Why subprograms enable code reuse in VHDL - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a subprogram in VHDL.

VHDL

procedure [1] is
begin
  null;
end procedure;

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

Amy_subprogram

Bentity

Carchitecture

Dsignal

Attempts:

3 left

💡 Hint

Common Mistakes

Using VHDL keywords like 'architecture' or 'entity' as subprogram names.

Forgetting to name the subprogram after 'procedure'.

The name of the subprogram is declared after the keyword procedure. Here, my_subprogram is the correct name.

2fill in blank

medium

Complete the code to call a subprogram named 'compute' inside a process.

VHDL

process
begin
  [1];
  wait;
end process;

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

Acompute

Bcompute()

Ccall compute

Drun compute

Attempts:

3 left

💡 Hint

Common Mistakes

Calling the subprogram without parentheses.

Using non-VHDL keywords like 'call' or 'run'.

In VHDL, to call a procedure, you use its name followed by parentheses (). So compute() is correct.

3fill in blank

hard

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

VHDL

function add_numbers(a : integer; b : integer) [1] integer is
begin
  return a + b;
end function;

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

Areturns

Breturns integer

Creturn

Dreturning

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'returns' instead of 'return'.

Including the return type twice like 'returns integer' as a single token.

The correct keyword to specify the return type of a function in VHDL is return.

4fill in blank

hard

Fill both blanks to complete the subprogram call with parameters.

VHDL

process
begin
  [1](5, [2]);
  wait;
end process;

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

Acompute_sum

B10

C20

Dcalculate

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong subprogram names like 'calculate'.

Passing incorrect parameter values.

The subprogram name is compute_sum and the second parameter passed is 20.

5fill in blank

hard

Fill all three blanks to create a function that returns the maximum of two integers.

VHDL

function max_val(a : integer; b : integer) [1] integer is
begin
  if a [2] b then
    return a;
  else
    return [3];
  end if;
end function;

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

Areturn

B>

Cb

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'returns' instead of 'return' in the function header.

Using the wrong comparison operator.

Returning the wrong variable.

The function uses return to specify the return type, compares a > b, and returns b if the condition is false.

Practice

1. Why do subprograms enable code reuse in VHDL?

easy

A. Because you can write the code once and call it multiple times

B. Because they automatically optimize the hardware

C. Because they replace the need for signals

D. Because they only work inside processes

Solution

1. Step 1: Understand what code reuse means

   Code reuse means writing a piece of code once and using it many times without rewriting it.

2. Step 2: Relate subprograms to code reuse

   Subprograms like procedures and functions allow you to write code once and call it wherever needed.

3. Final Answer:

   Because you can write the code once and call it multiple times -> Option A

4. Quick Check:

   Code reuse = write once, use many times [OK]

Hint: Subprograms let you reuse code by calling it repeatedly [OK]

Common Mistakes:

• Thinking subprograms optimize hardware automatically
• Confusing subprograms with signals
• Believing subprograms only work inside processes

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

easy

A. procedure add(a : integer; b : integer) is return integer begin return a + b; end procedure;

B. procedure add(a : integer; b : integer) return integer is begin return a + b; end procedure;

C. function add(a, b : integer) is return integer begin return a + b; end function;

D. function add(a : integer; b : integer) return integer is begin return a + b; end function;

Solution

1. Step 1: Identify correct function syntax

   In VHDL, a function is declared with 'function name(parameters) return type is' followed by the body.

2. Step 2: Check each option

   function add(a : integer; b : integer) return integer is begin return a + b; end function; matches correct syntax: parameters with types, return type, and body with return statement.

3. Final Answer:

   function add(a : integer; b : integer) return integer is begin return a + b; end function; -> Option D

4. Quick Check:

   Function syntax = function add(a : integer; b : integer) return integer is begin return a + b; end function; [OK]

Hint: Functions use 'return' keyword and specify return type [OK]

Common Mistakes:

• Using 'procedure' instead of 'function' for returning values
• Omitting return type in function declaration
• Incorrect parameter syntax

3. Given the following VHDL code, what is the output of the function call multiply(3, 4)?

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

medium

A. 7

B. 12

C. 34

D. Error: function does not return a value

Solution

1. Step 1: Understand the function logic

   The function multiply returns the product of x and y.

2. Step 2: Calculate multiply(3, 4)

   3 multiplied by 4 equals 12.

3. Final Answer:

   12 -> Option B

4. Quick Check:

   3 * 4 = 12 [OK]

Hint: Multiply inputs to get output from multiply function [OK]

Common Mistakes:

• Adding instead of multiplying
• Concatenating numbers as strings
• Assuming function returns sum

4. Identify the error in this VHDL procedure that aims to increment a value by 1:

procedure increment(signal val : in integer) is
begin
  val := val + 1;
end procedure;

medium

A. Cannot assign to an 'in' mode parameter

B. Procedure must return a value

C. Signal keyword is not allowed in procedure parameters

D. Missing 'return' statement

Solution

1. Step 1: Check parameter mode

   The parameter 'val' is declared as 'in', which means it is read-only inside the procedure.

2. Step 2: Identify assignment error

   Assigning 'val := val + 1;' tries to modify a read-only parameter, causing an error.

3. Final Answer:

   Cannot assign to an 'in' mode parameter -> Option A

4. Quick Check:

   'in' parameters are read-only [OK]

Hint: 'in' parameters cannot be changed inside procedures [OK]

Common Mistakes:

• Thinking procedures must return values
• Using 'signal' keyword incorrectly in parameters
• Expecting 'return' in procedures

5. You want to create a reusable subprogram in VHDL that calculates the average of two integers and returns the result. Which approach correctly uses subprograms to enable code reuse and proper functionality?

hard

A. Write a function that takes no parameters and returns a fixed average value

B. Write a procedure that takes two integers as input and modifies a global signal with the average

C. Write a function that takes two integers as input and returns their average as an integer

D. Write a procedure that returns the average value using a return statement

Solution

1. Step 1: Determine subprogram type for returning a value

   Functions return values and are suitable for calculations like average.

2. Step 2: Check options for correct usage

   Write a function that takes two integers as input and returns their average as an integer uses a function with inputs and returns the average, enabling reuse and correctness.

3. Final Answer:

   Write a function that takes two integers as input and returns their average as an integer -> Option C

4. Quick Check:

   Functions return values; procedures do not [OK]

Hint: Use functions to return values for reusable calculations [OK]

Common Mistakes:

• Using procedures to return values
• Not passing parameters to subprograms
• Using global signals instead of parameters