Addressing Modes: Definition, Types & Examples

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  • 0:03 What Is an Addressing Mode?
  • 1:18 Types of Addressing Modes
  • 4:52 Lesson Summary
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Lesson Transcript
Instructor: Allesha Fogle

Allesha has graduate degrees in software engineering and computer science. She has over 15 years' experience in software and application development.

In this lesson, we define what an addressing mode is in computer architecture and identify several different addressing modes. Additionally, we demonstrate how an operand would be fetched in each of the addressing modes.

What Is an Addressing Mode?

The addressing mode is the method to specify the operand of an instruction. The job of a microprocessor is to execute a set of instructions stored in memory to perform a specific task. Operations require the following:

  1. The operator or opcode which determines what will be done
  2. The operands which define the data to be used in the operation

For example, if we wanted to add the numbers 1 and 2 and get a result, mathematically we would likely write this as 1 + 2. In this case, our operator is (+), or the addition, and our operands are the numbers 1 and 2.

In a microprocessor, the machine needs to be told how to get the operands to perform the operation. The effective address is a term that describes the address of an operand that is stored in memory. There are several methods to designate the effective address of those operands or get them directly from the register. These methods are known as addressing modes.

In this lesson, we're going to define several addressing modes that can be used to indicate how to retrieve operands. We'll also illustrate examples of how each addressing mode is used and point out some characteristics of these addressing modes.

Types of Addressing Modes

Let's take a look at the different types of addressing modes, one at a time now.

1. Immediate

With immediate addressing mode, the actual data to be used as the operand is included in the instruction itself. Let's say we want to store operand 1 into a register and then add operand 2. With immediate addressing mode, the data values 1 and 2 would be part of the instruction itself as shown below.

Figure 1. Immediate
Immediate addressing mode

This would be a relatively fast option since there is no memory access required to get the operand. The operand, however, is limited to the size of one word.

2. Direct Addressing

When using direct addressing mode, the address of the operand is specified in the instruction. The processor will retrieve the data directly from the address specified in the instruction. In this figure, the example shows how the instruction tells the processor where to get the data from in memory. The variable addr_of_2 is a pointer to the effective address of the operand.

Figure 2. Direct
direct addressing

There are no calculations required to retrieve the operand since the effective address (the address of the operand) is addressed directly. Like immediate addressing mode, the operand is limited to the size of 1 word (8 or 16 bits).

3. Register Addressing

Register addressing mode indicates the operand data is stored in the register itself, so the instruction contains the address of the register. The data would be retrieved from the register. Here's how this would work:

Figure 3. Register
Register addressing mode

Retrieving data from the register is fast and the instructions are shorter because no memory is involved.

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