Signal assignment conditional

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 ... 

On synthesizing a mux structure using the concurrent selected signal assignment statements you find that one of the conditions of the primary inputs is not used (that is, left unconnected). Why ... 

DATAFLOW4 is an example of another concurrent statement, a selected signal assignment. This performs a similar function to die conditional sig-... 

Conditional signal assignment architecture DATAFLOWS of MUX4TOI is begin... 

Figure 4.4 Multiplexer created by a conditional signal assignment statement.

The second architecture illustrates an alternative approach that uses an If-Else statement. This sequentially evaluates each expression in the construct tmtil it finds one diat is true and then executes the associated sequence of actions. This sequential statement is similar to the conditional signal assignment in that each possible two-way branch is evaluated in series, generating a multiplexer tree. Like the Case statement, the If-Else can initiate any number of actions within each brandv unlike its concurrent equivalent. 

Those that evaluated every condition sequentially but still did not verify that only one of them was TRUE. These circuits contained Component instantiations or Concurrent signal assignments. 

Those that evaluated each condition sequentially until one was found to be TRUE. These circuits contained If statements or Conditional signal assignments. 

Those that evaluated all conditions concunendy but only allowed one to be valid. These circuits contained Case statements or Selective signal assignments. 

The third category created the smallest circuit as the Case and Selective signal assignment statements ensure that conditions are not duplicated by checking at compile time and not in hardware. Logic is required by the second category of circuit to inhibit the evaluation of subsequent conditions once one is found to be valid. 

In a conditional signal assignment, the evaluation of the condition in each When clause only occurs if the previous branch failed. The construct therefore generates a hardware multiplexer tree in which each branch contains a new two-input multiplexer. Ihe same applies to a nested If-Else statement. This must be remembered when constructing the conditional expression for each branch. For example, consider the following statement ... 

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 conditional signal assignment statement is similar to an If-Else statement. The former is executed concurrently and the latter sequential (inside a process). 

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