Combinational section signal assignment

For a combinational process, it has been shown in [12] how to eliminate the internal variables, and compute the transition function of all signals assigned in the process, as a set of concurrent signal assignments. In this section we will only illustrate a conditional form on a simple example in the two languages ... 

The keyword Unaffected can be used in certain concurrent signal assignment statements to ensure that there is no change in the value of a signal given a particular condition. In this respect it is similar to the Null statement in sequential statement sections. Its use therefore implies the existence of feedback around the combinational logic. In some circuits this coxild possibly lead to hazardous conditions and so it should be used with caution. 

A signal assigned inside a synchronous section is synthesized as a flip flop. If it is assigned inside a combinational section it will be synthesized as a signal on a wire. 

Although only one synchronous section is shown here, any number of them could be described in a single process. In this example, the signal S is assigned a value inside the synchronous section whereas A and C are assigned values outside in the combinational part. The compiler will there-... 

If the signal (or variable) infers a storage element (in a synchronous section elsewhere) then the logic of the assignment becomes part of the asynchronous combinational input logic to the storage element. 

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