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Logic is combinatorial if outputs at a specified time are a function only of the inputs at that time. Examples of combinatorial logic functions are decoders, multiplexers, and adders.

Combinatorial logic is implemented in VHDL with Concurrent Signal Assignment Statements or with Process Statements that describe purely combinatorial behavior, that is, behavior that does not depend on clock edges.

Both Concurrent Signal Assignment and Process Statements should be placed in an Architecture Body, as shown below.

ARCHITECTURE a OF and_gate IS
BEGIN
   <concurrent signal assignments>
   <process statements>
   <other concurrent statements>
END a;

This section includes the following topics:

• Using Concurrent Signal Assignment Statements
  • Using Simple Concurrent Signal Assignments
  • Using Conditional Signal Assignments
  • Using Selected Signal Assignments
• Using Process Statements

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