Auxiliary Carry Flag in 8086 Microprocessor

Auxiliary Carry Flag (AF) is one of the six status flags in the 8086 microprocessor. 

• This flag is used in BCD (Binary-coded Decimal) operations.
• The status of this flag is updated for every arithmetic or logical operation performed by ALU.
• This flag is set to one if there is a CARRY from the lower nibble or BORROW for the lower nibble in binary representation.
• Else it is set to zero.

Note : Auxiliary Carry Flag is to set to one when there is a carry from the units place in hexadecimal representation. Same as the lower nibble in binary representation.

Example :

In the below image, you can see a carry takes place from the units digit. Hence auxiliary carry flag is set to one here. Here ‘H’ represents a hexadecimal number.

Let us consider the same example in binary representation. 

   29H =  0010 1001

+4CH =  0100 1100

   75H =  0111 0101

                    ^ here there is carry generated and forwarded to next nibble, so the auxiliary carry flag is set to one.