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  All ROM BIOS services are invoked by interrupts. Each interrupt
  instruction selects a particular entry in the interrupt vector table in
  low memory. The addresses of all ROM BIOS service routines are stored in
  this table. This design makes it possible for any program to request a
  service without knowing the specific memory location of the ROM BIOS
  service routine. It also allows the services to be moved around, expanded,
  or adapted without affecting the programs that use the services. Although
  IBM has tried to maintain the absolute memory location of some parts of
  the ROM BIOS, it would be foolish to use these addresses because they may
  change in the future. The standard, preferred, and most reliable way to
  invoke a ROM BIOS service is to use its interrupt rather than its absolute
  address.

  The ROM BIOS services could be supervised by one master interrupt, but
  instead they are divided into subject categories, each with its own
  controlling interrupt. This design lets each interrupt handler be easily
  replaced. For example, if a hardware manufacturer created a radically
  different video display that operated under a completely new ROM BIOS
  program, the manufacturer could provide the new ROM BIOS program along
  with the hardware. The new ROM BIOS program might be stored in RAM, and it
  would replace the one part of IBM's ROM BIOS that was used with the old
  hardware. By making the ROM BIOS modular, IBM has made it easier to
  improve and extend the capabilities of its computers.