List the predefine software interrupts available in 8086.

Software Interrupts: Some instructions are inserted at the desired position into the program to create interrupts. These interrupt instructions can be used to test the working of various interrupt handlers. It includes:

INT - Interrupt instruction with type number: It is 2-byte instruction First byte provides the op-code and the second byte provides the interrupt type number. There are 256 interrupt types under this group its execution includes the following steps –

·    Flag register value is pushed on to the stack.

·    CS value of the return address and IP value of the return address are pushed on to the stack.

·    IP is loaded from the contents of the word location type number × 4

·    CS is loaded from the contents of the next word location.

·    Interrupt Flag and Trap Flag are reset to 0

The starting address for type 0 interrupt is 000000H, for type 1 interrupt is 00004H similarly for type 2 is 00008H and ... so on. The first five pointers are dedicated interrupt pointers. i.e. –

·    TYPE 0: interrupt represents division by zero situation.

·    TYPE 1: interrupt represents single step execution during the debugging of a program.

·    TYPE 2: interrupt represents non-mask able NMI interrupt.

·    TYPE 3: interrupt represents break-point interrupt.

·    TYPE 4: interrupt represents overflow interrupt.

The interrupts from Type 5 to Type 31 are reserved for other advanced microprocessors, and interrupts from 32 to Type 255 are available for hardware and software interrupts.

INT 3-Break Point Interrupt Instruction: It is a 1-byte instruction having op-code is CCH. These instructions are inserted into the program so that when the processor reaches there, then it stops the normal execution of program and follows the break-point procedure. Its execution includes the following steps -

·    Flag register value is pushed on to the stack.

·    CS value of the return address and IP value of the return address are pushed on to the stack.

·    IP is loaded from the contents of the word location 3 × 4 = 0000CH

·    CS is loaded from the contents of the next word location.

·    Interrupt Flag and Trap Flag are reset to 0

INTO - Interrupt on overflow instruction: It is a 1-byte instruction and their mnemonic INTO. The op-code for this instruction is CEH. As the name suggests it is a conditional interrupt instruction, i.e. it is active only when the overflow flag is set to 1 and branches to the interrupt handler whose interrupt type number is 4. If the overflow flag is reset then, the execution continues to the next instruction.

Its execution includes the following steps –

·    Flag register values are pushed on to the stack.

·    CS value of the return address and IP value of the return address are pushed on to the stack.

·    IP is loaded from the contents of word location 4 × 4 = 00010H

·    CS is loaded from the contents of the next word location.

·    Interrupt flag and Trap flag are reset to 0.