Assembly: FE + FB

Question

What will the carry flag (CF), the overflow flag (OF), the sign flag (SF) and the zero flag (ZF) be set to if the following arithmetic is done in an 8-bit register?

0xFE + 0xFB

1. CF=1, OF=0, SF=1, ZF=0
2. CF=1, OF=1, SF=0, ZF=0
3. CF=1, OF=0, SF=1, ZF=1
4. CF=1, OF=1, SF=1, ZF=0
5. None of the above.

Thanks

Exercise question from William Stallings - Computer organization and architecture.

Answer 1

CF = 1 because there is an overflow (the unsigned result is 0x1F9 which does not fit in 8 bits)

OF = 0 because we add 2 negative numbers and the result is also negative

SF = 1 because the result is negative (bit 7 of the result is set)

ZF = 0 because the result is not zero

So the answer is 1. Also see about assembly CF(Carry) and OF(Overflow) flag