This slide contains only Flag register for Assembly Language (8086)

1. Flag Registers
Assembly Language
Submitted To
Fahmida Afrin
Lecturer
Dept. of Computer Science &
Engineering
Daffodil International University
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Submitted By
1. Anwar Hasan Shuvo (151-15-5506)
2. Nobanul Hasan (151-15-5035)
3. Prosenjit Roy (151-15-4810)
4. Redwan Naeem Manik (151-15-5039)
5. Md. Shamsuzzaman (151-15-4861)

2. What is Flag Register?
 For the 8086 processor, the processor state is implemented as nine individual bits called Flags.
 Each decision made by the 8086 is based on the values of these flags.
 The flags are placed in the FLAG registers.
 They are classified as: Status Flags or Control Flags
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3. The Flags Register
 Figure: The Flags Register
The following figure manifests the Flags register.
 Status Flags in bit: 0, 2, 4, 6, 7
 Control Flags in bit: 8,9,10
Rest of the bits have no significance.
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15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
OF DF IF TF SF ZF AF PF CF

4. Carry Flag
 CF = 1 if there is a carry out from the most significant bit (msb) on addition, or
there Is a borrow into the msb on subtraction; [msb = most significant bit]
 Otherwise, it is 0. CF is also affected by shift and rotate Instructions.
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5. Parity Flag
 PF = 1 if the low byte of a result has an even number of one bits (even parity). It
is 0 if the low byte has, odd parity. For example, if the result of a word addition
is FFFEh, then the low byte contains 7 one bits, so PF=0.
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6. Auxiliary Flag
 AF = 1 if there is a carry out from bit 3 on addition, or a borrow into bit 3 on
subtraction.
 AF is used in Binary Coded Decimal (BCD) operations.
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7. Signed Flag
 SF means Signed Flag
 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 .
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8. Zero Flag
 ZF = 1 for a zero result, and ZF = 0 for a nonzero result.
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9. Overflow
Signed and unsigned overflows are independent phenomena. When we perform
an arithmetic operation such as addition, there are four possible outcomes:
 (1) no overflow,
 (2) signed overflow only,
 (3) unsigned overflow only, and
 (4) both signed and unsigned overflows.
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10. Unsigned Overflow
As an example of unsigned overflow.
AX contains FFFFh, BX contains 0001 h
ADD AX,BX is executed
 If we are giving an unsigned interpretation, the correct ansis 1OOOOh =
65536, but this is out of range for a word operation. A 1 is carried out of the
msband the answer stored in AX, OOOOh, is wrong, so unsigned overflow
occurred. But the stored answer is correct as a signed number, for FFFFh= -1.
OOO1h = 1, and FFFFh+ OOO1h = -1 + 1 = 0,
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11. Signed Overflow
Suppose AX and BX both contain 7FFFh,we execute
ADD AX,BX
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 The signed and unsigned decimal interpretation of 7FFFh is32767. Thus for
both signed and unsigned addition, 7FFFh + 7FFFh = 32767 + 32767 = 65534.
 This is out of range for signed numbers; the signed interpretation of the
stored answer FFFEh is 2. so signed overflow occurred. However, the unsigned
interpretation of FFFEh is 65534, which is the right answer, so there no

12. How Instructions Effect The Flags 12

13. Example 1 13

14. Example 3 14

15. Example 2 15

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