Linux Device Drive Question

Hi all:
I need sm help in understanding how Device Drivers are called, how does the Kernal know which Device Driver to look into when the UserMode appication requests a device to perform some operation.
Eg: If the user program gives a command to the kernal to draw a circle on the screen, the kernal will locate the device driver for the screen and then call the method that draws on the screen.
There are many Drivers installed, when an operation request arrives, how does it decide which Device Driver to use.
Are there any tutorials that i coud look into.
Thanks,
Preetham.

I don't know the answer to your question (sorry :-)) but if I'm not mistaken there's a developer mailinglist (or atleast a mailinglist) on www.kernel.org and I'm sure they're able to tell you the answer pretty quick.

[edit]
http://www.kernel.org/pub/linux/docs/lkml/
should prove useful to you
[/edit]