The DAA (Decimal Adjust after Addition) instruction allows addition of numbers represented
in 8bit packed BCD code. It is used immediately after normal addition instruction operating
on BCD codes. This instruction assumes the AL register as the source and the destination,
and hence it requires no operand.
The effect of DAS (Decimal Adjust after Subtraction) instruction is similar to that of DAA,
except that it is used after a subtraction instruction.
For example in the following program, that NUM1 and NUM2 are decimal numbers
coded in BCD format, the result should be 61
DAA Instructions 
.MODEL SMALL
.STACK 200
.DATA
NUM1 DB 27H
NUM2 DB 35H
.CODE
.STARTUP
MOV AL, NUM1 ;load AX with number NUM1
ADD AL, NUM2 ;AL = AL + NUM2 i.e. AL = 5CH = 92 in decimal
;The expected result is 62 in decimal
DAA ; AL = 62
.EXIT
END

DAA operation:
For the processor there is no difference between a BCD and a
hexadecimal number, all numbers are seen as hexadecimal numbers.
After performing an addition and the result is saved in the AL register,
conversion to decimal is carried out as follows:
if the digit in the lower four nibbles of AL is greater than 10 (decimal),
then subtract 10 and
add 1 to the digit in the higher four nibbles of AL.
Example:
Suppose that the result obtained after adding 27 to 35, is 5CH. To convert this to the
value that we would expect after a decimal addition, the DAA instruction is used.
assume result = AL = 5CH
digit in the low four nibbles of AL = C = 12
then 12  10 = 2
hence keep the 2 and
add 1 to the digit in the higher four nibbles of AL 5 + 1 = 6
The result is thus: 62
The DAS instruction works in a similar fashion after a SUB instruction.