Function and procedure in packages in VHDL - Step-by-Step Execution

Concept Flow - Function and procedure in packages

Define Package

↓

Declare Function

↓

Declare Procedure

↓

Package Body

↓

Implement Function

↓

Implement Procedure

↓

Use Package in Design

↓

Call Function or Procedure

This flow shows how to define a package with functions and procedures, implement them in the package body, and then use them in your VHDL design.

Execution Sample

VHDL

package MyPkg is
  function add(a, b : integer) return integer;
  procedure show_sum(a, b : integer);
end MyPkg;

This code declares a package with a function 'add' and a procedure 'show_sum'.

Execution Table

Step	Action	Evaluation	Result
1	Define package MyPkg with function and procedure declarations	No evaluation	Package interface ready
2	Implement function add in package body	add(2, 3)	Returns 5
3	Implement procedure show_sum in package body	show_sum(2, 3)	Outputs sum message
4	Use package in design and call add(2,3)	Call function	Value 5 returned
5	Call procedure show_sum(2,3)	Call procedure	Message 'Sum is 5' displayed
6	End of usage	No further calls	Execution stops

💡 No more calls to function or procedure, execution ends

Variable Tracker

Variable	Start	After Step 2	After Step 4	Final
a	undefined	2	2	2
b	undefined	3	3	3
result	undefined	5	5	5

Key Moments - 2 Insights

Why do we declare functions and procedures in the package before implementing them?

The package declaration (see Step 1 in execution_table) tells VHDL what functions and procedures exist so other parts of the design can use them before their actual code is written in the package body.

What is the difference between a function and a procedure in a package?

A function returns a value (like add returns 5 in Step 2), while a procedure performs actions but does not return a value (like show_sum outputs a message in Step 3).

Visual Quiz - 3 Questions

Look at the execution_table, what does the function 'add' return when called with (2,3)?

A 2

B 6

C 5

D No value

💡 Hint

Check Step 2 and Step 4 in the execution_table where add(2,3) returns 5

At which step does the procedure 'show_sum' output the sum message?

A Step 2

B Step 3

C Step 4

D Step 5

💡 Hint

Look at Step 3 in the execution_table where show_sum(2,3) outputs the message

If the function 'add' was not declared in the package, what would happen when calling it in the design?

A Compilation error due to missing declaration

B It would work normally

C It would return zero

D It would call the procedure instead

💡 Hint

Refer to Step 1 in execution_table: declaration is needed before use

Concept Snapshot

VHDL packages group reusable code.
Declare functions and procedures in the package.
Implement them in the package body.
Functions return values; procedures perform actions.
Use packages by including them in your design.
Call functions/procedures by name with parameters.

Full Transcript

In VHDL, functions and procedures are declared inside packages to organize reusable code. The package declaration lists the functions and procedures available. Their actual code is written in the package body. Functions return values, while procedures perform actions without returning values. After defining a package, you can use it in your design by calling these functions and procedures. This visual trace showed defining a package with a function 'add' and a procedure 'show_sum', implementing them, and then calling them with values 2 and 3. The function returned 5, and the procedure displayed a message with the sum. Declaring functions and procedures before use is necessary to avoid compilation errors.

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