Microprocessors And Microcontrollers – Interrupts And Programmable Interrupt Controllers: Microprocessors And Microcontrollers

Contents at a glance:

• 8086 Interrupts and Interrupt Responses
• Introduction to DOS and BIOS interrupts
• 8259A Priority Interrupt Controller

8086 Interrupts and Interrupt Responses:

• Sometimes it is necessary to have the computer automatically execute one of a collection of special routines whenever certain conditions exist within a program or in the microcomputer system.
• For example,, it is necessary that microcomputer system should give response to devices such as keyboard, sensor and other components when they request for service.
• The most common methods for servicing such device are Polling Method Interrupt method

Polling Method:

• In this method the processor must test each device in sequence and in effect ‘ask’ each one if it needs communication with the processor.
• It easy to see that a large portion of the main program is looping through this continuous polling cycle.
• Such a method would have a serious and decrementing effect on system throughput.

Interrupt Method:

• Another method would be the one that allows the microprocessor to execute its main program and only stop to service peripheral devices when it is told to do so by the device itself.
• In this method the processor would provide an external asynchronous input that would inform the processor that it should complete whatever instruction that currently being executed and fetch a new routine that will service the requesting device.
• Once this servicing is completed, the processor would resume exactly where it left off.
• This method is called Interrupt Method.
• This method increases the system throughput
• When a microprocessor is interrupted, it stops executing its current program and calls special routine which services the interrupt.
• The event that causes the interruption is call interrupt and the special routine executed to service the interrupt is called interrupt service routine (ISR) / Procedures.
• An interrupt caused by an external signal is referred as Hardware interrupt.
• A condition interrupts or interrupts caused by special instructions are called software interrupts.

Other type of interrupts are

1. Vectored and Non vectored interrupts

• The interrupts which are having fixed address location for ISR are called vectored interrupts.
• The interrupts which are not having fixed address location for ISR are called non vectored interrupts.

2. Maskable and Non maskable interrupts

• The interrupts which can be ignored are called maskable interrupts.
• The interrupts which cannot be ignored are called non maskable interrupts.
• An 8086 interrupt can come from any one the three sources:
  1. Exernal signal: An 8086 can get interrupt from an external signal applied to the nonmaskable interrut (NMI) input pin, or the interrupt (INTR) input pin.
  2. Special instruction: An execution of the Interrupt instruction (INT). This is referred as software interrupt.
  3. Condition produced by Instruciton: An 8086 is interrupted by some condition produced in the program by the execution of an instruction.
  4. For example divide by zero: program execution will automatically be interrupted if you attempt to divided an operand by zero.
• At the end of each instruction cycle, 8086 checks to see if any interrupts have been requested.