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Microprocessor and Assembly

Language
CSC-321
Sheeza Zaheer
Lecturer
COMSATS UNIVERSITY ISLAMABAD
LAHORE CAMPUS
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Array and Array

Addressing
 OUTLINE
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■ Arrays
○ Introduction, Syntax and Examples
■ Addressing Modes
○ Register Mode
○ Immediate Mode
○ Register Direct Mode
○ Register Indirect Mode

● References
■ Chapter 10, Ytha Yu and Charles Marut,
“Assembly Language Programming and
Organization of IBM PC
 Arrays
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● An ordered list of elements
● Syntax
Array_name type value1, value2, value3
● Data Definition Directives
■ DB, DW, DD, DQ, DT

Data Definition Directives

myArray dw 1000h,2000h
Data dw 3000h,4000h
name

Remember: you can skip array name!

4 Values
 Contd..
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Examples Bytes Description Pseudo-ops

array1 DB 10, 20,30,40 1 Define Byte DB

array2 DW 1000, 2000 2 Define Word DW

Var3 DD -214743648 4 Define Double Word DD

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 Arrays
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● Sequence of memory bytes or words
● Example 1:
B_ARRAY DB 10h, 20h, 30h
Symbol Address Contents

B_ARRAY 0200h 10h

B_ARRAY+1 0201h 20h
B_ARRAY+2 0202h 30h

*If B_ARRAY is assigned offset address 0200h by assembler

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 Example 2
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● W_ARRAY DW 1000, 40, 29887, 329
*If W_ARRAY is assigned offset address 0300h by assembler

Symbol Address Contents

W_ARRAY 0300h 1000d

W_ARRAY+ 2 0302h 40d
W_ARRAY+ 4 0304h 29887d
W_ARRAY+ 6 0306h 329d
 High & Low Bytes of a Word
WORD1 DW 1234h
 Low Byte = 34h, symbolic address is WORD1
 High Byte = 12h, symbolic address is WORD1+1
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 The DUP Operator
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● Used to define an array if initial value of elements are same.

● Examples:

GAMMA DW 100 DUP (0) ;sets an array of 100 elements initialized to 0

DELTA DB 20 DUP (?) ; creates an array with 20 uninitialized bytes

● Nested DUP:

LINE DB 5, 4, 3 DUP (2, 3 DUP (0), 1)

Which is equivalent to:
LINE DB 5, 4, 2, 0, 0, 0, 1 , 2, 0, 0, 0, 1 , 2, 0, 0, 0, 1

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 Location of Array Elements
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● Address of an array element = base address + constant
● Suppose A is an array, S denotes no. of bytes in each element (S =
1 for a byte array, S = 2 for a word array).
● Position of elements in array A:

Position Location
1 A
2 A= 1 *S
. .
. .
N A = (N-1) * S

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 Example 10.1
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● Exchange the 10th and 25th elements in a word

array W

● Solution:
MOV AX, W + 18 ; 10th element = 9 * 2
XCHG W + 48, AX ; 25th element = 24 * 2
MOV W + 18, AX

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 11

ADDRESSING MODES

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 Default Segment and Offset Registers

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● CS: IP
● SS: SP, BP
● DS: BX, DI, SI

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 Addressing Mode
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● The way an operand is specified is known as its addressing
mode.
● Modes

1. Register Mode (Operand is register)

2. Immediate (Operand is constant)
3. Direct (Operand is variable)
4. Indirect
1. Register Indirect
2. Based
3. Indexed
4. Based Indexed (used with 2D array)
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 Register Mode
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● Operand = Register
● Can be 8 or 16 bit register
● Efficient as no memory access required.
● Example
mov ax, bx
mov cl, al

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 Immediate Mode
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● Operand = Constant Expression

● Constant Expression can be a number or a character.
● Example
mov ax, 5
mov dl,’a’

Important: Destination operand cannot be in immediate mode.

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 Direct Mode
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● Direct operand refers to the contents of memory at a location

identified by the label in the data segment.

● Example
.data
count db 20
wordList dw 1000h,2000h
.code
mov al, count
mov bx, wordList

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 Contd..
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.data
array db 10,20,30,40
.code
mov al, array
mov bl, array+1
mov cl, array+2
mov dl, array+3

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 Offset Operator
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● Used to move the offset of a label into a register or variable.

● Example
■ Assume offset of aWord is 0200H

.data
aWord dw 1234
.code
mov bx, offset aWord

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 Register Indirect Mode
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● Register contains the offset of data in memory.
● So, the register becomes a pointer to the memory location.
● Syntax:

[register]
● BX, SI, DI with default segment DS
● BP with default segment SS

● Remember the difference:

MOV AX, SI
MOV AX, [SI]
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 Example
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.data
aString db "ABCDEFG“
.code
mov bx,offset aString
add bx,5
mov dl,[bx]

0200 0205
A B C D E F G ...........

aString [bx]

Also solve Example 10.2, 10.3 and 10. 4 from textbook

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 Based and Indexed Modes
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● In these modes, the operand's offset address is obtained by adding a number called a
displacement to the contents of a register. Displacement may be any or the following:
■ the offset address of a variable
■ a constant (positive or negative)
■ the offset address of a variable plus or minus a constant
● If A is a variable, examples of displacements are:
■ A ( offset address of a variable)
■ -2 (constant) .
■ A + 4 (offset address of a variable plus a constant)
● Register may be: SI, DI, BX, or BP.
● Based: If BX or BP used
● Indexed: IF SI or DI used
● Can be written as:
displacement [register]
displacement + [register]
[register] + displacement
[displacement + register]
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[register + displacement ]
 Example
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 Example
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 Adding 8-bit Integers
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.data
aList db 10h,20h,30h
sum   db 0
.code
mov bx,offset aList   
mov al,[bx] ; AL = 10h
inc bx
add al,[bx] ; AL = 30h
inc bx
add al,[bx] ; AL = 60h
mov si,offset sum ; get offset of sum
mov [si],al ; store the sum

If you want to paste a code example such as this into a program,

remember that the code segment must always begin with the
following statements:
mov ax,@data
mov ds,ax
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 Adding 16-bit Integers
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.data
wordList dw 1000h,2000h,3000h, 0
.code
mov bx,offset wordList
mov ax,[bx] ; first number
add ax,[bx+2] ; second number
add ax,[bx+4] ; third number
mov [bx+6],ax ; store the sum

1000 2000 C 3000 (sum)

[bx] [bx+2] [bx+4] [bx+6]

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 Displaying a String
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.data
string db "This is a string."
COUNT = ($–string)  ; calculate string length

.code
mov   cx,COUNT ; loop counter
mov   si,offset string 
L1: 
mov   ah,2 ; DOS function: display char
mov   dl,[si] ; get character from array
int   21h ; display it now
inc   si ; point to next character
Loop  L1 ; decrement CX, repeat until 0

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 Two-Dimensional Array Example
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Each row of this table contains five bytes. BX points to

the beginning of the second row:
.data
ROWSIZE = 5
array  db  2h, 16h, 4h, 22h, 13h
db 19h, 42h, 64h, 44h, 88h
.code
mov bx,ROWSIZE
mov al,array[bx]     ; AL = 19h
0200 0205
...............
02 16 04 22 13 19 42 64 44 88
.

array [BX] (BX = 0005)

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 Based-Index Operands
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Add the value of a base register to an index
register, producing an effective address of
0157:
BX = 0155, SI = 0002
0150 0155 0157
10 20 30 40 50 60 70 80 90 A0 B0 C0 D0 E0 F0

[BX] [BX + SI]

Example...

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 Base-Index Example
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.data
ROWSIZE = 5
array  db  10h, 20h, 30h, 40h, 50h
db 60h, 70h, 80h, 90h,0A0h
       db 0B0h,0C0h,0D0h,0E0h,0F0h

.code
mov  bx,offset array ; point to the array at 0150
add  bx,ROWSIZE ; choose second row
mov  si,2 ; choose third column
mov  al,[bx + si] ; get the value at 0157

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Base-Index with Displacement
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.data
ROWSIZE = 5
array db  10h, 20h, 30h, 40h, 50h
db 60h, 70h, 80h, 90h,0A0h
      db 0B0h,0C0h,0D0h,0E0h,0F0h
.code
mov bx,ROWSIZE ; row 1
mov si,2 ; column 2
mov dl,array[bx + si] ; DL = 80h

0150 0155 0157

10 20 30 40 50 60 70 80 90 A0 B0 C0 D0 E0 F0

[BX] [BX + SI]

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