[lug] IRQ"s and LINUX

Michael J. Pedersen marvin at netinfra.net
Tue Nov 16 22:23:00 MST 1999

On Mon, Nov 15, 1999 at 03:10:01PM -0700, David Willcox wrote:
> Does anyone happen to know what IRQ's a mouse can be set at in linux besides
> ttyS0, ttyS1 because I need COM1 for a Multiport Digiboard and COM2 for a
> Modem. Now during install what should I select for a serial mouse or would you
> suggest a bus mouse?

Well, strictly speaking, a mouse can be set at any irq that you can set the
serial port to.  However, there are some common misconceptions about mice and
windows, which may be part of your question.  I'm going clear them up for
anybody that may be interested.  Hopefully, you're one of them :)

Every device in the computer actually has two "addresses" (or more, but com
ports only use two) for it.  The first part is the irq, and the second part is
the memory area which is used to talk to the device.  The layout for the com
ports (default) looks like this:

Port	Address		IRQ
COM1	0x3F8		4
COM2	0x2F8		3
COM3	0x3E8		4
COM4	0x2E8		3

This arrangement was used at a time when 300 baud modems were the norm, and
2400 was blindingly fast. As such, it worked, and worked well.  Then came
Windows, and everything went to shit.  People configured their mouse for COM1,
their internal modem for COM3, and found out that they couldn't use their
mouse at the same time as their modem.  When they switched it to COM4, the
problem went away.  Or at least, looked like it did.  What they didn't realize
was that they couldn't use a device on COM2 at the same time as their modem,
since COM2 and COM4 share the irq.  COM1 and COM3 share an irq as well, so
that was the source of the lockup when people went to windows.

Modem manufacturers wised up to this problem, and began shipping modems that
could be assigned to different irq's than the standard.  Using jumpers, the
user was able to use a modem on com3, and a mouse on com1, and never have a
conflict.  Then came plug and play, which supposedly removed the need for
jumpers (personally, I've found jumpers to be still easier to use than the
crappy pnp stuff I've seen so far).

Now, here's where your question actually gets answered: The easiest option you
will have is to configure the digiboard for com3 and up, using IRQ's other
than 3 and 4. You can then plug your modem in on com2, and your mouse in on

If you have a special requirement which places the digiboard on com1, simply
go and get a multi-i/o board which has jumpers (I've seen them up at CompUSA
for about $30), and configure a com3 on irq 5 (assuming you don't have a sound
card on irq 5).  Once that's done, use the setserial command (man setserial.
Sorry, I haven't used it in a while, so can't help you parse it) to tell linux
that com3/ttyS2 is on irq 5, and you're set to go.  Tell any program which
needs it that the mouse is on that serial port, and it should be fine and

Yes, I'm using the dos device names, and I do apologize for that. They're just
more familiar to many linux newbies, which is where I tried to aim my little
dissertation :)

> Also while were Not on the subject what would be the best Backup Device I
> could use with Linux 5.0 - Linux 6.1 for stoarage for 200MB for some installs and
> 400MB for others but I would like to try to stay with the same device for multiple
> business installs. I like the 'tar' command for years in Xenix. Do you think I would
> be jumping in to soon for a Burner or are thay hard to get installed. I don't have a
> lot of time, about 45 days for 9 installs.

Well, hardware wise, a simple scsi tape drive would be the easiest to make
work for you.  tar will work beautifully with it (I can promise you that,
since I've used it many times myself).

As far as cd burners go, I've not had the chance to install any, so I don't
know how difficult they are to install.

However, here's another option for you to consider: The program amanda is a
setup which can back up windows clients, and linux clients.  A decent sized
tape drive attached to one server, with clients on all the other machines,
should let you do centralized backups (always a good thing).  Will these 9
machines be networked together in any special way, or are they all on separate
installs for separate companies?

If you need help, please feel free to contact me.  9 installs in 45 days is a
very doable task for me, so I can definitely help out on weekends at least.

Michael J. Pedersen
WhoDP: whodp://earth.activerse.com/pedersen
Check out Ding! at http://www.activerse.com
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