The FLAGS register contains status information about the processor's state. It has status flags like the carry flag, parity flag, zero flag, and sign flag that indicate the outcome of arithmetic operations. It also has control flags like the interrupt flag and direction flag that control interrupt handling and string processing direction. The document discusses each of these flags in detail, explaining what they indicate and their purpose.

1. FLAG REGISTERS
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By: Hamza Akash

2. What is a FLAGS Register??
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 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. Its successors, the
EFLAGS and RFLAGS registers, are 32 bits and 64 bits
wide, respectively. The wider registers retain compatibility
with their smaller predecessors. We are going to discuss
about the FLAGS registers here.

3. Category of FLAGS Registers
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There are 2 types of FLAGS register.
• Status Flags
• Control Flags

4. Status Flags
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There are 6 status flags in 8086 processor.
• Carry Flag
• Parity Flag
• Adjust Flag
• Zero Flag
• Sign Flag
• Overflow Flag

5. Carry Flag
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In 8086 processors the carry flag (usually indicated as
the C flag) is a single bit in a system status (flag)
register used to indicate when an arithmetic carry or
borrow has been generated out of the most significant
ALU bit position. The carry flag enables numbers larger
than a single ALU width to be added/subtracted by carrying
(adding) a binary digit from a partial addition/subtraction
to the least significant bit position of a more significant
word. It is located in the 0 bits if a 8086 processor

6. Parity Flag
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In 8086 processors the parity flag indicates if
the number of set bits is odd or even in the binary
representation of the result of the last operation. It
is normally a single bit(2) in a processor status
register. For example, assume a machine where a
set parity flag indicates even parity. If the result of
the last operation were 26 (11010 in binary), the
parity flag would be 0 since the number of set bits
is odd. Similarly, if the result were 102 (1100110 in
binary) then the parity flag would be 1.

7. Adjust Flag & Auxilary Flag
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 The Adjust flag (also known as the Auxiliary flag) is a
flag stored in the FLAGS register on all 8086
processors. It is bit 4. It is used to indicate when an
arithmetic carry or borrow has been generated out
of the 4 least significant bits. It is primarily used in
BCD arithmetic.
 Auxiliary flag is set (AF=1) if there is a carry from low
nibble to high nibble or a borrow from a high nibble to
low nibble of the low order 8-bit of a 16-bit
number.(for example, in BCD addition or subtraction.)

8. Zero Flag
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The Zero Flag is a single bit flag located in
6 bit of a 8086 processor. The zero flag is
used to check the result of an arithmetic
operation, including bitwise logical
instructions. It is set if an arithmetic result
is zero, and reset otherwise.

9. Sign Flag
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In a 8086 processor the sign flag or negative
flag is a single bit in a system status (flag) register
used to indicate whether the result of the last
mathematical operation resulted in a value whose
most significant bit was set. In a two's
complement interpretation of the result, the
negative flag is set if the result was negative.
For example, in an 8-bit signed number system, -37
will be represented as 1101 1011 in binary (the most
significant bit is 1), while +37 will be represented as
0010 0101 (the most significant bit is 0).

10. Overflow flag
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In 8086 processors, the overflow flag (sometimes
called V flag) is usually a single bit in a system status
register used to indicate when an arithmetic overflow has
occurred in an operation, indicating that the signed two's-
complement result would not fit in the number of bits used
for the operation (the ALU width).

11. Control Flags
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There are 3 Control flags in 8086 processor.
• Trap Flag
• Interrupt Enable Flag
• Direction Flag

12. Trap flag
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A trap flag permits operation of a processor in single-step
mode. If such a flag is available, debuggers can use it to step
through the execution of a computer program. The 8086
has no instruction to directly set or reset the trap flag..

13. Interrupt Flag
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 Interrupt Flag is a system flag bit in the 8086
processor’s FLAGS register, which determines whether
or not the CPU will handle hardware interrupts.
 The bit, which is bit 9 of the FLAGS register, may be
set or cleared by programs with sufficient privileges, as
usually determined by the Operating System. If the flag
is set to 1, hardware interrupts will be handled. If
cleared (set to 0), such interrupts will be ignored. IF
does not affect the handling of interrupts or software
interrupts generated by the INT instruction.

14. Direction flag
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 The direction flag is a flag that controls the left-to-right or
right-to-left direction of string processing, stored in the FLAGS
register on all 8086 processors. It is bit number 10.
 When it is set to 0 (using the clear-direction-flag instruction
CLD),it means that instructions that auto increment the source
index and destination index (like MOVS) will increase both of
them. In case it is set to 1 (using the set-direction-flag
instruction STD), the instruction will decrease them.
 This flag is used to determine the direction (forward or
backward) in which several bytes of data will be copied from one
place in the memory, to another. The direction is important
mainly when the original data position in memory and the target
data position overlap.

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Thank You!!