Turn-Off Dual-issue

for FU740-C000 that has Dual-issue in-order 64-bit execution pipeline

Is it possible to turn Dual-issue OFF by SW ?

I don’t see any information on disabling dual-issue in any of the current resources I can find for the Hifive Unmatched and FU740-C000. But based on what I can glean from the FU740-C000 manual, I would hazard a guess that it is not something that can be disabled. I do see on Unmatched product page that there is still a hardware reference manual and firmware that are still grayed out. Perhaps either of those might provide information on doing so when they are released.

Out of curiosity, why would you like to turn dual-issue off? I believe doing so would most likely lower the overall performance of the processor.

Curiosity killed the cat :slight_smile:
I would like to do some performance evaluation to the Dual Issue feature. By the Way, SweRV-EH1 do have this option available in SW (CSR Reg)

Fair enough. Maybe someone else has the answer you are looking for.

thanks for the awesome information.

maybe just get your compiler to emit NOPs every other instruction?

I just checked with our design team. We intentionally don’t offer a way to do this because — in our opinion — the modest power savings does not justify a substantial increase in energy per instruction. We believe that the DVFS (dynamic voltage and frequency scaling) mechanism is a better way to navigate this tradeoff.

As @zBeeble suggested, you could probably effectively cripple dual-issue with a clever selection of NOPs. I don’t know anyone, internally or externally, who has seriously attempted this.

Just out of curiosity, does the x86-64 have this capability?

IDK how apocryphal it is, but DEC was said to have issued updated faster “accelerated CPU” ROMS for some VAXen that were later identified as simply taking the NOPs out of the microcode of the CPU board.

I have, kicking around here somewhere, a board from an 11/750 that supposedly had these modifications for the FPU portion of the CPU — and cost the company (back-in-the-day) $250k.