# Difference between Procedure, Function and Impure Function

| Criteria | Procedure | Function | Impure Function |
| --------- | :--- | :--- | :--- |
| **Purpose** | Perform tasks without returning a value. | Return a value from a calculation. | Produce an unpredictable value or one that depends on external factors. |
| **Arguments** | Can have input and/or output arguments. | Can have input arguments only. | Can have input arguments only. |
| **Return Value** | Does not return a value (void). | Returns a value from a calculation. | Returns a value from a calculation. |
| **Usage** | Used to organize tasks or operations. | Used for calculations or data processing. | Used when the function's result depends on external factors. |
| **Example** | `vhdl procedure SetFlag(flag: out boolean);` | `vhdl function Add(a, b: integer) return integer;` | `vhdl function RandomNumber return integer;` |
| **Synthesis** | Can be synthesized if it is deterministic and synchronous. | Can be synthesized if it is deterministic and synchronous. | Not suitable for synthesis because the result is unpredictable or depends on external factors. |