explanation required

Posted on 2011-03-03
Last Modified: 2012-06-27
I went through the following question / answer :

Assume that AX = BBBBh  and BX = BBBBh. What will be the contents
   of the AX register after the instruction:

      ADD AL,BL

   is executed.

   A. BBBBh    B. BBF6h   C. BB76h    D. BB78h    E. None of these

   Answer: C

   What will the condition code bits be after the instruction is

   A. SF=0 ZF=0 OF=0 CF=1      B. SF=0 ZF=0 OF=1 CF=1

   C. SF=0 ZF=1 OF=0 CF=1      D. SF=1 ZF=0 OF=1 CF=1    E. None of the above

   Answer: B

I didn't understand why the OF is set to 1 can someone explain to me that ?

Question by:bachra04
  • 3
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LVL 53

Expert Comment

ID: 35031669
>> I didn't understand why the OF is set to 1 can someone explain to me that ?

OF is the overflow flag, and it is set if the previous instruction caused an (arithmetic) overflow.

In this case, ADD AL,BL caused an overflow, so the flag is set.

Author Comment

ID: 35031964
I understand that :

BBh + BBh caused a carry that's why CF is set ;

But what confuses me is when do we say that there is Overflow; do you mean every time we have a carry we'll have overflow ?
Is there a case when we have overflow and not a carry ? or vice versa ?

LVL 53

Accepted Solution

Infinity08 earned 500 total points
ID: 35032253
There is an important distinction between the carry flag (CF) and the overflow flag (OF) :

CF : set if the LSB (least significant bit) of the neighboring more significant value would have been set by the instruction (useful for unsigned accition eg.)
OF : set if the MSB (most significant bit) changes due to the instruction (useful for signed addition eg.)

For example (in binary) :

        11111111 + 11111111 = 111111110

so, there is carry (because an extra 9th bit is set), but there is no overflow (because the 8th bit hasn't changed - it was 1 before and after).

And also :

        01111111 + 01111111 = 11111110

so there is no carry (the result still fits in the 8 bits), but there is overflow (because the 8th bit has changed from 0 to 1).

Author Comment

ID: 35034040
Can the carry flag be set 1 in a substraction operation :

e.g : sub 11h, 44h
LVL 53

Expert Comment

ID: 35034149
>> Can the carry flag be set 1 in a substraction operation :

In the case of a subtraction, the CF is set in case of what is called a borrow :

        00000001 - 00000010 = 11111111

In order for this (unsigned) subtraction to work, we need a borrow, to add a 9th 1 bit to the first operand :

        100000001 - 00000010 = 11111111

so the CF flag is set.

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