Chapter 11 Interrupt Interface of the
8088 and 8086 Microcomputer
• 11.1 Interrupt Mechanism, Type, and
Priority
• 11.2 Interrupt Vector Table
• 11.3 Interrupt Instructions
• 11.4 Enabling/Disabling of Interrupts
• 11.5 External Hardware-Interrupt Interface
Signals
• 11.6 External Hardware-Interrupt Sequence
Chapter 11 Interrupt Interface of the
8088 and 8086 Microcomputer
• 11.7 82C59A Programmable Interrupt
Controller
• 11.8 Interrupt Circuit Using the 82C59A
• 11.9 Software Interrupts
• 11.10 Nonmaskable Interrupt
• 11.11 Reset
• 11.12 Internal Interrupt Functions
11.1 Interrupt Mechanism, Type, and
Priority
• Interrupts provide a mechanism for quickly
changing program environment.
• Transfer of program control is initiated by
the occurrence of either an event internal to
the microprocessor or an event in its
external hardware.
11.1 Interrupt Mechanism, Type, and
Priority
• When an interrupt signal occurs in external hardware
indicating that an external device, such as a printer,
requires service, the MPU must suspend what it is
doing in the main part of the program and pass control
to a special routine (interrupt-service routine) that
performs the function required by the device. In the
case of our example of a printer, the routine is usually
called the printer driver, which is the piece of
software when executed drives the printer output
interface.
Interrupt mechanism
• The 8088 and 8086 microcomputers are
capable of implementing any combination
of up to 256 interrupts.
• As Fig. 11-2, they are divided into five
groups: external hardware interrupts,
nonmaskable interrupt, software interrupts,
internal interrupts, and reset.
Figure 11.2 Types of interrupts and their
priority
Interrupt mechanism
• The user defines the function of the external
hardware, software, and nonmaskable
interrupt. For instant, hardware interrupt are
often assigned to devices such as the
keyboard, printer, and timers. On the other
hand, the function of the internal interrupts
and reset are not user defined. They perform
dedicated system functions.
Interrupt mechanism
• Hardware, software, and internal interrupts are
serviced on a priority basis. Priority is achieved in
two ways. First, the interrupt-processing sequence
implemented in the 8088/8086 tests the occurrence
of the various groups based on the hierarchy
shown in Fig. 11-2. Thus, we see that internal
interrupts are the highest-priority group, and the
external hardware interrupts are the lowestpriority group.
Interrupt mechanism
• Second, each of the interrupts is given a different
priority level by assigning it a type number. Type
0 identifies the highest-priority interrupt, and type
255 identifies the lowest-priority interrupt.
Actually, a few of the type numbers are not
available for use with software or hardware
interrupts. This is because they are reserved for
special interrupt functions of the 8088/8086, such
as internal interrupts.
Interrupt mechanism
• For instant, within the internal interrupt
group, the interrupt known as divide error is
assigned to type number 0. Therefore, it has
the highest priority of the internal interrupts.
Another internal interrupt, called overflow,
is assigned the type number 4, Overflow is
lowest-priority internal interrupt.
Interrupt mechanism
• The importance of priority lies in the fact
that, if an interrupt-service routine has been
initiated to perform a function assigned to s
specific priority level, only devices with
higher priority are allowed are allowed to
interrupt the active service routine. Lowerpriority devices will have to wait until the
current routine is completed before their for
service can be acknowledged.
Interrupt mechanism
• For hardware interrupts, this priority
scheme is implemented in external
hardware. For this reason, the user normally
assigns tasks that must not be interrupted
frequently to higher-priority levels and
those can not be interrupted to lowerpriority levels.
Interrupt mechanism
• An example of a high-priority service
routine that should not be interrupted is that
for power failure. Once initiated, this
routine should be quickly run to completion
to assure that the microcomputer goes
through an orderly power-down.
Interrupt mechanism
• A keyboard should also be assigned to a
high-priority interrupt. This will assure that
the keyboard buffer does not get full and
lock out additional entries. On the other
hand, device such as the floppy disk or hard
disk controller are typically assigned to a
lower priority level.
11.2 Interrupt vector table
• An interrupt vector table is used to link the interrupt
type numbers to the location of their service routine
in the program-storage memory.
• Fig. 11-2 contains 256 address pointers (vectors),
which are identified as vector 0 through vector 255.
That is, one pointer corresponds to each of the
interrupt types 0 through 255. These pointers
identify the starting location of their service routines
in program memory. The contents of this table may
be either held as firmware in EPROMs or loaded
into RAM as part of the system initialization routine.
11.2 Interrupt vector table
• Fig. 11-3 starts at address 0000016 and end at
003FE16. This represent the first 1Kbytes of
memory.
• Each of the 256 pointers requires two words (4
bytes) of memory and is always stored at an evenaddress boundary. The higher-addressed word of
the two-word vector is called the base address. IT
identifies the program memory segment in which
the service routine resides.
Figure
11.3
11.2 Interrupt vector table
• For example, the offset and base address for
type number 255, IP255 and CS255, are stored
at word addresses 003FC16 and 003FE16,
respectively. When loaded into the MPU, it
points to the instruction at CS255:IP255
11.2 Interrupt vector table
• Looking more closely at the table in Fig. 11-3, we
find that the first 31 pointers either have dedicated
functions or are reserved. For instance, pointers 1,
2, 3, and 4 are used by the 8088’s and 8086’s
internal interrupts: divide error, single step,
breakpoint, and overflow. Pointer 2 is used to
identify the starting location of the nonmaskable
interrupt’s service routine. The next 27 pointers, 5
through 31, represent a reserved portion of the
pointer table and should not be used.
11.2 Interrupt vector table
• The remainder of the table , the 224 pointers in the
address range 0008016 through 003FF16, is
available to the user for storage of software or
hardware interrupt vectors. These pointers
correspond to type number 32 through 255. In the
case of external hardware interrupts, each type
number (priority level) is associated with an
interrupt input in the external interrupt interface
circuitry.
Ex 11.1
11-3 Interrupt instructions
• The first interrupt instructions, STI and CLI,
permit manipulation interrupt flag through
software. STI stands for set interrupt enable
flag. Execution of this instruction enables
the external interrupt request (INTR) input
for operation-that is, it sets interrupt flag
(IF). On the other hand, execution of CLI
(clear interrupt enable flag) disables the
external interrupt input by resetting IF.
11.3 Interrupt Instructions
Figure 11.4
11.External
HardwareInterrupt
Interface
Siganl
Figure 11.5
Figure 11.6
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Figure 11.12
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11.15
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Figure
11.16
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Figure 11.17
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Figure 11.24