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  An interrupt is an indication to the microprocessor that its immediate
  attention is needed. The 8086-family microprocessors can respond to
  interrupts from either hardware or software. A hardware device can
  generate an interrupt signal that is processed by the programmable
  interrupt controller (PIC) and passed to the microprocessor; in software,
  the INT instruction generates an interrupt. In both cases, the
  microprocessor stops processing and executes a memory-resident subroutine
  called an interrupt handler. After the interrupt handler has performed its
  task, the microprocessor resumes processing at the point the interrupt
  occurred.

  The 8086 supports 256 different interrupts, each identified by a number
  between 00H and FFH (decimal 255). The segmented addresses of the 256
  interrupt handlers are stored in an interrupt vector table that starts at
  0000:0000H (that is, at the very beginning of available memory). Each
  interrupt vector is 4 bytes in size, so you can locate the address of any
  interrupt handler by multiplying the interrupt number by 4. You can also
  replace an existing interrupt handler with a new one by storing the new
  handler's segmented address in the appropriate interrupt vector.