## Input/Output Device Driver Reference

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### Parallel Port IRQ Performance

The Programmable Interrupt Controller (PIC) under OS/2 2.1, IRQ3 (serial port) has the highest priority in the system. This has given IRQ7 (parallel port) a much higher priority than it would otherwise have. It is now higher in priority than the keyboard or the mouse. A system unit running multiple parallel port devices, that can receive data quickly on a combination of serial/parallel port devices at a high rate of speed (for example, a laser printer with several megabytes of memory) can experience slow user input performance from the mouse and keyboard.

Several alternatives exist to improve user input performance. One alternative, for attended print servers, is to ensure the system unit supports a direct memory access (DMA) parallel or serial port. The parallel port device driver takes advantage of DMA, which reduces the number of hardware interrupts in the system. Fewer high priority interrupts in the system allow the processor to spend more time processing lower priority interrupts.

Another alternative is to convert a parallel port device to run using the serial port and to reduce the bit rate to the serial device. The ASYNC adapter must support extended hardware buffering. For more information on this approach, refer to the Physical ASYNC Communications Device Driver in the OS/2 Physical Device Driver Reference.

A last alternative for those situations where user responsiveness is extremely important is to reduce the number of print files being sent to the print server by moving some of the devices to other system units (thus balancing the workload among several system units available) or to reduce the user installable memory within the device.