Function and procedure in packages in VHDL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a function call from a package

What is the output of the following VHDL code snippet when the function add_one is called with input 5?

VHDL

package math_pkg is
  function add_one(x : integer) return integer;
end package;

package body math_pkg is
  function add_one(x : integer) return integer is
  begin
    return x + 1;
  end function;
end package body;

architecture behavior of test is
  signal result : integer;
begin
  process
  begin
    result <= math_pkg.add_one(5);
    wait;
  end process;
end behavior;

A result = 6

B result = 5

C result = 0

D Compilation error due to missing package use clause

💡 Hint

Remember that the function adds 1 to the input value.

The function add_one defined in the package adds 1 to the input integer. Calling it with 5 returns 6, which is assigned to result.

❓ Predict Output

intermediate

Procedure call effect on signal in package

Given the following VHDL code, what is the value of signal out_val after the procedure set_value is called with input 10?

VHDL

package util_pkg is
  procedure set_value(signal s : out integer; val : integer);
end package;

package body util_pkg is
  procedure set_value(signal s : out integer; val : integer) is
  begin
    s <= val;
  end procedure;
end package body;

architecture behavior of test is
  signal out_val : integer := 0;
begin
  process
  begin
    util_pkg.set_value(out_val, 10);
    wait;
  end process;
end behavior;

A Compilation error due to procedure call syntax

B out_val = 0

C out_val = undefined

D out_val = 10

💡 Hint

Procedures can assign values to signals passed as out parameters.

The procedure set_value assigns the input value to the signal passed as an out parameter. Calling it with 10 sets out_val to 10.

🧠 Conceptual

advanced

Error caused by missing package body for function

What error will occur if a function declared in a VHDL package is used in design but its body is missing in the package body?

A Linker error: unresolved external symbol for the function

B Syntax error during compilation

C Runtime error: function returns default value

D No error; function returns zero by default

💡 Hint

Think about what happens when the function implementation is missing.

If the function is declared but not defined in the package body, the compiler accepts the declaration but the linker cannot find the function implementation, causing a linker error.

❓ Predict Output

advanced

Output of recursive function in package

What is the output of the following recursive function factorial defined in a package when called with input 4?

VHDL

package math_pkg is
  function factorial(n : integer) return integer;
end package;

package body math_pkg is
  function factorial(n : integer) return integer is
  begin
    if n = 0 then
      return 1;
    else
      return n * factorial(n - 1);
    end if;
  end function;
end package body;

architecture behavior of test is
  signal result : integer;
begin
  process
  begin
    result <= math_pkg.factorial(4);
    wait;
  end process;
end behavior;

A Runtime error due to infinite recursion

B result = 24

C result = 0

D result = 10

💡 Hint

Factorial of 4 is 4*3*2*1.

The recursive function calculates factorial correctly. factorial(4) = 4*3*2*1 = 24.

🔧 Debug

expert

Identify the error in procedure parameter passing

What error will the following VHDL code produce when compiling, considering the procedure parameter mode?

VHDL

package util_pkg is
  procedure update_val(signal s : in integer; val : integer);
end package;

package body util_pkg is
  procedure update_val(signal s : in integer; val : integer) is
  begin
    s <= val;
  end procedure;
end package body;

architecture behavior of test is
  signal data : integer := 0;
begin
  process
  begin
    util_pkg.update_val(data, 5);
    wait;
  end process;
end behavior;

A No error; signal data updated to 5

B Runtime error: signal assignment ignored

C Compilation error: cannot assign to signal with mode 'in'

D Syntax error: missing 'out' keyword in procedure declaration

💡 Hint

Signals declared as 'in' mode cannot be assigned inside procedures.

The procedure parameter s is declared as in mode, which means it is read-only. Assigning to it causes a compilation error.

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