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assemblybinaryadditionalucarryflag

how to set auxiliary flag for 16bits binary addition

I know that when performing an 8-bit binary addition, the auxiliary flag is set to 1 if there is a carry from 3rd bit to 4th bit; but what about the addition of 2 16-bit numbers? i can't see any clear answer on the web.

I'm studying intel 8086 microprocessor...

For example, when i add these 2 numbers, 0x30a2 and 0xf1ac

0011 0000 1010 0010 + 1111 0001 1010 1100

CF=1
ZF=0
PF=1
SF=0
OF=1
AF=?

I'm not sure where to check it

Solution

  • From "Programming the 8086/8088" by James W. Coffron:

    AF auxiliary carry flag. If this flag is set, there has been a carry of the low nibble to the high nibble or a borrow from the high nibble to the low. The high or low nibble refers to the low order byte of a 16-bit value.

    In my day we would write a short piece of code to observe the processor behaviour. You could check it out by adding or subtracting two 16-bit numbers, followed by a pushf and pop ax to examine the status flags at your pleasure.

    EDIT.

    (Another way to get the flags is with LAHF which loads 5 bits of AH with flags, AF going to bit 4.)

    So AF represents the carry out from bit 3 to bit 4, whatever the size of the operands.

    Note that there are no branch instructions dependent on AF. It is used internally by the DAA instruction to do a decimal adjustment immediately after an ADD instruction, typically with AL.