INTERRUPTS OF 8086 MICROPROCESSOR

• An Interrupt is either a Hardware
generated CALL (externally
derived from a hardware signal)
OR
• A Software-generated CALL
(internally derived from the
execution of an instruction or by
some other internal event

 An interrupt is used to cause a temporary
halt in the execution of program.
 The meaning of ‘interrupts’ is to break the
sequence of operation.
 While the Microprocessor is executing a
program, an ‘interrupt’ breaks the normal
sequence of execution of
instructions, diverts its execution to some
other program called Interrupt Service
Routine (ISR).
• After executing ISR, IRET returns the control
back again to the main program. Interrupt
processing is an alternative to polling.

The keyboard controller can hold only a
single keystroke. Therefore, the keyboard
controller must be freed before the next
keystroke arrives.
The keystroke is passed to the CPU
by putting it in the keyboard buffer. So, the
keyboard controller keeps on passing the
keystroke input to the CPU,
but how does the CPU attend to it? The
CPU is not at the disposal of the keyboard
controller; it is usually busy doing several
other operations. So, we need some
mechanism to indicate to the CPU that a
keystroke has arrived. How is this done?
There are two approaches to making sure
that the CPU pays attention:

 The CPU executes other program, as soon as a
key is pressed, the Keyboard generates an
interrupt. The CPU will response to the
interrupt – read the data. After that returns to
the original program. So by proper use of
interrupt, the CPU can serve many devices at
the “same time”

Interrupts are useful when interfacing I/O
devices at relatively low data transfer
rates, such as keyboard inputs.
Interrupt processing allows the processor to
execute other software while the keyboard
operator is thinking about what to type next.
When a key is pressed, the keyboard encoder
debounces the switch and puts out one pulse
that interrupts the microprocessor.

– a time line shows typing on a keyboard,
a printer removing data from
memory, and a program executing
– the keyboard interrupt service
procedure, called by the keyboard
interrupt, and the printer interrupt service
procedure each take little time to execute

TYPES OF INTERRUPT
 SOFTWARE INTERRUPTS: There are instructions in 8086 which
cause an interrupt.
• INT instructions with type number specified.
• INT 3, Break Point Interrupt instruction.
• INTO, Interrupt on overflow instruction.
 HARDWARE INTERRUPTS:
The primary sources of interrupts, however, are the PCs timer
chip, keyboard, serial ports, parallel ports, disk drives, CMOS real-
time clock, mouse, sound cards, and other peripheral devices.

The interrupt vector table contains 256 four
byte entries,containg the CS:IP
Interrupt vectors for each of the 256 possible
interrupts. The table is used to locate the
interrupt service routine addresses for each
of those interrupts.
The Interrupt vector table is located in the
first 1024 bytes of memory at addresses
000000H-0003FFH.It contains the
address(segment and offset)of the interrupt
service provider

The interrupt vector table for the microprocessor and (b) the contents
of an interrupt vector.
– the first five interrupt vectors are identical
in all Intel processors
– Intel reserves the first 32 interrupt vectors
– the last 224 vectors are user-available
– each is four bytes long in real mode and
contains the starting address of the
interrupt service procedure.
– the first two bytes contain the offset
address
– the last two contain the segment address

TYPE 0
The divide error : whenever the results
from a division overflows or an attempt is
made to divide by zero.

Type 2
The non-maskable
interrupt occurs
when a logic 1 is
placed on the NMI
input pin to the
microprocessor. non-
maskable—it cannot
be disabled

Type 3
A special one-byte instruction (INT 3)
that uses this vector to access its
interrupt-service procedure. often
used to store a breakpoint in a
program for debugging

TYPE 4
Overflow is a special vector used with the INTO
instruction. The INTO instruction interrupts the
program if an overflow condition exists.

TYPE 5
The BOUND instruction compares a register
with boundaries stored in the memory. If the
contents of the register are greater than or
equal to the first word in memory and less
than or equal to the second word, no interrupt
occurs because the contents of the register are
within bounds. if the contents of the register
are out of bounds, a type 5 interrupt ensues as
reflected by the overflow flag (OF)

Type 7
The coprocessor not available
interrupt occurs when a
coprocessor is not found, as
dictated by the machine
status word (MSW or CR0)
coprocessor control bits. if an
ESC or WAIT instruction
executes and no coprocessor
is found, a type 7 exception
or interrupt occurs

Type 8
A double fault
interrupt is
activated when
two separate
interrupts
occur during
the same
instruction.

In computing, a double fault
is a serious type of error that
occurs when a central
processing unit (CPU) cannot
adequately handle a certain
type of system event that
requires the CPU’s immediate
attention. Double faults may
cause computer crashes and
error messages, automatic
restarting of the machine, and
the loss of any unsaved data.
They are often caused by
problems in the computer’s
hardware such as a bad
memory module or
overheating CPU.

Type 9
The coprocessor segment overrun
occurs if the ESC instruction
(coprocessor opcode) memory
operand extends beyond offset
address FFFFH in real mode.

 Type 10
An invalid task state segment interrupt
occurs in the protected mode if the TSS
is invalid because the segment limit field
is not 002BH or higher. usually because the
TSS is not initialized
 Type 11
The segment not present interrupt occurs
when the protected mode P bit (P = 0) in
a descriptor indicates that the segment is
not present or not valid.

• Type 12
A stack segment overrun
occurs if the stack segment is
not present (P = 0) in the
protected mode or if the limit of
the stack segment is
exceeded.

Type 13
The general protection fault occurs
for
most protection violations in
80286–Core2 in protected mode
system.
These errors occur in Windows as
general protection faults. A list of
these protection violations follows.

• Type 13
PROTECTION VIOLATIONS
o (a) Descriptor table limit exceeded
o (b) Privilege rules violated
o (c) Invalid descriptor segment type loaded
o (d) Write to code segment that is protected
o (e) Read from execute-only code segment
o (f) Write to read-only data segment
o (g) Segment limit exceeded
o (h) CPL = IOPL when executing
CTS, HLT, LGDT, LIDT, LLDT, LMSW, or LTR
o (i) CPL > IOPL when executing
CLI, IN, INS, LOCK, OUT, OUTS, and STI
(cont.)

Type 14
Page fault interrupts occur for any page
fault memory or code access in
80386, 80486, and Pentium–Core2
processors.
Type 16
Coprocessor error takes effect when a
coprocessor error (ERROR = 0) occurs
for ESCape or WAIT instructions for
80386, 80486, and Pentium–Core2 only.

Type 17
Alignment checks indicate word and
double word data are addressed at an
odd memory location (or incorrect
location, in the case of a double
word).
interrupt is active in 80486 and
Pentium–Core2

Type 18
A machine check activates a
system memory management
mode interrupt in Pentium–
Core2.

INTERRUPTS OF 8086 MICROPROCESSOR

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INTERRUPTS OF 8086 MICROPROCESSOR