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  It takes software to make a computer go. And getting a computer going and
  keeping it going is much easier if some of that software is permanently
  built into the computer. That's what the ROM programs are all about. ROM
  stands for read-only memory--memory permanently recorded in the circuitry
  of the computer's ROM chips, that can't be changed, erased, or lost.

  PCs and PS/2s come with a substantial amount of ROM that contains the
  programs and data needed to start and operate the computer and its
  peripheral devices. The advantage of having a computer's fundamental
  programs stored in ROM is that they are right there--built into the
  computer--and there is no need to load them into memory from disk the way
  that DOS must be loaded. Because they are permanent, the ROM programs are
  very often the foundation upon which other programs (including DOS) are
  built.

  There are four elements to the ROM in IBM's PC family: the start-up
  routines, which do the work of getting the computer started; the ROM
  BIOS--an acronym for Basic Input/Output System--which is a collection of
  machine-language routines that provide support services for the continuing
  operation of the computer; the ROM BASIC, which provides the core of the
  BASIC programming language; and the ROM extensions, which are programs
  that are added to the main ROM when certain optional equipment is added to
  the computer. We'll be examining each of these four major elements
  throughout the rest of this chapter.

  The ROM programs occupy addresses F000:0000H through F000:FFFFH in the
  PC/XT/AT family and the PS/2 models 25 and 30, and E000:0000H through
  F000:FFFFH in the other PS/2s. However, the routines themselves are not
  located at any specific addresses in ROM as they are in other computers.
  The address of a particular ROM routine varies among the different members
  of the PC/XT/AT and PS/2 families.

  Although the exact addresses of the ROM routines can vary, IBM provides a
  consistent interface to the ROM software by using interrupts. Later in
  this book we'll show you exactly how to use interrupts to execute the ROM
  routines.