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  This service lets a device driver alert an operating system to the
  beginning of an input or output operation. An operating system's interrupt
  15H handler processes this information (for example) by preventing
  subsequent input/output to the device until the device signals, with
  service 91H, that it is no longer busy.

  The ROM BIOS device drivers for disks, the keyboard, and the printer all
  issue appropriate service 90H (decimal 144) interrupts. Each device is
  identified by a value in register AL. (See Figure 12-13.) These ID values
  are selected according to the following guidelines:

  .  00H-7FH: Non-reentrant devices that can process only one I/O request at
     a time sequentially.

  .  80H-BFH: Reentrant devices that can handle multiple I/O requests at
     once.

  .  C0H-FFH: Devices that expect the operating system to wait for a
     predetermined period of time before returning control to the device.
     The operating system's interrupt 15H handler must set the carry flag to
     indicate that the wait has been carried out.

  Value                    Meaning
  --------------------------------------------------------------------------
  00H                      Fixed disk
  01H                      Diskette
  02H                      Keyboard
  03H                      Pointing device
  80H                      Network
  FCH                      PS/2 fixed-disk reset
  FDH                      Diskette-drive motor start
  FEH                      Printer
  --------------------------------------------------------------------------

  Figure 12-13.  Device identification values for interrupt 15H, services
  90H and 91H.