Types of Addressing Modes

Types of addressing modes are an important topic in computer architecture. Questions are always asked in the GATE (CS/IT) and UGC NET(CS) Exam. Students who are preparing for the GATE exam are requested to read this tutorial completely.

After reading this tutorial, they will able to solve the questions asked in GATE.

Addressing mode is an important topic for students preparing for GATE(CS/IT) and UGC NET(CS/IT). In this addressing mode tutorial, we will discuss different types of addressing modes in computer architecture.

In this tutorial, we will also learn about different types of addressing modes in computer organization. In this addressing modes tutorial, we are going to cover the following topics in detail.

Let us start with the introduction of the addressing mode.

Note Point – Students are advised to study Types of Registers in Computer Architecture Tutorial before studying this addressing mode tutorial for better understanding.

What is Addressing Mode?

• Addressing mode is a way of specifying the Operand of an  instruction.
• To understand the concept of addressing modes in computer architecture, we need to know its background detail. When an instruction is fetched from memory, then it is stored in an Instruction Register.
• Instruction Register is connected to a decoder. The decoder decodes this instruction. The instruction format has mainly three parts, as shown in the following figure.

What are Different Types of Addressing Mode ?

Different Types of Addressing Modes are Listed as follow –

• Immediate Mode.
• Direct Addressing Mode.
• Indirect Addressing Mode.
• Register Direct Mode.
• Register Indirect Mode.
• Displacement Addressing Mode.
• Relative Addressing Mode.
• Base Register Addressing Mode.

All computer architectures provide more than one of these addressing modes. The question arises as to how the control unit can determine which addressing mode is being used in a particular instruction. Several approaches are used.

Often, the different OPCODE will use different addressing modes.

The value of the mode field determines which addressing mode is to be used. What is the interpretation of effective address?

In a system without virtual memory, the effective address will be either the main memory address or a register.

In a virtual memory system, the effective address is a virtual address or a register. The actual mapping to a physical address is a function of the paging mechanism and is invisible to the programmer.

There are the following types of addressing modes.

(1) Immediate Addressing Mode

The simplest form of addressing is immediate addressing mode, in which the operand is itself present in the instruction:

The address field part of the instruction is nothing but the operand itself. Immediate addressing mode can be used to define and use constants or set initial values of variables. Immediate addressing mode example is explained here.

Example: Consider the instruction LOAD 5. So in this instruction, five is itself is an operand.

The advantage of the immediate addressing mode is that no memory reference other than the instruction fetch is required to obtain the operand.

The disadvantage of immediate addressing mode is that the size of the number is restricted to the size of the address field, which, in most instruction sets, is small compared with the word length.

(2) Direct Addressing Mode

When we say direct address mode, it is understood that operand is stored in the memory, not in the register. A very simple form of addressing is direct addressing, in which the address field contains the effective address of the operand.

This mode is also known as the absolute addressing mode. It requires only one memory reference and no special calculation.

With direct addressing, the length of the address field is usually less than the word length, thus limiting the address range.

One solution is to have the address field refer to the address of a word in memory, which contains a full-length address of the operand. This is known as the indirect addressing mode.

Register direct addressing is similar to direct addressing. The only difference is that the address field refers to a register rather than the main memory address.

It means the operand is stored in the register as specified by the address field of the instruction. This register is inside the CPU.

The advantages of registers direct addressing are that only a small address field is needed in the instruction, and no memory reference is required.

The disadvantage of registers direct addressing is that the address space is very limited.

Register indirect addressing is similar to indirect addressing, except that the address meaning field refers to a register instead of a memory location. It requires only one memory reference and no special calculation.

Register indirect addressing mode uses one less memory reference than indirect addressing. Because the first information is available in a register, which is nothing but a memory address, and at that memory address, the operand is stored.

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A very powerful mode of addressing combines the capabilities of direct addressing and register indirect addressing, which is broadly categorized as displacement addressing:

EA = A + (R)

Displacement addressing requires that the instruction have two address fields, at least one of which is explicit. The value contained in one address field (value = A) is used directly.

The other address field, or an implicit reference based on opcode, refers to a register whose contents are added to A to produce the effective address.

For relative addressing, the implicitly referenced register is the program counter (PC). That is, the current instruction address is added to the address field to produce the EA. Thus, the effective address is a displacement relative to the address of the instruction.

6.2 Base-Register Addressing Mode

The reference register contains a memory address, and the address field contains a displacement from that address. The register reference may be explicit or implicit.

In some implementations, a single segment/base register is employed and is used implicitly. In others, the programmer may choose a register to hold the base address of a segment, and the instruction must reference it explicitly.