INT N : Interrupt Type N:-

In the interrupt structure of 8086/8088, 256 interrupts are distinct equivalent to the types from OOH to FFH. When an instruction INT N is executed, the TYPE byte N is multiplied by value 4 and the contents of IP and CS register of the interrupt  service routine will be taken from hexadecimal multiplication (Nx4) as offset address and 0000 as the segment address. In other terms, the multiplication of type N by value 4 (offset) points to a memory block in the 0000 segment, which have the IP and CS register values of the interrupt service routine.

For the execution of this instruction, the IF ought to be enabled.

Example :

Therefore the instruction INT 20H will find out the address of the interrupt service routine as follows:

INT       20H

Type* 4 = 20 * 4 = 80H

Pointer to CS and IP of the ISR is 0000: 0080 H

Given figure shows the arrangement of CS and IP  register addresses of the ISR in the interrupt vector table.

Posted Date: 10/12/2012 4:06:18 AM | Location : United States

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