IRQ conflict with parport and other devices (Re: Wireless issues...)

John dingo at
Mon Sep 20 03:09:02 UTC 2004

Matt Zimmerman wrote:

>On Mon, Sep 20, 2004 at 09:53:35AM +0800, John wrote:
>>If lp is loaded, it should be using IRQ 7 or (I think) 5, depending on 
>>which I/O port it's using.
>>As I've already stated, there are generally BIOS settings to workaround 
>>software that can't cope with this, but I do not believe it's a BIOS 
>>It _can_ arise if you install a conflicting ISA device, but ISA PnP and 
>>expecially PCI should never conflict with this. That's why one of my 
>>suggestions was to change the PnP OS setting. _I_ always have it turned 
>>on for Linux, but while it works for the hardware I use it might not 
>>work for others.
>Are you saying that it should be possible for PCI devices to share IRQ7 with
>the parallel port?  If so, it could be argued that Linux is at fault here,
>but my impression was that this is not the case.  The BIOS is assigning
>IRQ7 to PCI devices when it is also in use by the parallel port.
I'm saying it's never possible to share IRQ 7. If anything tries to, 
then as soon as two (or more) of those devices become active there will 
be lost interrupts.

I've not noticed anything in the discussion to the effect the BIOS is 
making the conflicting assignment, but if it is then configuring PnP OS 
(or Windows) should fix it.

There's also a page in many BIOS setups where you can explicitly reserve 
IRQs so nothing but the device you specify can use it. The acronym ICU 
may appear on the page.

I'm being a little short on specifics because the details vary wildly 
between AMI, Award, Phoenix & other vendors' BIOSes, and sometimes 
between versions of the same vendors' BIOS.

IRQ is for the printer. That was so in the original PC, and AFAIK 
nothing has changed that.

I've had a peek at my Gateway.

Two printer settings:
378   7     3
278   5     1

The first is the usual primary printer (LPT1: in DOS)

There's also a page, "Resource Allocation" where one may reserve areas 
of RAM (for ISA network cards' boot ROMs I think) and IRQs for ISA devices.

Normally this stuff happens automatically, but when it doesn't you can 
use this page to prevent automatic assignments of IRQs.

Remember that the onboard serial ports and printer ports are ISA 
devices. Not even PnP.

You _can_ get by with polling the printer port, and I know OS/2 when I 
used it and Linux distroes I've used until quite recently have done just 
that, but the whole idea of using interrupts is to reduce CPU usage 
while improving performance.

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