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  One secret of successful programming for the PC family lies in the
  effective use of the software that is built right into the machine: the
  ROM BIOS services. Conceptually, the ROM BIOS services are sandwiched
  between the hardware and the high-level languages (including the operating
  system). They work directly with the computer's hardware and peripheral
  devices, performing some of the system's most fundamental tasks, such as
  reading and writing individual bytes of data to the display screen or
  disk. DOS services and programming-language services are often built from
  these basic functions and enhanced to make a particular process more
  efficient. You can enhance your programs in the same way by plugging them
  directly into the ROM BIOS, thereby gaining access to an extremely
  powerful set of tools and using your computers in the way that IBM
  intended them to be used.

  That last point is worth emphasizing. IBM has gone to considerable lengths
  to create a clean and well-defined method for directing the operation of
  the computer through the ROM BIOS services. As each new PC model is
  designed, IBM (and any other computer maker who is faithfully extending
  the PC family) makes sure its ROM BIOS services are thoroughly compatible
  with those of the other members of the family. As long as you control your
  computers through the ROM BIOS, whether directly or indirectly, you are
  safe from any compatibility problems. If you bypass the ROM BIOS and
  program directly to the hardware, you are not only asking for trouble, but
  you are also severely limiting the range and viability of your programs.

  That's not to say that you should always use ROM BIOS services when
  they're available. The input/output functions provided in DOS and in
  high-level programming languages often provide the same services as the
  ROM BIOS, but in a form that is easier to use within your programs.
  However, when a program needs more direct access to the computer's
  input/output devices than DOS or your programming language can provide,
  the ROM BIOS services are usually the answer.

  The next five chapters discuss the ROM BIOS service routines. Fortunately,
  the routines fall naturally into groups derived from the hardware devices
  they support, so the video services, disk services, and keyboard services
  can all be reviewed separately. But before you take a closer look at the
  individual services, you need to find out how to incorporate them into
  your programs. This chapter sets the stage by explaining what goes into
  writing an interface routine, the bridge between programming languages and
  the ROM BIOS services. First, a word on how the ROM BIOS operates.