Continuous assignment statement

Figure 2-1 Combinational circuit from continuous assignment statement.

The continuous assignment statement describes an inverter that has its input connected to Statin and whose output is StatOut. Delays, if any, specified in a continuous assignment statement are usually ignored by a synthesis system. For example, in the continuous assignment ... 

The logical operators get directly mapped onto primitive logic gates in hardware. Here is a model of a full-adder using continuous assignment statements. 

In the above examples on continuous assignments, there is a one-to-one correlation between a continuous assignment statement and its synthesized logic. This is because a continuous assignment implicitly describes the structure. 

Notice the structure of the synthesized circuit is very similar to that of the continuous assignment statements. 

Boolean equations represent combinational logic. Boolean equations are best represented using continuous assignment statements. Here is an example of a Gray code to binary code convertor using boolean equations. 

Here is a model of a different simple arithmetic-logic-unit. This logic unit performs four functions add, nand, greater-than and exclusive-or. A continuous assignment statement with a conditional expression is used to model the arithmetic-logic-unit. 

Here is an example of a simple 2-by-4 decoder circuit. This is a combinational circuit modeled purely using continuous assignment statements. Delays specified with the assignment statements, if any, are typically ignored by a synthesis system. 

Here is a model of a 4-by-l multiplexer circuit. In this case, a bit-select in a continuous assignment statement has been used to model the combinational logic. 

The input to the Workbench is a mixed behavioral and structural description. In Verilog, sequential behavior is described with the always statement, combinational behavior is described with the continuous assignment statement assign), and hierarchical structural is described with module definitions, and module and gate instantiations. For the purposes of the Workbench, the synthesis tools treat the always statement as a single process description of the behavior to be synthesized into a structural data path and controller. For each always statement, a separate controller and data path is generated. 

You may list the addresses of assignment statements executed, the respecifications of the y and what must be the value of T(y ) if the computation is to continue, making allowances for choices (compare our treatment of Example II - 3). Or you may draw a version of the execution sequence tree (as in Example III - 1), recording at each node only important information such as the address of the executed statement and the new values of the y for an assignment or the values tested for a test statement since we always have val(y, j) = f x) for some n, you can save space by recording only n. ... 

Workbench. That is, these structural components are not synthesized by the Workbench. However, they define the implicit ports (connections to a continuous assignment and instantiated submodules), and explicit ports (declared ports of the current module) between the behavior described in the always statement and these structural entities. 

We wish to see that for any choice of X = a and Y = b each path verification condition in W(P,A,B,I) holds. That is, we must examine each V(P,a,At r,I)(a,b) where o is a consistent path from tagged point t to tagged point r not passing through any other tagged point en route from t to r. If the hypothesis of the conditional expression V(P,a,A, A r,I)(a,b) is false, then the verification condition is vacuously true. If it is true, then A (a,b) is true and by definition of Aj, A(a) is true and computation (P,I,a) at some point enters tagged point t with Y = b. Further a is the continuation of this computation and reaches r with Y = b. So there is certainly a time when computation (P,I,a) reaches r with this specification of Y. Now if r is not a STOP statement, inductive assertion was assigned by our definition and thus Ar(a,b ) holds by definition. 

We close this section with a reminder of a fnndamental issue in electrochemistry Not all the quantities in Equations 13.8 throngh 13.13 are accessible to measurement by electrochemical or thermodynamic methods. Only the electrochemical potential ( i ), the work function (W ) or equivalently the real potential (a ) and the Volta potential ( / ) are. Equations 13.9, 13.11, and 13.13 are therefore formal resolutions. It is not possible to assign actual values to the separate terms, the chemical potential ( t ), the Galvani potential (cp ), nor the surface potential (x ), without making extrathermodynamic assumptions. These quantities must therefore be considered unphysical, at least from the point of view of thermodynamics. This statement, which is called the Gibbs-Guggenheim Principle in [42], is often met with disbelief from theoretical and computational chemists, particularly in the case of the chemical potential (Equation 13.10). The standard chemical potential is essentially the (absolute) solvation free energy AjG of species i. One would hope that a molecular simulation contains all information needed to compute AjG . Indeed, there seems to be a way around this thermodynamic verdict for computation and also mass spectroscopic. This continues to be, however, hazardous territory, particularly for DFT calculations in periodic systems. ... 

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