Procedure declaration and usage in VHDL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a procedure named 'display' that takes an integer input.

VHDL

procedure display([1] : in integer) is
begin
  null;
end procedure;

Drag options to blanks, or click blank then click option'

Anum

Binput_val

Cvalue

Ddata

Attempts:

3 left

💡 Hint

Common Mistakes

Leaving the parameter name blank

Using invalid characters in the parameter name

The procedure parameter name can be any valid identifier. Here, 'value' is used as the parameter name.

2fill in blank

medium

Complete the code to call the procedure 'display' with the integer 10.

VHDL

begin
  [1](10);
end;

Drag options to blanks, or click blank then click option'

Adisplay

Bprint

Cshow

Doutput

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different procedure name

Missing parentheses when calling the procedure

To call the procedure named 'display', use its exact name followed by parentheses with the argument.

3fill in blank

hard

Fix the error in the procedure declaration by completing the missing keyword.

VHDL

procedure display(value : in integer) [1]
begin
  null;
end display;

Drag options to blanks, or click blank then click option'

Adeclare

Bas

Cbegin

Dis

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'begin' instead of 'is' after the parameter list

Omitting the keyword entirely

In VHDL, the procedure declaration uses the keyword 'is' before the body begins.

4fill in blank

hard

Fill both blanks to declare a procedure 'increment' that takes an integer input and increments it by 1.

VHDL

procedure increment([1] : inout integer) [2]
begin
  [1] := [1] + 1;
end procedure;

Drag options to blanks, or click blank then click option'

Anum

Bis

Cas

Dvalue

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'in' instead of 'inout' for the parameter mode

Using 'as' instead of 'is' after the parameter list

The parameter name 'num' is used, and the keyword 'is' starts the procedure body.

5fill in blank

hard

Fill all three blanks to declare and call a procedure 'multiply' that multiplies an integer by 2.

VHDL

procedure multiply([1] : inout integer) [2]
begin
  [1] := [1] * 2;
end procedure;

begin
  [1] := 5;
  multiply([1]);
end;

Drag options to blanks, or click blank then click option'

Aval

Bis

Dnum

Attempts:

3 left

💡 Hint

Common Mistakes

Using different names for parameter and variable

Omitting the 'is' keyword

Not passing the variable correctly to the procedure

The parameter and variable name is 'num', the keyword 'is' starts the procedure body, and 'num' is used in the call and assignment.

Practice

1. What is the main purpose of a procedure in VHDL?

easy

A. To group a set of instructions for reuse

B. To declare a new signal

C. To define a new entity

D. To assign values to constants

Solution

1. Step 1: Understand what a procedure does

   A procedure groups instructions to perform a specific task that can be reused multiple times in the code.

2. Step 2: Compare with other options

   Declaring signals, entities, or assigning constants are different VHDL tasks, not the main purpose of procedures.

3. Final Answer:

   To group a set of instructions for reuse -> Option A

4. Quick Check:

   Procedure = reusable instruction group [OK]

Hint: Procedures bundle instructions for reuse [OK]

Common Mistakes:

• Confusing procedures with signal declarations
• Thinking procedures define entities
• Assuming procedures assign constants

2. Which of the following is the correct way to declare a procedure in VHDL?

easy

A. procedure my_proc() begin
-- code
end procedure;

B. procedure my_proc is
begin
-- code
end procedure;

C. procedure my_proc is
-- code
end;

D. procedure my_proc is
begin
-- code
end;

Solution

1. Step 1: Recall VHDL procedure syntax

   A procedure declaration uses the syntax: procedure name is begin ... end procedure;.

2. Step 2: Check each option

   procedure my_proc is begin -- code end procedure; matches the correct syntax with is, begin, and end procedure;. Options B, C, and D have syntax errors or missing keywords.

3. Final Answer:

   procedure my_proc is begin -- code end procedure; -> Option B

4. Quick Check:

   Procedure syntax = procedure my_proc is begin -- code end procedure; [OK]

Hint: Procedure ends with 'end procedure;' [OK]

Common Mistakes:

• Omitting 'procedure' keyword at the end
• Using parentheses after procedure name
• Ending with just 'end;' instead of 'end procedure;'

3. What will be the output of the following VHDL procedure call?

procedure add_one(signal x : inout integer) is
begin
  x <= x + 1;
end procedure;

signal val : integer := 5;
begin
  add_one(val);
  -- What is val now?
end;

medium

A. 5

B. 7

C. 6

D. Error: procedure call syntax

Solution

1. Step 1: Understand procedure behavior

   The procedure add_one takes a signal x as inout and increments it by 1.

2. Step 2: Apply procedure to signal val

   Initially, val is 5. After calling add_one(val);, val becomes 5 + 1 = 6.

3. Final Answer:

   6 -> Option C

4. Quick Check:

   5 + 1 = 6 [OK]

Hint: Inout parameter changes value inside procedure [OK]

Common Mistakes:

• Assuming input parameters cannot change
• Confusing procedure call syntax
• Forgetting procedure modifies signal

4. Identify the error in this procedure declaration:

procedure multiply_by_two(x : in integer; y : out integer) is
begin
  y := x * 2;
end;

medium

A. Procedure must end with 'end procedure;' not just 'end;'

B. Procedure body cannot assign values to 'out' parameters

C. Parameters cannot have modes like 'in' or 'out'

D. Missing 'procedure' keyword before 'multiply_by_two'

Solution

1. Step 1: Check procedure ending syntax

   In VHDL, a procedure must end with end procedure; not just end;.

2. Step 2: Verify other parts

   The procedure keyword is present, parameter modes are allowed, and assigning to 'out' parameters is valid.

3. Final Answer:

   Procedure must end with 'end procedure;' not just 'end;' -> Option A

4. Quick Check:

   Procedure ends with 'end procedure;' [OK]

Hint: Procedure must end with 'end procedure;' [OK]

Common Mistakes:

• Ending procedure with just 'end;'
• Thinking parameter modes are not allowed
• Confusing procedure and function endings

5. You want to create a procedure that swaps two integer signals a and b. Which of the following is the correct procedure declaration and usage?

hard

A. procedure swap(signal x, y : in integer) is
variable temp : integer;
begin
temp := x;
x := y;
y := temp;
end procedure;

-- Usage: swap(a, b);

B. procedure swap(signal x, y : out integer) is
variable temp : integer;
begin
temp := x;
x := y;
y := temp;
end procedure;

-- Usage: swap(a, b);

C. procedure swap(x, y : integer) is
variable temp : integer;
begin
temp := x;
x := y;
y := temp;
end procedure;

-- Usage: swap(a, b);

D. procedure swap(signal x, y : inout integer) is
variable temp : integer;
begin
temp := x;
x <= y;
y <= temp;
end procedure;

-- Usage: swap(a, b);

Solution

1. Step 1: Determine correct parameter modes for swapping

   To swap values, both parameters must be inout or signal with mode allowing read and write. 'inout' is correct.

2. Step 2: Check procedure declaration and usage

   procedure swap(signal x, y : inout integer) is variable temp : integer; begin temp := x; x <= y; y <= temp; end procedure; -- Usage: swap(a, b); declares parameters as signal x, y : inout integer and uses a temporary variable to swap values correctly. Usage is correct.

3. Final Answer:

   procedure swap(signal x, y : inout integer) is variable temp : integer; begin temp := x; x <= y; y <= temp; end procedure; -- Usage: swap(a, b); -> Option D

4. Quick Check:

   Swap needs inout parameters [OK]

Hint: Use 'inout' mode to modify both parameters [OK]

Common Mistakes:

• Using 'in' mode which is read-only
• Using 'out' mode which cannot read input
• Omitting 'signal' keyword for signals