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  The ROM BIOS is the part of ROM that is in active use whenever the
  computer is at work. The role of the ROM BIOS is to provide the
  fundamental services that are needed for the operation of the computer.
  For the most part, the ROM BIOS controls the computer's peripheral
  devices, such as the display screen, keyboard, and disk drives. When we
  use the term BIOS in its narrowest sense, we are referring to the device
  control programs--the programs that translate a simple command, such as
  read-something-from-the-disk, into all the steps needed to actually
  perform the command, including error detection and correction. In the
  broadest sense, the BIOS includes not only routines needed to control the
  PC's devices, but also routines that contain information or perform tasks
  that are fundamental to other aspects of the computer's operation, such as
  keeping track of the time of day.

  Conceptually, the ROM BIOS programs lie between programs that are
  executing in RAM (including DOS) and the hardware. In effect, this means
  that the BIOS works in two directions in a two-sided process. One side
  receives requests from programs to perform the standard ROM BIOS
  input/output services. A program invokes these services with a combination
  of an interrupt number (which indicates the subject of the service
  request, such as printer services) and a service number (which indicates
  the specific service to be performed). The other side of the ROM BIOS
  communicates with the computer's hardware devices (display screen, disk
  drives, and so on), using whatever detailed command codes each device
  requires. This side of the ROM BIOS also handles any hardware interrupts
  that a device generates to get attention. For example, whenever you press
  a key, the keyboard generates an interrupt to let the ROM BIOS know.

  Of all the ROM software, the BIOS services are probably the most
  interesting and useful to programmers--as a matter of fact, we have
  devoted six chapters to the BIOS services in Chapters 8 through 13. Since
  we deal with them so thoroughly later on, we'll skip any specific
  discussion of what the BIOS services do and instead focus on how the BIOS
  as a whole keeps track of the computer's input and output processes.