Ascii adjust & decimal adjust

ASCII ADJUST AND
DECIMAL ADJUST

1

INSTRUCTIONS
 AAA - ASCII Adjust After Addition
 AAS - ASCII Adjust After Subtraction
 AAM - ASCII Adjust After Multiply
 AAD - ASCII Adjust Before Division
 DAA - Decimal Adjust for Addition
 DAS - Decimal Adjust for Subtraction

2

INTRODUCTION
The PC supports BCD format,
Uses of BCD
1)No loss of precision
2)Simpler to perform arithmetic
operation on small values from keyboard
BCD can be stored in two way:
Unpacked BCD
Packed BCD
3

Unpacked BCD Data
Unpacked BCD representation contains only
One decimal digit per byte. The digit is
stored in the least significant 4 bits; the
most significant 4 bits are not relevant to
the value of the represented number.

Example: Representing 1527
01 05 02 07h

4

Packed BCD Data
Packed BCD representation packs two
Decimal digits into a single byte.

Example: Representing 1527
15 27h

5

ASCII Adjust After Addition
Adjusts the result of the addition of two
unpacked BCD values to create a unpacked
BCD result.

Operation 1:
In AL
If rightmost nibble is >9 (ie)A to F
Or AuxilaryFlag=1
ADD 6 to rightmost nibble
6

Operation 2:
Clear left nibble form AL.

Operation 3:
In AH
ADD 1

Operation 4:
Set Carry and AuxilaryCarry
7

.model small
.data
b1 dw 38h
b2 dw 34h
.code
mov ax,@data
mov ds,ax
mov ax,b1 ;moving unpacked BCD into ax
mov bx,b2 ;moving unpacked BCD into bx
add ax,bx
aaa ;adjusting unpacked BCD after addition
or ax,3030h
end

8

ASCII Adjust After Subtraction
Adjusts the result of the subtraction of two
unpacked BCD values to create a unpacked BCD
result.

Operation 1:
a)AAS checks the rightmost nibble in AL
b)If rightmost nibble is >9 (ie)A to F
Or AuxilaryFlag=1
c)Then Subtract 6 from rightmost nibble

9

Operation 2:
Clear left nibble in AL.

Operation 3:
Subtracts 1 from AH

Operation 4:
Set Carry and AuxilaryCarry

10

Example :
d1 contains 34h , d2 contains 38h of byte
type
Ax AF
Mov AL, d1 ; 0034
Sub AL, d2; 00fc 1
AAS ; ff06 1
 since the rightmost digit of AL is c ,
subtract 6 from AL
 Subtract 1 from ah, set AF and CF flags

 The answer is -4, is ff06 in 10’s

complement 11

.model small
.data
b1 dw 38h
b2 dw 34h
.code
mov ax,@data
mov ds,ax
mov ax,b1 ;moving unpacked BCD into ax
mov bx,b2 ;moving unpacked BCD into bx
sub ax,bx
aas ;adjusting unpacked BCD after subtraction
or ax,3030h
end

12

ASCII Adjust After Multiplication
 For multiplication and Division of ASCII
numbers require that the numbers first be
converted into unpacked BCD format.
 Before Multiplication First clear the leftmost
nibble in each Byte.
 After Multiplication
 AAM performs the following operations
1) Divides AL value by 10 (0AH)
2) Stores Quotient in AH
3) Store Remainder in AL
13

Example:
AL contains 35H and CL contains 39H
Instruction comment AX CL
and CL, 0Fh; Convert CL to 09 0035 39
and AL,0Fh; Convert AL to 05 0005 09
mul CL; Multiply AL by CL 002D
AAM ; Convert to unpacked 0405
BCD
Or AX,3030h; covert to ASCII 3435

14

 Mul operation generates 45 (002Dh) in
AX

 AAM divides this value by 10, so quotient
of 04 in AH and remainder of 05 in AL

 OR instruction converts the unpacked
BCD to
ASCII format
15

.model small
.data
b1 dw 39h ; 9 in ASCII Format
.code
mov ax,@data
mov ds,ax
mov ax,b1 ;AX=0039h
and AL,0fh ;AX=0009h
mov bx,b2 ;BX=0035h
and bl,0fh ;BX=0005h
mul bx ;AX=002Dh
aam
or ax,3030h
end

16

ASCII Adjust Before Division
 AAD allows for a 2-Byte Dividend in AX. The
divisor can be only a single Byte(0-9)
 Before Division First clear the leftmost nibble in
each Byte.
 Operations done by AAD instruction
1) AAD multiplies the AH by 10(0Ah).
2) Then adds the product to AL and clears
the AH
 After AAD , division is performed.

17

Example:
AX contains 3238 (28) - Dividend
CL contains 37 (07) – Divisor
Instruction comment AX CL
and CL,0Fh; convert to unpacked 3238 07
BCD
and AX,0F0Fh; convert to unpacked 0208 07
BCD
AAD; convert to binary 001C
div CL; divide by 7 0004

18

 AAD multiplies the AH by 10(0Ah)
 Adds the product 20(14h) to the AL and
clears
the AH
 The result is 001Ch, is hex
representation of decimal 28
 Then division is performed.
 Remainder stored in AH, Quotient stored
in AL

19

model small
.data
b2 db 37h ; 7 in ASCII Format
.code
mov ax,@data
mov ds,ax
mov ax,b1 ;AX=3238h
and ax,0f0fh ;AX=0208h
mov cl,b2 ;CL=37h
and cl,0fh ;CL=07h
aad ; AX= 001c
div cl ; AX=0004
or ax,3030h; AX=3034
end

20

Decimal Adjust
To adjust the result of packed BCD
numbers which stored in AL.

DAA - Decimal Adjust for Addition
DAS - Decimal Adjust for Subtraction

21

Decimal Adjust for Addition
DAA performs the following operations:
Operation 1:
If AL is >9 (ie) A to F
or AuxilaryCarry=1 then
ADD 6 to AL
Set AuxilaryCarry=1

22

Operation 2:
If AL contains value > 99
or Carry=1 then
ADD 60 to AL and set Carry =1

Otherwise
AuxilaryCarry=0 ,carry=0

23

AL contains 45h
BL contains 45h
Instruction Comment AL
ADD AL,BL ; AL=AL+BL 8Ah
DAA ; Adjust packed BCD 90h
addition

24

 DAA checks the rightmost nibble of AL, it
is A so add 6 to AL
 Now AL = 90

25

.model small
.data
d1 dw 45h ;moving 45 as packed BCD
d2 dw 45h ;moving 45 as packed BCD
.code
mov ax,@data
mov ds,ax
mov ax,d1
mov bx,d2
add ax,bx
daa ;adjusting the packed BCD after addition
end

26

Decimal Adjust for Subtraction
Operations performed by DAS :
If AL is >9 (ie) A to F
or AuxilaryCarry=1 then
Subtract 6 from AL
Set Carry=1

Otherwise
AuxilaryCarry=0 ,carry=0

27

AL contains 80h
BL contains 49h
Instruction Comment AL
SUB AL,BL ; AL=AL-BL 37h
DAS ; Adjust packed BCD 31h
Subtraction

28

.model small
.data
d1 dw 57h
d2 dw 48h
.code
mov ax,@data
mov ds,ax
mov ax,d1 ;moving 45 as packed BCD
mov bx,d2 ;moving 45 as packed BCD
sub ax,bx
das ; adjusting the packed BCD after subraction
end

29

Ascii adjust & decimal adjust

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Ascii adjust & decimal adjust