This document describes several x86 assembly language instructions used for arithmetic operations on binary-coded decimal (BCD) values. It provides examples of how ADD, ADC, SUB, SBB, DAA, DAS, AAA, and AAS instructions work when performing decimal arithmetic on packed BCD values in registers. These instructions allow adding, subtracting, and adjusting BCD values to account for decimal carry/borrow in the same way decimal arithmetic is performed on paper.

1. (4)

4. INST. OPERANDS FUNCTION
ADD
D.OPERAND,S.OPERAND
REG, memory
memory, REG
REG, REG
memory, immediate
REG, immediate
FUNCTION:
D.OPERAND = D.OPERAND + S.OPERAND
ADC
O, S, Z, A, C & P FLAGS are Affected
Example:
MOV AL, 6
ADD AL, -3
FUNCTION:
D.OPERAND = D.OPERAND + S.OPERAND + CF
Example:
MOV AL, 6
ADC AL, -3

5. MOV BH, 26
ADD BH, 77 ; BH = 103
; CF = 0, ZF = 0
; OF = 0
(103 < 128, sign bit: 0→0)
ADD BH, 39 ; BH = 142
; CF = 0, ZF = 0
; OF = 1
(142 > 128, sign bit: 0→1)
ADD BH, 142 ; BH = 28
; CF = 1, ZF = 0
(142+142 = 256 + 28)
; OF = 1
(28 < 128, sign bit: 1→0)

6. EEL 3801C
Unsigned Integers
• Unsigned integers are easy – they use all 8 or
16 bits in the byte or word to represent the
number.
• If a byte, the total range is 0 to 255
(00000000 to 11111111).
• If a word, the total range is 0 to 65,535
(0000000000000000 to 1111111111111111).

7. EEL 3801C
Signed Integers
• Are slightly more complicated, as they can only
use 7 or 15 of the bits to represent the number.
The highest bit is used to indicate the sign.
• A high bit of 0  positive number
• A high bit of 1  negative number
• The range of a signed integer in a byte is
therefore, -128 to127, remembering that the
high bit does not count.
• The range of a signed integer in a word is
–32,768 to 32,767.

8. INST. OPERANDS FUNCTION
NO OPERAND
Decimal adjust After Addition.
Corrects the result of addition of two packed
BCD values.
Algorithm:
if low nibble of AL > 9 or AF = 1 then:
AL = AL + 6
AF = 1
if AL > 9F or CF = 1 then:
AL = AL + 60h
CF = 1
DAA
Example:
MOV AL, Fh ; AL = 0Fh
DAA ; AL = 15h
O, S, Z, A, C & P FLAGS are Affected

9. 8086 Assembly 9
BCD Packing
• When typing a digit from the keyboard, it is
represented in ASCII values 30H thru 39H
– 00110000 thru 00111001
– if we replace the 0011 in the upper nibble, we
are left with BCD code for the digit
– this is an unpacked BCD number because we
are using 8 bits to represent it
• We can combine 2 BCD digits in a single
byte…this is a packed BCD number

10. EXAMPLE 1:
AL contains 25 (packed BCD)
BL contains 65 (packed BCD)
ADD AL, BL ;AL=8Ah
DAA ;AL=90h (8A+06)
EXAMPLE 2:
AL contains 45 (packed BCD)
BL contains 65 (packed BCD)
ADD AL, BL ;AL=AAh
DAA ;AL=?

11. INST. OPERANDS FUNCTION
NO OPERAND
ASCII Adjust after Addition.
Corrects result in AH and AL after addition
when working with BCD values.
Algorithm:
if low nibble of AL > 9 or AF = 1 then:
AL = AL + 6
AH = AH + 1
AF = 1 & CF = 1
else AF = 0 & CF = 0
In both cases: clear the high nibble of AL
AAA
A& C FLAGS are Affected
Example: MOV AX, 8h
ADD AX, 7h ; AH = 00, AL =0Fh
AAA ; AH = 01, AL = 05

12. EXAMPLE 1:
AL contains 32 (ASCII code for number 2)
BL contains 34 (ASCII code for number 4)
ADD AL,BL ;AL=66h
AAA ;AL=06 AH=00
?
EXAMPLE 2:
AL contains 32 (ASCII code for number 2)
BL contains 38 (ASCII code for number 8)
ADD AL, BL ;AL=6Ah
AAA ;AL=? AH=?
?

13. INST. OPERANDS FUNCTION
SUB
D.OPERAND,S.OPERAND
REG, memory
memory, REG
REG, REG
memory, immediate
REG, immediate
FUNCTION:
D.OPERAND = D.OPERAND - S.OPERAND
SBB
O, S, Z, A, C & P FLAGS are Affected
Example:
MOV AL, 6
SUB AL, 3
FUNCTION:
D.OPERAND = D.OPERAND - S.OPERAND - CF
Example:
MOV AL, 6
SBB AL, 3

14. INST. OPERANDS FUNCTION
NO OPERAND
Decimal adjust After Subtraction.
Corrects the result of subtraction of two
packed BCD values.
Algorithm:
if low nibble of AL > 9 or AF = 1 then:
AL = AL - 6
AF = 1
if AL > 9F or CF = 1 then:
AL = AL - 60h
CF = 1
DAS
Example: MOV AL, 0
SUB AL, 01h ; AL = 0FFh (-1)
DAS ; AL = 99h, CF=1
S, Z, A, C & P FLAGS are Affected

15. EXAMPLE 1:
MOV AL,83h
MOV BL,28h
SUB AL,BL ;AL=5Bh (83h-28h)
;= (83h+TC(28h))
;=(83h+D8h)
DAS ;AL=55h (5Bh-6)
EXAMPLE 2:
MOV AL, 16h
MOV BL,28h
SUB AL, BL ;AL=EEh (16h-28h)
DAS ;AL=?

16. INST. OPERANDS FUNCTION
NO OPERAND
ASCII Adjust after Subtraction.
Corrects result in AH and AL after subtraction
when working with BCD values.
Algorithm:
if low nibble of AL > 9 or AF = 1 then:
AL = AL - 6
AH = AH - 1
AF = 1 & CF = 1
else AF = 0 & CF = 0
In both cases: clear the high nibble of AL
AAS
A& C FLAGS are Affected
Example: :
MOV AX, 02FFh ; AH = 02, AL = FFh
AAS ; AH = 01, AL = 09

17. EXAMPLE 1:
MOV AX,38h
SUB AL,39h ;AX=00FFh (38h-39h)
;= (38h+TC(39h))
;=(38h+C7h)
AAS ;AX=FF09h (Ten’s
;complement of -1)
EXAMPLE 2:
MOV AX, 38h
SUB AL, 35h ;AX=0003h (38h-35h)
;= (38h+TC(35h))
;=(38h+??h)
AAS ;AX=????h ( ? )

18. Examples
; DX;AX = 1303F0h + 22A00h = 152DF0h
MOV AX, 03F0h
MOV DX, 0013h
ADD AX, 2A00h ; 03F0h + 2A00h = 2DF0h
ADC DX, 0002h ; 0013h + 0002h ; +0(no carry) = 0015h
; AX = 398 – 32 = 366 (= 016Eh)
MOV AX, 398; 018Eh
SUB AX, 32 ; AX = 018Eh – 0020h
; DX;AX = 1303F0h – 22A00h = 10D9F0h
MOV AX, 03F0h
MOV DX, 0013h
SUB AX, 2A00h ; 03F0h - 2A00h = D9F0h
SBB DX, 0002h ; 0013h - 0002h
; -1(borrow) = 0010h
MOV AL, 125
ADD AL, 15 ; AL = 125 + 15 (= 8Ch)