How does an interrupt work?
   The 8086 reserves the lowest 1024 bytes of memory for a table 
   containing the addresses for each of the 256 possible 
   interrupts.  When an interrupt occurs (hardware or software), 
   the processor multiplies its number by 4 and looks at the 
   resulting memory location to find the address of the piece of 
   code which handles the interrupt.  It then places the current 
   address in the program and the processor flags on the stack, 
   and jumps to the beginning of the interrupt handler.

   When the interrupt handler finishes, it invokes a special 
   instruction to return from the interrupt.  This instruction 
   takes the previously saved flags and program address off of 
   the stack and places them back in the appropriate registers in 
   the CPU.

   The interrupt handler has to be careful to preserve any 
   registers that it uses which are not used to communicate 
   results to the program that invoked the interrupt.  If the 
   interrupt can be triggered by a hardware interrupt (only 
   certain ones can on IBM PC's, XT's, and AT's), then the 
   interrupt handler has to preserve ALL registers, since the 
   interrupt could have happened anywhere.


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