Windows NT Parallel Ports Do Not Use Interrupts

In Windows NT, parallel ports do not use any hardware interrupt request lines (IRQs).

The Windows NT parallel port driver is thread-based, and the port is polled at background priority. This eliminates a number of potential hardware conflicts with other devices that might want to use IRQ 7 or 5, that used to be the parallel port interrupts.

If you have applications that run very slow due to large print jobs, you can reduce the thread priority of the port print function by adding the following registry entry:

WARNING: Using the Registry Editor incorrectly can cause serious problems that may require you to reinstall your operating system. Microsoft cannot guarantee that problems resulting from the incorrect use of Registry Editor can be solved. Use Registry Editor at your own risk.

Windows NT 3.51

1. Start Registry Editor (Regedt32.exe) and locate the following key:
   
   HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Print
2. Add the following value:
   
   

         Key:   PortThreadPriority
         Type:  REG_DWORD
         Value: Thread_Priority_Below_normal
   						

3. Quit Registry Editor.

NOTE: If you start OS/2 or MS-DOS, you still need to check for IRQ conflicts; the Windows NT port driver will not be used.

Windows NT 4.0

1. Start Registry Editor (Regedt32.exe), and locate the following key:
   
   HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Print
2. Add or change the following value:
   
   

         Key:   PortThreadPriority
         Type:  REG_DWORD
         Value: -1
   						

3. Quit Registry Editor.

The default value in Windows NT 4.0 for PortThreadPriority is 0, which equals THREAD_PRIORITY_ NORMAL. The value for THREAD_PRIORITY_ABOVE_NORMAL is 1 and THREAD_PRIORITY_BELOW_NORMAL is -1.