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Motivation for using a four-valued logic.
Why use a four-valued logic instead of a simple, two-valued, Boolean logic with just T and F?
Having an X value is fundamental to the semantics of esim, our
circuit evaluator. Loosely speaking, when we begin simulating a circuit, all
of the internal wires are given X as their value. We then evaluate all of the
submodule occurrences. Because of
In the context of symbolic simulation, X values are sometimes also useful as a way to ignore certain signals. For instance, if we think some inputs to a particular circuit are not even involved in the property we wish to prove, we may leave them as X.
X values also allow us to model some circuits which cannot be expressed with just Boolean logic. For instance, imagine a scenario like:
Diagram Verilog
____
| \ assign C = ~A;
A ---| o------+ assign C = ~B;
|____/ |
____ |-- C
| \ |
B ---| o------+
|____/
Here the wire C is being driven by two separate sources. When these sources have different values, e.g., suppose A is T and B is F, then C is simultaneously driven to both F and T. We do not know which value will "win," or, really, whether there will even be a winner. So in this case we just say the value of C is X. Without an X value, we would not be able to model this circuit.
The Z value allows us to model additional circuits, even beyond those circuits that are possible to model using Xes. In the circuit above, we did not need a Z value because not-gates always drive at least some value onto C. But other kinds of circuits do not necessarily drive their output. For instance, in Verilog one might describe a mux whose selects must be one-hot as follows:
assign C = S1 ? A : 1'bz; assign C = S2 ? B : 1'bz; ...
By adopting Z into our logic, we can model these kinds of assignments directly, e.g., see 4v-tristate and 4v-res.