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  Before leaping into the following chapters, we should discuss the design
  philosophy behind the PC family. This will help you understand what is
  (and what isn't) important or useful to you.

  Part of the design philosophy of the IBM personal computer family centers
  around a set of ROM BIOS service routines (see Chapters 8 through 13)
  that provide essentially all the control functions and operations that IBM
  considers necessary. The basic philosophy of the PC family is: Let the ROM
  BIOS do it; don't mess with direct control. In our judgment, this is a
  sound idea that has several beneficial results. Using the ROM BIOS
  routines encourages good programming practices, and it avoids some of the
  kludgy tricks that have been the curse of many other computers. It also
  increases the chances of your programs working on every member of the PC
  family. In addition, it gives IBM more flexibility in making improvements
  and additions to the line of PC computers. However, it would be naive for
  us to simply say to you, "Don't mess with direct control of the hardware."
  For good reasons or bad, you may want or may need to have your programs
  work as directly with the computer hardware as possible, doing what is
  colorfully called "programming down to the bare metal."

  Still, as the PC family has evolved, programmers have had the opportunity
  to work with increasingly powerful hardware and system software. The newer
  members of the PC family provide faster hardware and better system
  software, so direct programming of the hardware does not necessarily
  result in significantly faster programs. For example, with an IBM PC
  running DOS, the fastest way to display text on the video display is to
  use assembly-language routines that bypass DOS and directly program the
  video hardware. Video screen output is many times slower if you route it
  through DOS. Contrast this with a PC/AT or PS/2 running OS/2, where the
  best way to put text on the screen is to use the operating system output
  functions. The faster hardware and the efficient video output services in
  OS/2 make direct programming unnecessary.

  As you read the programming details we present in this book, keep in mind
  that you can often obtain a result or accomplish a programming task
  through several means, including direct hardware programming, calling the
  ROM BIOS, or using a DOS service. You must always balance portability,
  convenience, and performance as you weigh the alternatives. The more you
  know about what the hardware, the ROM BIOS, and the operating system can
  do, the better your programs can use them.