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  The IBM PC family, like all computers based on the Intel 8086 family of
  microprocessors, is controlled largely through the use of interrupts,
  which can be generated by hardware or software. The BIOS service routines
  are no exception; each is assigned an interrupt number that you must call
  when you want to use the service.

  When an interrupt occurs, control of the computer is turned over to an
  interrupt-handling subroutine that is often stored in the system's ROM (a
  BIOS service routine is nothing more than an interrupt handler). The
  interrupt handler is called by loading its segment and offset addresses
  into registers that control program flow: the CS (code segment) register
  and the IP (instruction pointer) register--together known as the CS:IP
  register pair. Segment addresses that locate interrupt handlers are called
  interrupt vectors.

  During the system start-up process, the BIOS sets the interrupt vectors to
  point to the interrupt handlers in ROM. The interrupt vector table starts
  at the beginning of RAM, at address 0000:0000H. (See Chapter 2 for more
  about interrupts and interrupt vectors.) Each entry in the table is stored
  as a pair of words, with the offset portion first and the segment portion
  second. The interrupt vectors can be changed to point to a new interrupt
  handler simply by locating the vector and changing its value.

  As a general rule, PC-family interrupts can be divided into six
  categories: microprocessor, hardware, software, DOS, BASIC, and general
  use.

  Microprocessor interrupts, often called logical interrupts, are designed
  into the microprocessor. Four of them (interrupts 00H, 01H, 03H, and 04H)
  are generated by the microprocessor itself, and another (interrupt 02H,
  the nonmaskable interrupt) is activated by a signal generated by certain
  hardware devices, such as the 8087 math coprocessor.

  Hardware interrupts are built into the PC hardware. In PCs, XTs, and PS/2
  models 25 and 30, interrupt numbers 08H through 0FH are used for hardware
  interrupts; in ATs and PS/2 models 50, 60, and 80, interrupt numbers 08H
  through 0FH and 70H through 77H are reserved for hardware interrupts. (See
  Chapter 2 for more about hardware interrupts.)

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