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  This chapter is about the IBM PC and PS/2 keyboards. The first part of
  this chapter explains how the keyboard interacts with the computer on a
  hardware and software level. In the second part, we'll describe how the
  ROM BIOS treats keyboard information and makes it available to programs.

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  NOTE:
    If you plan to play around with keyboard control, we urge you to read
    the comments on page 140 first and then apply the information in this
    chapter to your programs only if you have a reason to do so (for
    example, if you are creating a keyboard-enhancer program to modify the
    operation of the keyboard; see the sidebar on page 133 for more
    information on such programs). If you have any such application in mind,
    take a look at the ROM BIOS keyboard services in Chapter 11.
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  The keyboard has undergone several modifications since the IBM PC was
  released. The original IBM PC keyboard had 83 keys. The PC/AT was
  introduced with an 84-key keyboard that changed the locations of several
  keys on the 83-key keyboard and added one new key, the Sys Req key.

  IBM later upgraded the AT with a 101/102-key keyboard that provided extra
  function keys and a new keyboard layout. The 101/102-key keyboard became
  standard equipment in the PS/2 series. The 101/102-key layout includes two
  extra function keys (F11 and F12), a number of duplicate shift and control
  keys, and modifications to several keys and keyboard combinations found in
  the 83- and 84-key layouts (Pause, Alt-Sys Req, and Print Screen).

  A trend in IBM's keyboard design has been to increase the similarity
  between the PC and PS/2 keyboards and the keyboards on their mainframe
  display terminals. For example, the 101/102-key keyboard's 12 function
  keys (F1 through F12) are reminiscent of the Program Function (PF) keys on
  IBM mainframe display terminals. Similarly, the Sys Req key is like the
  Sys Req key in IBM mainframe terminals: A mainframe terminal-emulator
  program running on a PC or PS/2 could use the Sys Req key for the same
  purpose a mainframe terminal would--to switch among terminal sessions or
  to initiate a keyboard reset function.

  Another trend in IBM's keyboard design has been to accommodate non-English
  alphabets in the keyboard layout. The English-language version of the
  101/102-key keyboard released in the United States and United Kingdom has
  101 keys, but for other languages the same keyboard has an extra key next
  to the left Shift key, a different arrangement of keys around the Enter
  key, and a different map of ASCII characters to key locations. From a
  programmer's point of view, however, these two keyboards are so similar
  that IBM describes them together in its technical documentation--and we'll
  do the same in this chapter.