Microprocessor flags

2.  The rightmost five flags and the overflow
flag change after many arithmetic and
logic instructions execute.
 flags never change for any data transfer
or program control operation
 Some of the flags are also used to
control features found in the
microprocessor.
15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
- NT IOPL OF DF IF TF SF ZF - AF - PF - CF

3. Carry Flag (Bit 0)
 holds the carry after addition or the
borrow after subtraction.
 indicates error conditions, as dictated by
some programs and procedures.

4. Parity Flag (Bit 2)
 PF = 1 even number of 1’s
 PF = 0 odd number of 1’s

5. Auxiliary Flag (Bit 4)
 holds the carry (half-carry) after addition or
the borrow after subtraction between bits
positions 3 and 4 of the result
Addition:
Bit 3 – Bit4
AF = 1 > Carry
AF = 0 > No Carry
Subtraction:
Bit 4 – Bit 3
AF = 1 > Borrow
AF = 0 > No Borrow

6. Zero Flag (Bit 6)
 shows that the result of an arithmetic or
logic operation is zero
 memory or register is equal to zero, Z=1
 Z = 0, the result is not zero or greater
than zero

7. Sign Flag (Bit 7)
 2’s complement
 Sign bit = 1, negative # in 2’s
complement

8. Trap Flag
 enables trapping through an on-chip-
debugging feature
 T =1, microprocessor interrupts the flow
of the program on conditions as
indicated by the debug registers and
control registers.
 T=0, the trapping (debugging) feature is
disabled.

9. Interrupt Flag
 controls the operation of the INTR
(interrupt request) input pin
 IF=1, INTR is enabled
 IF=0, INTR pin is disabled
 state of the flag bit is controlled by the
STI (set I flag) and CLI (clear I flag)
instructions.

10. Direction Flag
 selects either the increment or
decrement mode for the DI and/or SI
registers during string instructions
 D=1, registers decrement (Backward
Branching)
 D=0, registers increment (Forward
Branching)
 set with the STD (set direction)
 cleared with the CLD (clear direction)
instructions

11. Overflow Flag
 occurs when signed numbers are added
or subtracted
 indicates that the result has exceeded
the capacity of the machine.
 For signed numbers
OF = 1, exceed the limit
OF = 0, did not exceed the limit

12. INPUT/OUTPUT PRIVILEGE LEVEL
 Priority of current task

13. NESTED TASK
 indicates if current task is nested