An interrupt is a special condition that arises during the working of a microprocessor. The microprocessor services it by executing a subroutine called Interrupt Service Routine (ISR).

Dedicated interrupts:

1. Type 0: Divide by Zero Interrupt

   • 8086 supports division (unsigned/signed) instruction. 8086 will automatically do a type 0 interrupt if the result of DIV or IDIV operation is too large to fit in destination register.
   • When type 0 interrupt is internally generated, microprocessor will
     • Push flag register.
     • Reset TF and IF.
     • Push CS and IP (i.e. return address).
     • Get NEW CS and NEW IP. For this, microprocessor takes type no i.e. ‘0’, multiply by 4. Therefore we get 0 x 4 = 000H. So microprocessor gets NEW IP from 00000H/00001H location and NEW CS from 00002H/00003H location.
     • NEW CS and NEW IP will be loaded into the CS and IP register. Thus we get branching to ISR routine.
     • After returning from ISR, microprocessor will pop CS and IP (OLD CS/OLD IP). Microprocessor will also pop flag register.
   • Type 0 is automatic and cannot be disabled i.e. non mask-able.
   • Users have to account it in the program where he/she uses DIV/IDIV instruction.
   • Normally user will write an interrupt service procedure which takes desired action when an invalid division occurs.
   • To avoid this interrupt, user can check before division that divisor is not zero.

2. Type 1: Single Step Interrupt (INT1)

   • The microprocessor executes this interrupt after every instruction if the TF is set.
   • It puts microprocessor in single stepping mode i.e. the microprocessor pauses after executing every instruction. This is very useful during debugging.
   • Its ISR generally displays contents of all registers. Its ISR address is stored at location 1 x 4 = 00004H in the IVT.

3. Type 2: NMI (Non Mask-able Interrupt) (INT2)

   • This is the highest priority hardware interrupt and is non mask-able. The input is edge triggered but is synchronized with the CPU clock and must be active for two clock cycles to generate recognition.
   • The interrupt signal may be removed prior to entry to the service routine.
   • Since the input must make a LOW to HIGH transition to generate an interrupt, spurious transition on the input should be suppressed.
   • If the input is normally HIGH, the NMI low time to guarantee triggering is two CPU clock times.
   • Its ISR address is stored at location 2 x 4 = 00008H in the Interrupt Vector Table (IVT).
   • Basically NMI interrupt input is used for catastrophic failures for example power failure, time out of system watchdog timer.

4. Type 3: One Byte Interrupt/Breakpoint Interrupt (INT3)

   • This type is invoked by a special form of the software interrupt instruction which requires a single byte of code space i.e. CCH (INT3).
   • This interrupt is primarily used as a breakpoint interrupt for software debug.
   • When we insert a breakpoint in program, the system executes instructions up to the breakpoint and then goes to the breakpoint procedure.
   • When user informs debugger program to insert breakpoint at some point in program, they actually do it by temporarily replacing the instruction byte at that address with CCH i.e. code for INT3 instruction.
   • Thus this single byte instruction can be mapped into the smallest instruction for absolute resolution in setting breakpoints.
   • Its ISR address is stored at location 3 x 4 = 0000CH in the IVT.
   • A breakpoint ISR routine usually saves all the register contents on the stack.

5. Type 4: Interrupt on Overflow (INTO)

   • This interrupt occurs if the overflow flag (OF) is set in the flag register.
   • The OF flag is set if the signed result of an arithmetic operation on two signed number is too large to be represented in destination register or memory location. Thus this interrupt is used to capture overflow errors.
   • Its ISR address is stored at location 4 x 4 = 00010H in the IVT.