1. The FLAGS Register An x bit means an unidentified value 9/12/2018
CAP 221

2. Status Flags
The Carry Flag (C): This flag is set when the result of an unsigned arithmetic operation is too large to fit in the destination register. CF=1 if a carry from most significant bit (msb) on addition, or a borrow into msb on subtraction; otherwise CF=0.
9/12/2018
CAP 221

3. Status Flags
The Overflow Flag (O): This flag is set when the result of a signed arithmetic operation is too large to fit in the destination register (i.e. when an overflow occurs). Overflow can occur when adding two numbers with the same sign (i.e. both positive or both negative). A value of 1 = overflow and 0 = no overflow.
9/12/2018
CAP 221

4. Status Flags
The Sign Flag (S): This flag is set when the result of an arithmetic or logic operation is negative. This flag is a copy of the MSB of the result (i.e. the sign bit). A value of 1 means negative and 0 = positive.
9/12/2018
CAP 221

5. Status Flags
The Zero Flag (Z): This flag is set when the result of an arithmetic or logic operation is equal to zero. A value of 1 means the result is zero and a value of 0 means the result is not zero.
9/12/2018
CAP 221

6. Status Flags
The Auxiliary Carry Flag (A): This flag is set when an operation causes a carry from bit 3 to bit 4 (or a borrow from bit 4 to bit 3) of an operand. A value of 1 = carry and 0 = no carry.
9/12/2018
CAP 221

7. Status Flags
The Parity Flag (P): This flags reflects the number of 1s in the low byte of a result of an operation. If the number of 1s is even its value = 1 and if the number of 1s is odd then its value = 0.
9/12/2018
CAP 221

8. Overflow Flag and Carry Flag
Both are indicators of out-of-range condition.
Overflow flag is used to evaluate an out-of-range condition of signed number operations .
Carry flag is used in unsigned number operations.
9/12/2018
CAP 221

9. Overflow Flag and Carry Flag
Since a binary number can represent an unsigned number or signed number, the processor computes both flags and the user checks the appropriate flag to check if the result is out of range or not.
9/12/2018
CAP 221

10. Unsigned Overflow CF
Carry Flag CF is set to 1 if there is an end carry in an addition operation or there an end borrow in a subtraction operation. A value of 1 = carry and 0 = no carry.
9/12/2018
CAP 221

11. Signed Overflow OF
Overflow Flag is set to 1 when adding two numbers the same sign and the result has a different sign. Otherwise, OF is reset to 0.
Subtraction operation A - B can be performed as operation A + (-B). OF=1 if the result has a different sign than expected
OF=1 if the carries into and out of the msb don’t match
9/12/2018
CAP 221

12. Overflow Flag and Carry Flag
There are four possible conditions happen due to arithmetic operations.
OF=0, CF=0
OF=0, CF=1
OF=1, CF=0
OF=1, CF=1
9/12/2018
CAP 221

13. Example ADD AL,BL AL = 0Fh 00001111 BL = 08h 00001000
AL=AL+BL=17h
CF=0, OF=0,Both operands and the result are positive. PF=1, ZF=0, SF=0, AF=1
9/12/2018
CAP 221

14. Example ADD AL,BL AL = 0Fh 00001111 BL = F8h 11111000
AL=AL+BL= 07h
CF=1, OF=0,Operands have different sign bits. PF=0,
ZF=0, SF=0, AF=1
As a signed operation, 15 + (-8) = 7 (ok).
As an unsigned operation, = 263 > 255 (out-of-range).
9/12/2018
CAP 221

15. Example ADD AL,BL AL = 4Fh 01001111 BL = 40h 01000000
AL=AL+BL=8Fh
CF=0, OF=1, The result and operands have different sign bits. PF=0, ZF=0, SF=1, AF=0
As a signed operation, = 143 > 127 (out-of-range).
As an unsigned operation, 143 < 255 (ok).
9/12/2018
CAP 221

16. Example ADD AL,BL AL = F8h 11111000 BL = 81h 10000001
AL=AL+BL=79h
CF=1, OF=1, Operands are negatives, the result is positive. PF=0, ZF=0, SF=0, AF=0
As a signed operation, = -135 < -128 (out-of-range).
As an unsigned operation, = 377 > 255 (out-of-range).
9/12/2018
CAP 221

17. Example BX = 0001h 0000000000000001 AX=AX+BX=0000h 1 0000000000000000
ADD AX,BX
AX = FFFFh
BX = 0001h
AX=AX+BX=0000h
CF=1, OF=0,Operands have different sign bits. PF=1, ZF=1, SF=0, AF=1
As a signed operation, FFFFh+ 0001h = 10000h (out-of-range).
As an unsigned operation, FFFFh+0001h=-1+1=0 (ok).
9/12/2018
CAP 221

18. Example BX = 7FFFh 0111111111111111 AX=AX+BX=FFFEh 0 1111111111111110
ADD AX,BX
AX = 7FFFh
BX = 7FFFh
AX=AX+BX=FFFEh
CF=0, OF=1, The result and operands have different sign bits. PF=0, ZF=0, SF=1, AF=1
As a signed operation, 7FFFh+ 7FFFh = = = = FFFEh (out-of-range).
As an unsigned operation, 7FFFh+7FFFh = = (ok).
9/12/2018
CAP 221

19. How the processor indicates Overflow
The processor sets: OF=1 for signed overflow
CF =1 for unsigned overflow
The processor does not interpret the result as signed or unsigned
It is the programmer who interprets the results and take the appropriate action:
If signed interpretation OF is of interest
If unsigned interpretation CF is important
9/12/2018
CAP 221

20. How the processor determines that Overflow occurred
Unsigned overflow:
For Addition: CF=1 if there is a carry out of msb
For subtraction: CF=1 if there is a borrow into msb
Signed overflow:
OF=1 if the carries into and out of the msb don’t match
9/12/2018
CAP 221

21. Signed overflow
On addition of numbers with the same sign, or subtraction of numbers with different signs overflow can occur
Two conditions under which overflow can occur are:
(i) positive add positive gives negative
(ii) negative add negative gives positive
9/12/2018
CAP 221

22. How instructions affect the flag
Instruction Affects flags
MOV/XCHG none
ADD/SUB all
INC/DEC all except CF
NEG all (CF=1 unless result is 0,
OF=1 if word operand is
8000h, or byte operand is
80h)
9/12/2018
CAP 221

23. Example BX = + FFFFh 1 FFFEh= 1111 1111 1111 1110
ADD AX,BX
AX = FFFFh
BX = + FFFFh
1 FFFEh=
CF=1, OF=0, PF=0, ZF=0, SF=1
9/12/2018
CAP 221

24. Example ADD AL,BL AL = 80h BL = + 80h 1 00h
CF=1, OF=1, PF=1, ZF=1, SF=0
9/12/2018
CAP 221

25. Example
SUB AX,BX
AX = h
BX = h
7FFFh =
CF=0, OF=1, PF=1, ZF=0, SF=0
9/12/2018
CAP 221

26. Example INC AL, AL=FFh FFh + 1h 100h OF=0, PF=1, ZF=1, SF=0
CF unaffected
9/12/2018
CAP 221

27. Example MOV AX, -5 AX= -5= FFFBh, none of the flags are affected
NEG AX, AX=8000h
8000h =
2’s complement =
= 8000h
CF=1, OF=1, PF=1, ZF=0, SF=1
9/12/2018
CAP 221