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7.3.2.3. Module Loading
Device drivers compiled as modules may have aliases built into them. Aliases are visible in the output of the modinfo program and are usually related to the bus-specific identifiers of devices supported by a module. For example, the snd-fm801 driver supports PCI devices with vendor ID 0x1319 and device ID 0x0801, and has an alias of “pci:v00001319d00000801sv*sd*bc04sc01i*”. For most devices, the bus driver exports the alias of the driver that would handle the device via sysfs. E.g., the /sys/bus/pci/devices/0000:00:0d.0/modalias file might contain the string “pci:v00001319d00000801sv00001319sd00001319bc04sc01i00”. The default rules provided with Udev will cause udevd to call out to /sbin/modprobe with the contents of the MODALIAS uevent environment variable (which should be the same as the contents of the modalias file in sysfs), thus loading all modules whose aliases match this string after wildcard expansion.
In this example, this means that, in addition to snd-fm801, the obsolete (and unwanted) forte driver will be loaded if it is available. See below for ways in which the loading of unwanted drivers can be prevented.
The kernel itself is also able to load modules for network protocols, filesystems and NLS support on demand.