8086 Bcd arithmetic instructions

1. Microprocessor & its Applications
Module 3 Continued……
Dr. Girisha G S
Dept. of CSE
SoE, DSU, Bengaluru
1

2. • BCD AND ASCII ARITHMETIC Instructions
- DAA, DAS, AAA, AAD, AAM, AAS
Agenda

3. BCD (Binary-Coded Decimal) code :
- Four-bit code that represents one of the ten decimal digits from 0 to 9.
can not use hex digits A-F
- Let us take an Example:
Consider 10 as a decimal number
Its BCD will be 0001 0000 ( not the binary Form 1010)
- BCD can be stored in two way
- Unpacked BCD
- Packed BCD
Packed BCD
- Packed BCD data are stored as two digits per byte
E.g. 17d is stored as 17= 0001 0111
Unpacked BCD
- Unpacked BCD data are stored as one digit per byte
E.g. 17d is stored as 01 07 = 0000 0001 0000 0111

4. ASCII Code (American Standard Code for Information Interchange)
- ASCII code is a 7 bit code which assigns a binary value to letters, numbers &
other characters from 0 to 127 in decimal (00-7F in hexadecimal)
- In this code number 0 through 9 are represented as 30 through 39
- letters A through Z are represented as 41H through 5AH
- letters a through z are represented as 61H through 7AH
Example: 1234 is stored as 31 32 33 34H
ASCII for 1 is 31H, for 2 is 32H etc.

5. BCD Arithmetic
- 8086 Microprocessor allows arithmetic manipulation of BCD data
- This is accomplished by instructions that adjust the numbers for BCD
arithmetic
- used to correct the BCD addition/subtraction
- These instructions do not take any operands. The operand is assumed to
be in AL.
• DAA (decimal adjust after addition) instruction follows BCD addition.
• DAS (decimal adjust after subtraction) instruction follows BCD subtraction

6. DAA: Decimal adjust after addition
Syntax: DAA ; AL←AL converted for BCD addition
- This instruction is used to convert the result of addition of two packed BCD numbers to a valid
BCD number.
- The instruction should be issued after an ADD, ADC instruction. The result has to be only in AL
register.
- On execution of this instruction,
- If the lower nibble in AL register is greater than 9 or AF is set, then 6 will be added to the
AL register. & the AF is set.
- If the upper nibble in AL register is greater than 9 or CF is set, then 60 will be added to the
AL register. & the CF is set.
Flags affected: AF, CF, PF, ZF
Examples:
MOV AL,53H ; AL<-53 BCD = 01010011
MOV CL, 29H; CL<-29 BCD= 00101001
ADD AL, CL ; AL<-7CH = 01111100
DAA ; add 6 to AL because C>9
; AL←7C = 01111100
+6 = 00000110
; AL←82 (BCD)= 10000010

7. DAS: Decimal adjust after subtraction
- This instruction is used to convert the result of subtraction of two packed BCD numbers
to a valid BCD number.
- The instruction should be issued after an SUB, SBB instruction. The result has to be
only in AL register.
- On execution of this instruction,
- if the lower nibble in AL register is greater than 9 or AF is set, then 6 will be
subtracted from the AL register. & the AF is set.
- If the result of the upper nibble in AL register is greater than 9 or CF is set, then 60
will be subtracted from the AL register. & the CF is set.
Examples:
MOV AL,32H ; Load 32 BCD = 00110010
MOV CL, 17H; Load 17 BCD = 00010111
SUB AL, CL ; AL←32H-17H = 00011010
; AL←1BH
DAS ; subtract 6 from AL because B>9
; AL←1B-6 = 00011010
; - 00000110
; AL←15 (BCD) = 00010101

8. ASCII Arithmetic
- The ASCII arithmetic instructions function with ASCII-coded numbers.
- These numbers range in value from 30H to 39H for the numbers 0-9.
- There are four instructions used with ASCII arithmetic operations:
• AAA (ASCII adjust after addition)
• AAD (ASCII adjust before division)
• AAM (ASCII adjust after multiplication)
• AAS (ASCII adjust after subtraction)

9. AAA - ASCII adjust after addition
- The AAA instruction can be used after addition to get the correct result in unpacked BCD
form.
- The AAA instruction works only on AL register. The AAA instruction converts the
resulting content of AL to unpacked BCD form.
- On execution,
- this instruction clears the upper nibble of AL, AH register should be cleared before
adding.
- If lower nibble of AL>9 or AF=1, then it add 6 to the AL register, add 1 to AH register &
set AF & CF = 1.
- In all other cases, the upper nibble of AL is cleared.
- This instruction does not give exact ASCII codes of the SUM, but they can be obtained
by adding 3030H to AX.
E.g. ADD AL,CL ; Before : AL=34H, ASCII 4
; CL=38H, ASCII 8
; After: AL=6CH
AAA ; AL = C + 6
; AL = 12
; AH=01
; AL=02

10. AAS: ASCII adjust after subtraction
- The AAS instruction can be used after subtraction to get the correct result in
unpacked BCD form.
- The AAS instruction works only on AL register. The AAS instruction converts
the resulting content of AL to unpacked BCD form.
- The adjustment is needed only if the result is negative.
Example: SUB AL, CL ; Before : AL=38H, ASCII 8; CL=32H, ASCII 2
; After: AL=06
AAS ; AL=06 (BCD )
; AX=0006

11. AAD: ASCII adjust before division
- The instruction AAD can be used to convert the two unpacked BCD digit in AH & AL register
to the equivalent binary number in the register AL. i.e. AAD instruction adjusts the number
in AX before dividing two unpacked decimal numbers.
- This instruction should be issued before the division (DIV) instruction.
- The division instruction will place the quotient in AL register & remainder in the AH
register. The upper nibble of the AH & AL are filled with zero.
- It works as follows: Multiply AH by 10 (0AH) & add it to AL & set AH to 0
Example: AAD ; Before: AH= 02H , AL=07H, CL=07H
; After: AX=001BH
DIV CL

12. AAM: ASCII adjust after multiplication
- The instruction AAM can be used to convert the result of multiplication of two
valid unpacked BCD number in AX.
- This instruction should be issued after multiplication instruction.
- It works as follows: AL is divided by 10, quotient is stored in AH & remainder in
AL
Example: MUL CL ; Before : AL=04H, CL=06H
; After : AX=0018H (24D)
AAM ; AX=02 04 (unpacked BCD form)