Riyadh Philanthropic Society For Science
Prince Sultan College For Woman
Dept. of Computer & Information Sciences
CS 251
Introduction to Computer Organization
& Assembly Language
Lecture 18
Addressing Modes

Study from Chapter 10 (10.1,10.2.1, 10.2.2, example 10.4 not included)

Introduction

Addressing Modes
Register Indirect Mode
 Based and Indexed Addressing Modes

Addressing Modes
2

The way an operand is specified is known as its addressing mode.
 Addressing
modes:
 Register mode
 Immediate mode
 Direct mode
 Register Indirect
 Based
 Indexed
 Based Indexed
An operand is a register (ex. INC AX)
An operand is a constant (ex. ADD A, 5)
An operand is a variable(ex. ADD A, 5)
Address memory operands indirectly
Used with two dimensional arrays
Used with one dimensional arrays
Addressing Modes
3

The offset address of the operand is contained in a register.

I.e. The register acts as a pointer to the memory location.
Format:
[register]


The register must be one of the following:
 BX
 SI
The operand’s segment number is contained in DS
 DI
 BP
The operand’s segment number is contained in SS
Addressing Modes
4

Ex. suppose that SI contains 0100h, and the word at 0100h contains 1234h.

MOV AX, [SI]
The CPU:
1. Examines SI and obtains the offset address 100h.
2. Uses the address DS:0100h to obtain the value 1234h.
3. Moves 1234h to AX.



MOV AX, SI
The CPU will move the value of SI, namely 100h, into AX.
Addressing Modes
5
• Ex. Write some code to sum in AX the elements of the 10-element array W defined by
W DW 10,20,30,40,50,60,70,80,90,100
MOV
AX, 0
LEA
SI, W
MOV CX, 10
ADDNOS:
ADD
AX, [SI]
ADD
SI, 2
LOOP ADDNOS
; AX holds sum
; SI points to array W
; CX has number of elements
; sum = sum + element
; move pointer to the next element
; loop until done
Addressing Modes
6

The operand’s offset address is obtained by adding a number called a displacement to the
contents of a register.

Displacement may be:
 The offset address of a variable. (ex. A)
 A constant (positive or negative). (ex. -2)
 The offset address of a variable + or - a constant. (ex. A + 4)

Syntax:
 [register + displacement]
 [displacement + register]
 [register] + displacement
 displacement + [register]
 displacement [register]
Addressing Modes
7

The register must be one of the following:
 BX
The operand’s segment number is contained in DS
 SI
 DI
The operand’s segment number is contained in SS
 BP

The addressing mode is called based if BX (base register) or BP (base pointer) is used.

The addressing mode is called indexed if SI (source index) or DI (destination index) is
used.
Addressing Modes
8

Ex. Suppose W is a word array, and BX contains 4
MOV AX, W[BX]

The displacement is the offset address of variable W

The instruction moves the element at address W + 4 to AX.


(this is the third element in the array)
The instuction could also have been written in any of these forms:
 MOV AX, [W+BX]
 MOV AX, [BX+W]
 MOV AX, W+[BX]
 MOV AX, [BX]+W
Addressing Modes
9

Ex. suppose SI contains the address of a word array W
MOV AX, [SI+2]



The displacement is 2.
The instruction moves the contents of W + 2 to AX.
(this is the second element in the array)
The instruction could also have been written in any of these forms:

MOV AX, [2+SI]

MOV AX, 2+[SI]

MOV AX, [SI]+2

MOV AX, 2[SI]
Addressing Modes
10
• Ex. Write some code to sum in AX the elements of the 10-element array W defined by
W DW 10,20,30,40,50,60,70,80,90,100
XOR
XOR
MOV
ADDNOS:
ADD
ADD
LOOP
AX, AX
BX, BX
CX, 10
; AX holds sum. Can also use MOV AX,0
; clear base register
; CX has number of elements
AX, W[BX]
BX, 2
ADDNOS
; sum = sum + element
; index next element
; loop until done
Addressing Modes
11