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 * Concurrent Assignments

 * Process

 * Sequential Statements

 * Variable

  * User Defined Types

 

Process

Although process is concurrent statement, it contains a set of sequential statements. In this statement all the processes keep being executed till they stopped with sensitivity list or a wait statement.

Syntax:

Label: Process (Sensitivity list)
             contant declaration
             type declaration
             variable declaration
          Begin
             -- sequential statement
             -- sequential statement
                  色.
         end process;

All the expresions in a process are executed one by one from top to bottom. So the last assigned value stays on the object.

Example:

Process(a,b)
   Begin
     a<=5;
     b<=6;
     a<=3;
end process;

After this process executed, the value of "b" becomes 6 and the value of "a" becomes 3 since its the last assigned value to varable "a".

Sensitivity List

If there is a sensitivity list in a process, the suspended process becomes active at the time one of the signals listed change. Process may contain more than one wait expressions.

A sensiivity list and a wait expression can not e used in one process.

Example:

U1:process(input1,input2)
          Begin
             Output<=input1 and input2;
          end process;

U1:process
          Begin
            Output<=input1 and input2;
            Wait on (input1,input2)
         end process;

The two expressions above are identical. In the both processes, "input1 and input2" value is asssigned to output value. Asignment process is executed as soon as any change occurs on input1 or input2.

An architecute may contain more than one process. All the processes in an architecture are executed concurently.

 

 

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