The device driver does not attempt to initialize or support a port if it does not get the INIT request packet for the port's corresponding device name. If the physical device driver gets the INIT request packet for a given device name, it checks to see if a valid I/O address is in the BIOS 40: data area that corresponds to that device name. COM1 is in 40:0 and COM2 is in 40:2. If the 40: area does not have a valid I/O address, the physical device driver fails the port and will not support the port. Otherwise, the device driver attempts to get exclusively the interrupt level that corresponds to the I/O address for the port. If the interrupt level is not available, the physical device driver fails to initialize the port and will not support the port.

If the interrupt level is available, the physical device driver relinquishes the interrupt level. The physical device driver also initializes the port and sets up support for the port during this startup of the operating system.

In summary, in order for the physical device driver to support a port, the following must be TRUE:

• The device driver must get an INIT request packet for the device name.
  
• The 40: area that corresponds to the device name must have a valid I/O address.
  
• The appropriate interrupt level must be available for exclusive use, even though the physical device driver will not claim the interrupt level for exclusive use during initialization.

The physical device driver claims ownership of the port by not deinstalling the corresponding device name. Another device driver can cause this device driver not to claim a port by initializing before this device driver, and by doing at least one of the following:

• Not allowing this physical device driver to receive an INIT request packet for a given device name 
  Putting an invalid I/O address in the corresponding 40: area (for example, 0)
  
  Exclusively owning the appropriate interrupt level at initialization time

  The device driver does not attempt to initialize or support a port if it does not get the INIT request packet for the port's corresponding device name. If the physical device driver gets the INIT request packet for a given device name, it attempts to claim ownership of the specific LID position for the ASYNC Device ID that corresponds to the device name being initialized.

  For Micro Channel bus machines, if the LID is not available, the physical device driver fails to initialize the port and does not support the port. If the LID is available, the physical device driver initializes the port and sets up support for the port during this startup of the operating system. The LID for the port is relinquished; it is reclaimed during Open processing.

  For the AT-bus machine, COM.SYS still installs even though the LID is not present.

  In summary, for the physical device driver to support a port on an IBM PS/2 computer, the following must be TRUE:

  • The physical device driver must get an INIT request packet for the device name. 
    The ASYNC LID corresponding to the device name must be available.

    The physical device driver claims ownership of the port by not deinstalling the corresponding device name. Another device driver can cause this device driver not to claim a port by initializing before this device driver and doing at least one of the following:

    • Not allowing this device driver to receive an INIT request packet for a given device name 
      Claiming the appropriate ASYNC LID

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