What is subroutine instruction?

Subroutine instruction is defined as the instructions used in the programming language in a sequence form saved in memory. They are used to doing a specific task. Subroutine instruction is called a unit that instructs to perform some needed task. There are many programming languages in which subroutine instructions are used but have a different identity or name, such as method, function, subprogram, routine, etc. Subroutine or sub-program is designed or coded as they can be called multiple times while executing the program.

This figure shows how subroutine works in the main program.
Working of subroutine

Role of subroutine instruction

In programming or program execution, the codes are written in sequence. Many types of instructions are used repeatedly because these subroutine instructions are coded once. When needed again in the program, the program gets back to the subroutine. This forms a nest. Subroutine instruction is a time-saving process.

Meaning of nest or nesting and its advantages

Nest or nesting refers to the condition in which one program acquires another program code. This phenomenon is called a nest, and this process is known as nesting. For example, in a particular software program, the main program includes subroutine instructions; then, it is called nesting.

Advantages of nesting-

  • It decreases the code length.
  • Ease for programmers
  • Simple to code and debug

Ways to call subroutine

There are several ways to call subroutine instruction. They differ in the different programming languages. For this, several codes are there.

If we talk about early computers, their programs are loaded in memory by punched paper tape. In that case, the subroutine instruction or subroutine call is provided on a separate piece of paper that will slice before or after the main program. This subroutine tape would be used in many programs, called subroutine call.

Subroutine instructions or calls include many codes used at particular addresses or require an address. These collective codes are known as subroutine libraries or the collection of codes, tapes, etc.

In some other types of computers and software, the code indirect jump instruction or return address is used return address calls the location again and again, so programmers use indirect jump.

Moving further to the advanced method of calling subroutine instruction is jump to subroutine instruction. This code is a combined form of the return address and jumps call.

Use of a stack

Stack is somehow related to an array, or it can be described as a list of function calls in systematic form. The address of the next instruction is pushed onto the stack when the function is called. Stack works on the principle-like process of in and out as if the function was removed, then the address also popped out from the stack and, like this, continues the execution. The stack is defined as the sequenced data, which is in a linear sequence. Because of this, the topmost data is used first as a sequence.

Sometimes subroutine instruction or code needs more register space to store additional codes or call other subroutines. They save these registers in the register stack, which is a private location.

As the subroutines need another register or stack space, the most effective method for subroutine calls is to use the code called stack that performs subroutine calls and returns. It is an instruction to call the subroutine and return through the stack call.

It shows the working of the stack and how the stack is accessible.
CC-BY-SA-3.0 | Image credits: https://commons.wikimedia.org | R.Koot

Overall process of a program having subroutine instruction

It is necessary to know the quick summary of the parameter used in the whole execution of the program.

Main program

The main program is the most important part of the whole execution. It contains the information of the exact inputs according to which we get output. The main program is defined as the main code that has the main information or instruction of the program.


The term call is an instruction that addresses some other location code or file to be executed within the program.


This command or instruction allows the flow of the program to return to the main program.


Output is the desired information that a user or client wants after the proper execution of the program by software.

The overall process starts from the main program. First, we start to code the main program with sufficient input information. If there is a need for some predefined code or instruction written in a subroutine, then the call function is to address the desired subroutine. Sometimes subroutine itself contains more subroutines within it. In that case, the call function calls that particular subroutine for the desired operation. After executing subroutine instruction, the return function returns the execution to the main program and then continues to execute. After successfully executing the whole program parameter or code, the user or client gets his desired output.

This figure shows the whole process of how the program starts, calls a subroutine, executes that, and returns to the remaining main program.
Program process

Advantages of having subroutines

Advantages of having subroutine instructions are the following:

  • Reduce the coding load or dissociate the hard code or program into a simple form.
  • Rid of writing the same code again and again.
  • Easy to trace the steps.

Disadvantages of having subroutines

Disadvantages of having subroutines are the following:

  • Increases the use of call and return functions.

Common Mistakes

It is to be remembered the difference between stack and register.

Context and Applications

This is an important concept for coding students. This concept is mainly used in

  • Bachelor of Technology in Computer Science
  • Bachelor of Technology in Electronics
  • Bachelor of Computer Application
  • Loop
  • Array
  • Structure

Practice Problems

Q1. Which portion is editable in the stack?

  1. Bottom
  2. Top
  3. Middle
  4. None of these

Correct option- (b)

Explanation- Stack is like a box having books, one on the other, so it is only accessible from the top.

Q2. Is it required to have software that is compatible with the programming function?

  1. Yes
  2. No
  3. Doesn’t matter
  4. Can’t say

Correct option- (a)

Explanation – It is required to have compatible software which is familiar with the usable functions.

Q3. What is the benefit of a subroutine?

  1. Time-saving process
  2. Ease for the coder
  3. Less complexity
  4. All of the above

Correct option- (d)

Explanation- A subroutine is an advantageous process because of its benefits.

Q4. When do we use subroutine?

  1. At the end of the program
  2. Between the program
  3. Both
  4. None of these

Correct option- (b)

Explanation- A subroutine is used when writing the program to call some other instructions.

Q5. Which command is not a part of adding subroutine?

  1. Jump
  2. Return
  3. Call
  4. Back

Correct option- (d)

Explanation- The back term is not a command in terms of adding subroutine.

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