Question

Write procedure of DAA instruction to convert hexadecimal number in to Binary Coded Decimal (BCD) number with suitable example. 

Answer 1

The DAA (Decimal Adjust Accumulator) instruction is used to convert the hexadecimal result of the addition of two BCD numbers into a valid packed BCD number. It is used only after the ADD/ADC/ADI instruction.

Procedure

1. Perform addition: Add two BCD numbers using ADD/ADC/ADI. The result (in hexadecimal form) is stored in Accumulator (A).

2. Check lower nibble (units digit): If lower nibble > 9 OR Auxiliary Carry (AC) = 1 → Add 06H to accumulator.

3. Check upper nibble (tens digit): After step 2, if upper nibble > 9

   OR Carry flag (CY) = 1 → Add 60H to the accumulator.

4. Final result: The accumulator now contains a valid packed BCD number. If CY = 1, it indicates an extra digit (hundreds).

Add 68 and 85 (BCD numbers)

Perform addition → 68H + 85H = EDH

Lower nibble check → EDH + 06H = F3H

Upper nibble check → F3H + 60H = 153H

Final result 

A = 53H

CY = 1