The document discusses the flag register in processors like the 8086 and 8088. It explains that the flag register is a 16-bit register that contains status flags indicating the result of arithmetic operations like carry, zero, sign, overflow, and parity. It contains six status flags and three control flags that can be set or reset by programmers to control functions like interrupt handling and string instruction direction. The status flags provide information about the results of operations to determine subsequent instruction execution.

1. Flag Register
Prepared By:
Mr. Afraz Butt
Unit No. 3

2. What is register
A register (CPU register) is one of a small set of data holding
places that are part of the computer processor.
A register may hold an instruction, a storage address, or any
kind of data (such as a bit sequence or individual characters).
There are various types of Registers those are used for various
purpose. Among of the some Mostly used Registers named as
AC or Accumulator Register, Data Register or DR, the AR or
Address Register, program counter (PC), Memory Data Register
(MDR) ,Index register, Memory Buffer Register and Flag
Register.

3. What is Flag Register?
 For the 8086,8088 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
• 2

4. Flag Register
• A 16-bit register, also called Program Status Word (PSW).
Seven bits remain unused while the rest nine are used. Six are
status flags and three are control flags.
• The FLAGS register is the status register in Intel x86
microprocessors that contains the current state of the
processor. This register is 16 bits wide

5. Flag Register
The following figure manifests the Flags register.
• Status Flags in bit: 0, 2, 4, 6, 7,11
• Control Flags in bit: 8,9,10
Rest of the bits have no significance.

6. 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.
• 15 = 1111
15=1111
15=1111
14=1110

7. Carry Flag
• CF = 1 if there is a carry out from the most significant bit (msb)
on addition, or subtraction
• 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.
101
010
====
1 1 1

8. Auxiliary Carry 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.

9. 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 .

10. Zero Flag
• ZF (Zero Flag) is set (ZF=1) when result of an arithmetic or
logical operation is zero. For non-zero result this flag is reset
(ZF=0).
• 111
• 111
• ====
000

11. Overflow Flag
• OF (Overflow Flag) set to 1 when there is a signed overflow.
For example, when you multiply FFH by 11H (result is not one
byte).

12. Control Flags
The control flags can be set/reset by the programmer.
• 1. DF (Direction Flag) : controls the direction of operation of
string instructions.
(DF=0 Ascending order DF=1 Descending order)
2. IF (Interrupt Flag): controls the interrupt operation in 8086μP.
• (IF=0 Disable interrupt IF=1 Enable interrupt)
3. TF (Trap Flag): controls the operation of the microprocessor.
• (TF=0 Normal operation TF=1 Single Step operation)