Function and procedure in packages in VHDL - Mini Project: Build & Apply

Function and Procedure in Packages

📖 Scenario: You are designing a simple VHDL package to help with basic arithmetic operations. You want to create reusable functions and procedures inside a package so that other VHDL modules can use them easily.

🎯 Goal: Build a VHDL package named Math_Operations that contains a function to add two integers and a procedure to multiply two integers and return the result via an output parameter.

📋 What You'll Learn

Create a package declaration named Math_Operations.

Inside the package, declare a function Add_Two_Numbers that takes two integers and returns their sum.

Inside the package, declare a procedure Multiply_Two_Numbers that takes two integers and returns their product via an output parameter.

Create the package body implementing the function and procedure.

Write a simple test process that calls the function and procedure and prints the results.

💡 Why This Matters

🌍 Real World

Packages in VHDL help organize reusable code like functions and procedures, making designs cleaner and easier to maintain.

💼 Career

Understanding how to create and use packages with functions and procedures is essential for FPGA and ASIC design engineers working on modular and scalable hardware designs.

1

Create the package declaration

Write a VHDL package declaration named Math_Operations that declares a function Add_Two_Numbers with two integer inputs a and b returning an integer, and a procedure Multiply_Two_Numbers with two integer inputs a, b and one integer output result.

VHDL

package Math_Operations is
    -- Declare function Add_Two_Numbers
    -- Declare procedure Multiply_Two_Numbers
end package Math_Operations;

Hint

Start by writing package Math_Operations is and declare the function and procedure signatures inside.

2

Create the package body with implementations

Write the package body for Math_Operations implementing the function Add_Two_Numbers to return the sum of a and b, and the procedure Multiply_Two_Numbers to set result to the product of a and b.

VHDL

package Math_Operations is
    function Add_Two_Numbers(a : integer; b : integer) return integer;
    procedure Multiply_Two_Numbers(a : integer; b : integer; result : out integer);
end package Math_Operations;

package body Math_Operations is
    -- Implement Add_Two_Numbers function
    -- Implement Multiply_Two_Numbers procedure
end package body Math_Operations;

Hint

Use return a + b; inside the function and assign result := a * b; inside the procedure.

3

Write a test process to use the package

Write a VHDL architecture named Test_Arch with a process that declares integer signals sum_result and mul_result. Call the function Add_Two_Numbers with inputs 5 and 7 and assign to sum_result. Call the procedure Multiply_Two_Numbers with inputs 3 and 4 and output to mul_result.

VHDL

library ieee;
use ieee.std_logic_1164.all;
use work.Math_Operations.all;

entity Test_Entity is
end Test_Entity;

architecture Test_Arch of Test_Entity is
    signal sum_result : integer := 0;
    signal mul_result : integer := 0;
begin
    process
    begin
        -- Call Add_Two_Numbers with 5 and 7 and assign to sum_result
        -- Call Multiply_Two_Numbers with 3 and 4 and output to mul_result
        wait;
    end process;
end Test_Arch;

Hint

Use sum_result <= Add_Two_Numbers(5, 7); and Multiply_Two_Numbers(3, 4, mul_result); inside the process.

4

Print the results

Add lines inside the process to print the values of sum_result and mul_result using report statements with messages: "Sum is: " and "Product is: ".

VHDL

library ieee;
use ieee.std_logic_1164.all;
use work.Math_Operations.all;

entity Test_Entity is
end Test_Entity;

architecture Test_Arch of Test_Entity is
    signal sum_result : integer := 0;
    signal mul_result : integer := 0;
begin
    process
    begin
        sum_result <= Add_Two_Numbers(5, 7);
        Multiply_Two_Numbers(3, 4, mul_result);
        -- Print sum_result and mul_result using report statements
        wait;
    end process;
end Test_Arch;

Hint

Use report "Sum is: " & integer'image(sum_result); and report "Product is: " & integer'image(mul_result); to print the values.

Practice

1. What is the main difference between a function and a procedure in a VHDL package?

easy

A. A procedure returns a value, while a function does not.

B. A function returns a value, while a procedure does not.

C. Both return values but differ in syntax.

D. Neither returns a value; both perform actions only.

Solution

1. Step 1: Understand function behavior

   A function in VHDL always returns a value and can be used in expressions.

2. Step 2: Understand procedure behavior

   A procedure performs actions but does not return a value directly.

3. Final Answer:

   A function returns a value, while a procedure does not. -> Option B

4. Quick Check:

   Function returns value = B [OK]

Hint: Functions return values; procedures do not [OK]

Common Mistakes:

• Thinking procedures return values
• Confusing syntax differences with return behavior
• Assuming both can be used in expressions

2. Which of the following is the correct way to declare a function inside a VHDL package?

easy

A. function add(a : integer; b : integer) return integer;

B. procedure add(a : integer; b : integer) return integer;

C. function add(a, b : integer);

D. procedure add(a : integer; b : integer);

Solution

1. Step 1: Check function declaration syntax

   A function must specify parameters with types and a return type using return.

2. Step 2: Validate options

   function add(a : integer; b : integer) return integer; correctly declares a function with parameters and return type. Options A and B are procedures, and C misses return type.

3. Final Answer:

   function add(a : integer; b : integer) return integer; -> Option A

4. Quick Check:

   Function declaration needs return type = D [OK]

Hint: Functions must declare return type explicitly [OK]

Common Mistakes:

• Using 'procedure' keyword for functions
• Omitting return type in function declaration
• Adding return type to procedures

3. Given the following package body code, what is the output of calling result := math_pkg.multiply(3, 4);?

package body math_pkg is
  function multiply(x : integer; y : integer) return integer is
  begin
    return x * y;
  end multiply;
end math_pkg;

medium

A. 12

B. 7

C. Error: function not found

D. 0

Solution

1. Step 1: Understand function multiply

   The function returns the product of x and y, so multiply(3,4) returns 3*4 = 12.

2. Step 2: Confirm no errors

   The function is correctly declared and defined in the package body, so no errors occur.

3. Final Answer:

   12 -> Option A

4. Quick Check:

   3 * 4 = 12 [OK]

Hint: Multiply returns product of inputs [OK]

Common Mistakes:

• Confusing addition with multiplication
• Assuming function is undefined
• Expecting procedure behavior

4. Identify the error in this package declaration:

package util_pkg is
  procedure display_message(msg : in string);
  function get_length(msg : string) return integer;
end util_pkg;

medium

A. Package missing body keyword.

B. Procedure missing return type.

C. Function parameter missing mode (in/out).

D. Procedure parameter mode is incorrect.

Solution

1. Step 1: Check procedure parameter modes

   Procedure parameters must specify mode (in, out, inout). The procedure parameter msg correctly uses in.

2. Step 2: Check function parameter modes

   Function parameters must also specify mode explicitly. The function parameter msg : string lacks mode, which is required.

3. Final Answer:

   Function parameter missing mode (in/out). -> Option C

4. Quick Check:

   Function parameters need mode = A [OK]

Hint: Always specify parameter mode in functions and procedures [OK]

Common Mistakes:

• Omitting parameter mode in functions
• Confusing package and package body syntax
• Thinking procedures need return types

5. You want to create a VHDL package that includes a function to calculate the factorial of a number and a procedure to print the result. Which of the following is the best approach?

hard

A. Define factorial as a procedure modifying a global variable; print as a function returning string.

B. Define both factorial and print as functions returning integer.

C. Define factorial as a procedure returning integer; print as a procedure with no parameters.

D. Define the factorial as a function returning integer; define the print as a procedure taking integer input.

Solution

1. Step 1: Choose correct return types

   Factorial calculation returns a value, so it should be a function returning integer.

2. Step 2: Choose correct procedure for printing

   Printing is an action without return value, so it should be a procedure taking the integer to print.

3. Final Answer:

   Define the factorial as a function returning integer; define the print as a procedure taking integer input. -> Option D

4. Quick Check:

   Function returns value; procedure performs action = C [OK]

Hint: Use function for calculations, procedure for actions [OK]

Common Mistakes:

• Using procedures to return values
• Defining print as function with return value
• Not passing parameters to procedures