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leonardt / fault

A Python package for testing hardware (part of the magma ecosystem)
BSD 3-Clause "New" or "Revised" License
41 stars 13 forks source link

Use non-blocking assign for SV poke #191

Closed leonardt closed 4 years ago

leonardt commented 4 years ago

Verilator effectively uses non-blocking semantics because pokes are staged until eval is called. This updates the system verilog target to do the same by using non-blocking assign.

Kuree commented 4 years ago

I don't think the semantics is correct. If all the pokes are non-blocking, then the values will be updated in the NBA region, but a lot of semantics such as DPI calls and $display is evaluated in the active region. I won't recommend to force the evaluation behavior in SV simulator.

Notice that Verilator is not SV spec-compliant by choice. That means the test bench is not setup the same way as SystemVerilog test bench files. Matching a spec-compliant simulator to a non-compliant one seems to be an odd choice. Maybe I'm missing something here?

You can refer to Figure 4-1 in IEEE Std 1800-2017.

leonardt commented 4 years ago

The goal of fault is to provide an abstraction over both simulators, otherwise you wouldn't be able to write a generic test bench and target both simulators, in order to do this, the semantics for the actions need to be consistent across the generated test benches.