Procedure, Function and Impure Function Synthesis

In VHDL, both "function" and "procedure" can be used in hardware descriptions, but it should be understood that hardware synthesis is usually more suitable for implementations based on deterministic and synchronous behavior. Therefore, there are several limitations on the use of functions and procedures in the context of synthesis:

• Procedure: Procedures in VHDL are used to perform tasks without returning a value. They can be used in hardware descriptions to manage operations and organize code. Hardware synthesis will usually replace procedure calls with the corresponding physical actions in the target hardware. Therefore, deterministic procedures can be synthesized. However, there are some limitations on the use of procedures that depend on time flow or behavior that is difficult to predict. Some VHDL compilers may not support the synthesis of such procedures.
• Function: VHDL functions that do not have impure properties (e.g., they produce a deterministic value based on input arguments only) can usually be synthesized well.
• Impure Function: Impure functions, which produce unpredictable results or depend on external factors, are usually not suitable for deterministic hardware synthesis. Impure functions that depend on random or non-deterministic behavior will not be synthesizable, as the resulting hardware must be deterministic and predictable.

So, while functions and procedures can be used in hardware descriptions and can be synthesized if they meet certain requirements, impure functions are usually not suitable for VHDL synthesis.