---
title: Transfer of control - Procedure Call
description: Call procedure note. Author - Dzaqwan Amir
tags: assignment
---

# Transfer of Control

There are several types of transfer-of-control

- Branch
- Skip
- Call

## Call procedure

- Is the most important innovation in the development of programming language
- Two basic instruction in call procedure:
- `CALL`
- `RETURN`

### How it works in program


- In this figure, we can see there are 3 separate program parts (code blocks)
- Main
- Procedure 1 (Proc 1)
- Procedure 2 (Proc 2)

#### Program flow

1. At some point in main program, Proc 1 is called to be executed (`CALL Proc 1`). The program then jump to Proc 1
2. Then in Proc 1, Proc 2 is called. The program jump to Proc 2 (`CALL Proc 2`)
3. At the end of Proc 2, it hits instruction `RETURN`
4. The program then come back to where it left off when it called Proc 2, which is at address 4600 in Proc 1.
5. Program continues executing instruction in 4601 -- 4649, and met another `CALL Proc 2` instruction.
6. Proc 2 is executed again and return to where it was called, 4650
7. Proc 1 executed until it hit `RETURN` and go back to the main program in address 4100
8. The main program executed sequentially until the end.

### Stack

- Because we call a procedure from various places, the processor must somehow **save the return address** so that the return can take place appropriately
- It got complicated when there are multiple nested procedure call, where a procedure call another procedure
- One way to neatly save the correct return address is by using stack


- Every time a procedure is called, the address where the `CALL` is executed is saved **on top** of a stack.
- When a procedure reached `RETURN`, the program take the address on top of the stack to continue where it left off

### Benefit of call procedure

- **Economy**: A procedure allows the same piece code to be used many times
- **Modularity**: Large programming task can be divided into smaller units