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  In this chapter we'll describe how to communicate with DOS through
  interrupts. (See Figure 15-1.) DOS reserves all 32 interrupt numbers from
  20H through 3FH (decimal 32 through decimal 63) for its own use. DOS
  provides system services through five of these interrupts (20H, 21H, 25H,
  26H, and 27H). These interrupts can be called directly from a program with
  the INT instruction. DOS uses the interrupt vectors for four others (22H,
  23H, 24H, and 28H) to contain the addresses of routines called by DOS
  itself; you can substitute your own routines for the default DOS routines
  by updating one of these interrupt vectors. Interrupt 2FH is reserved for
  communication between memory-resident programs. The other 22 interrupts
  reserved by DOS are not intended for use in your programs.

  Interrupt  Number
  Hex        Dec        Description
  --------------------------------------------------------------------------
  20H        32         Program Terminate
  21H        33         General DOS Services
  22H        34         Terminate Address
  23H        35         Ctrl-C Handler Address
  24H        36         Critical Error-Handler Address
  25H        37         Absolute Disk Read
  26H        38         Absolute Disk Write
  27H        39         Terminate and Stay Resident
  28H        40         DOS Idle Interrupt
  2FH        47         Multiplex Interrupt
  --------------------------------------------------------------------------

  Figure 15-1.  DOS interrupts.

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  NOTE:
    You can use any of the 10 interrupts described in this chapter in your
    programs. Nevertheless, there is some overlap between the services
    provided through the separate interrupts described in this chapter and
    the functions available through interrupt 21H. When you have a choice,
    use the interrupt 21H functions described in Chapters 16 and 17. We'll
    point out why as we describe each DOS interrupt.
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