2006 NVIDIA Corporation Slide 3 Validation vs. Verification Validation: answers the question: “Does the model produce the correct output?” Verification: answers the question: “Does the DUT behavior match that of the reference model?”

4. © 2006 NVIDIA Corporation Slide 4 A Typical Verification Example Stimulus is applied to the C model and the DUT. The outputs are then compared.

5. © 2006 NVIDIA Corporation Slide 5 Causes of Mismatches Errors in: DUT C model stimulus checker code compiler or simulator Differences in the way models handle time

6. © 2006 NVIDIA Corporation Slide 6 Determinism / Non-Determinism According to Wikipedia: In a deterministic system, every action, or cause, produces a reaction, or effect, and every reaction, in turn, becomes the cause of subsequent reactions. The totality of these cascading events can theoretically show exactly how the system will exist at any moment in time. Thus, a non-deterministic system is one where knowing the state of a model and the stimulus, we cannot precisely know the next state and output At Nvidia – the term “non-determinism” is used for situations where the C model cannot predict the DUT’s behavior

7. © 2006 NVIDIA Corporation Slide 7 Causes of Non-Determinism Independence between C model and DUT Architectural vagueness Efficiency Points of arbitration (including memories and caches)

8. © 2006 NVIDIA Corporation Slide 8 Time and Non-Determinism Timed-events are the largest cause of non- determinism, including: Memory accesses Cache fills/hits/misses Time-outs Multiple clock domains Clock jitter in silicon

9. © 2006 NVIDIA Corporation Slide 9 Non-Determinism Work-Arounds Hints from DUT Cycle-accurate C models Smart checkers Don’t verify non-deterministic cases

10. © 2006 NVIDIA Corporation Slide 10 Non-Determinism Example MUX selects stimulus until: FIFO full, or Timer reaches time-out C model does not have concept of time – RTL sees time-outs that C model does not !=

11. © 2006 NVIDIA Corporation Slide 11 Hints from DUT DUT simulation: provides hints to untimed C model gates the C model weakens the independent check by C model ==

12. © 2006 NVIDIA Corporation Slide 12 Cycle-Accurate C Model Slows C model runs Slows C model development Problem modeling jitter between async clock domains Weakens the independent check of C model

13. © 2006 NVIDIA Corporation Slide 13 Smart Checkers Match out-of-order transactions Ignore don’t care bits Recognize acceptable extra transactions Score-board transactions to catch spurious transactions Statistical analysis needed sometimes

14. © 2006 NVIDIA Corporation Slide 14 Don’t Verify Non-Deterministic Cases Some cases too difficult to test Verify later-stage results Validate DUT Validate in silicon Avoid non-deterministic designs

15. © 2006 NVIDIA Corporation Slide 15 Conclusions C model and DUT behavior may vary for some stimulus, but both behaviors are correct Time-based behavior is especially problematic When non-determinism cannot be avoided: Smart checkers Cycle-accurate C models DUT hints Validate DUT or silicon