Block diagram reduction techniques in control systems.ppt
Cpe103 03 [processor flags]
1.
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