1. Morgan Kaufmann Publishers Computer Organization and Assembly Language
September 17, 2018
CS 206 D
Computer Organization and Assembly Language
Chapter 1 — Computer Abstractions and Technology

2. Array and Addressing Modes

3. Lecture Outline
Introduction
Addressing Modes
Register Indirect Mode
Based and Indexed Addressing Modes

4. Introduction Introduction
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)
Used with one dimensional arrays
Address memory operands indirectly
Used with two dimensional arrays
Addressing Modes

5. Register Indirect Mode
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 DI BP
The operand’s segment number is contained in DS
The operand’s segment number is contained in SS

6. Memory Address Generation
Physical Address (20 Bits)
Adder
Segment Register (16 bits)
Offset Value (16 bits)
The physical memory addresses
The Data Segment
Memory
Segment Register
Offset
Physical Address +
DS: SI
05C0
0050
05C00H
05C50H
DS:SI
00000H
0FFFFFH
Data is usually fetched with respect to the DS register.
The effective address (EA) is the offset.
The EA depends on the addressing mode.

7. Register Indirect Mode
Example 1:
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

8. Register Indirect Mode
Example 2:
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 ; AX holds sum
LEA SI, W ; SI points to array W
MOV CX, 10 ; CX has number of elements
ADDNOS:
ADD AX, [SI] ; sum = sum + element
ADD SI, 2 ; move pointer to the next element
LOOP ADDNOS ; loop until done

9. Based and Indexed Addressing Modes
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] R
opcode
instruction
Registers
operand
Memory A +
Addressing Modes

10. Based and Indexed Addressing Modes
The register must be one of the following: BX SI DI 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.
The operand’s segment number is contained in DS
The operand’s segment number is contained in SS
Addressing Modes

11. Based and Indexed Addressing Modes
Example 3: 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

12. Based and Indexed Addressing Modes
Example 4: . 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

13. Based and Indexed Addressing Modes
Example 5:
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 AX, AX ; AX holds sum. Can also use MOV AX,0
XOR BX, BX ; clear base register
MOV CX, 10 ; CX has number of elements
ADDNOS:
ADD AX, W[BX] ; sum = sum + element
ADD BX, 2 ; index next element
LOOP ADDNOS ; loop until done
Addressing Modes

14. Based and Indexed Addressing Modes
Example6 Suppose that ALPHA is declared as
ALPHA DW H,0456H,0789H,0ABCDH
In segment address by DS. Suppose also that
BX contains 2 offset 0002 contains 1084 h
SI contains 4 offset 0004contains 2BACh
DI contains 1
What will be the result of following instructions:
instruction
NUMBER MOVED
MOV AX,[ALPHA+BX]
0456H
MOV BX,[BX+2]
2BACH
MOV CX,ALPHA[SI]
0789H
MOV AX,-2[SI]
1084H
MOV BX,[ALPHA+3+DI]
MOV AX,[BX]2
illegal
Addressing Modes

15. Based and Indexed Addressing Modes
Example7. Replace each lower case letter in the following string by it’s upper case equivalence. Use index addressing mode
MSG DB ‘this is a message’
Solution:
MOV CX, ; NO of chars in string
XOR SI, SI ; clear SI indexes a char
TOP:
CMP MSG[SI],’ ‘ ;BLANK ?
JE NEXT ; yes skip over
AND MSG[SI],0DFH ; convert to upper case
NEXT:
INC SI ;Index next byte
LOOP TOP ; loop until done
Addressing Modes

16. Addressing modes Mode Meaning Pros Cons Implied Fast fetch
Limited instructions
Immediate
Operand=A
No memory ref
Limited operand
Direct
EA=A
Simple
Limited address space
Indirect
EA=[A]
Large address space
Multiple memory ref
Register
EA=R
Register indirect
EA=[R]
Extra memory ref
Displacement
EA=A+[R]
Flexibility
Complexity
stack
EA=stack top
Limited applicability