Answer to Question #159302 in Assembler for Hasan Khan

Question

Define Interrupt vector and Interrupt vector table.

How the 8086 microprocessor finds out the actual location of an interrupt service routine for an 

interrupt? Describe with proper diagram.

Answer 1

An interrupt is a condition that halts the microprocessor temporarily to work on a different task and then return to its previous task. Whenever an interrupt occurs the processor completes the execution of the current instruction and starts the execution of an Interrupt Service Routine (ISR) or Interrupt Handler. ISR is a program that tells the processor what to do when the interrupt occurs. After the execution of ISR, control returns back to the main routine where it was interrupted.

Each interrupt that requires special processing by the processor is assigned a unique identification number called a vector number

An interrupt vector table (IVT) is a data structure that associates a list of interrupt handlers with a list of interrupt requests in a table of interrupt vectors. Each entry of the interrupt vector table, called an interrupt vector, is the address of an interrupt handler.

Each interrupt type is given a number between 0 to 255 and the address of each interrupt is found by multiplying the type by 4 e.g. for type 11, interrupt address is 11 x 4 = 4410= 0002CH

When the 8086 responds to an interrupt, it automatically goes to the specified location in the Interrupt Vector Table in 8086 to get the starting address of interrupt service routine.