The document contains information about the FLAGS register in 8086 microprocessors. It discusses: 1. The FLAGS register contains the current state of the processor and indicates the status by setting bits called flags. There are 9 active flags in 8086. 2. The two kinds of flags are status flags and control flags. Status flags reflect the result of operations, while control flags enable or disable processor operations. 3. Important status flags are carry flag, parity flag, auxiliary carry flag, zero flag, and sign flag. The document provides details on how each flag is set based on results of operations.

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3. FLAGS resister
and
Their changes due to
of 8086

4. What is FLAGS register ??
FLAGS resister:
The FLAGS register is the status register in Intel x86
microprocessors that contains the current state of the processor.
 8086 has 16 flag registers among which 9 are active.
 The purpose of the FLAGS register is to indicate the status of
the processor.
 It does this by setting the individual bits called flags.

5. Types of FLAGS:
There are two kinds of FLAGS.
1. Status FLAGS
2. Control FLAGS

6. Types of FLAGS
Status FLAGS :
 Status FLAGS reflect the result of an operation executed by
the processor
Control FLAGS :
 The control FLAGS enable or disable certain operations of
the processor.

8. FLAG Register Bits

9. Status Flags
The Carry Flag (CF) :
 CF = 1 if there is a carry out
from the msb (most significant
bit) on addition, or there is a
borrow into the msb on
subtraction.
 CF = 0 otherwise
 CF is also affected by shift and
rotate instructions

10. Status Flags
The Parity Flag (PF):
PF = 1 if the low byte of
a result has an even
number of one bits (even
parity)
PF = 0 otherwise (odd
parity)

11. Status Flags
The Auxiliary Carry Flag
(AF):
AF = 1 if there is a carry out from
bit 3 on addition, or there is a
borrow into the bit 3 on
subtraction
AF = 0 otherwise
AF is used in binary-coded decimal
(BCD) operations

12. Status Flags
The Zero Flag (ZF) :
 ZF = 1 for a zero result
 ZF = 0 for a non-zero result
 In this example the result is not
zero. So ZF=0

13. Status Flags
The Sign Flag (SF) :
SF = 1 if the msb of a result
is 1; it means the result is
negative if you are giving a
signed interpretation
SF = 0 if the msb is 0
In this Example the msb of
the result is 1. So SF=1

14. Status Flags
The Overflow Flag (OF):
OF = 1 if signed overflow occurred
OF = 0 otherwise
Overflow flag is 2 types:
1.Signed
2. Unsigned

15. Control Flags
Direction Flag (DF) :
 - this flag is used by some instructions to process data chains,
when this flag is set to 0 - the processing is done forward, when
this flag is set to 1 the processing is done backward.
Interrupt enable Flag (IF):
 when this flag is set to 1 CPU reacts to interrupts from
external devices.
Trap Flag (TF):
Used for on-chip debugging.

16. How instruction affect the flags
Instruction affect 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)


17. Example1:
ADD AX,BX,where AX contains
FFFFh,BX contains FFFFh.
Solution:
FFFFh
+FFFFh
1FFFEh
The result stored in AX is FFFEh
=1111 1111 1111 1110

18. Example2:
ADD Al,Bl,where Al contains 80h,Bl contains
80h.
Solution:
80h
+80h
100h
Result stored in Al is 00h.

19. Example 3:
INC AL , where AL contains FFh.
Solution :
FFh
+ 1h
100h
The result stored in AL is 00h. SF = 0, PF =1, Zf =1. Even though there is a carry out,
CF is unaffected by INC. this means that if CF = 0 before the execution of the
instruction , CF will still be 0 afterward.
OF = 0 because numbers of unlike sign are being added (there is a carry into the
msb and also a carry out ).

20. Example 4:
MOV AX,-5
Solution:
The result stored in AX is -5=FFFBh.
None of the flags are affected by MOV.

21. Example 5 :
NEG AX , where AX contains 8000h
Solution :
8000h=1000 0000 0000 0000
One’s complement=0111 1111 1111 1111
+1
1000 0000 0000 0000=8000h
The result stored in AX is 8000h.
SF=1, PF=1 ,ZF=0.
CF=1, because for NEG CF is always 1 unless the result is 0.
OF=1, because the result is 8000h; when a number is negated, we
would expect a sign change, but because 8000h is its own two’s
complement , there is no sign change.

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