218

Basic System Administration

has no knowledge of what card the machine has installed. This information

is provided to the kernel in the /etc/modules file. An Adaptec 1542 SCSI

controller needs a line added to the modules file (anywhere after the  auto 

line) that declares this driver. In this example the /etc/modules file might

look like:

# /etc/modules:

kernel modules to load at boot time.

#

# This file should contain the names of kernel modules that are

# to be loaded at boot time, one per line.

Comments begin with

# a `#', and everything on the line after them are ignored.

#

auto

aha1542

This file will cause all the necessary modules that support the controller to

be loaded by kerneld, while the aha1542 driver is loaded at boot time and left

loaded until reboot.

The device driver that accesses the root file system must be built into the

kernel or the system will be unable to boot. If the device driver is a module,

the kernel has no driver to read the root file system, as this is where it expects

to find the module for that driver. This Catch 22 is, for instance, why the

Debian installation kernel has the MS DOS FAT driver built into the kernel,

rather than available in the modules installation section. Because the rescue

disk is an MS DOS format disk the kernel must have the FAT file system

drivers already installed before it can read the root file system from the floppy

(minix is also built in because the root.bin image file is a minix file system).

Any devices the kernel will need before the root file system has been mounted,

in order to get to the point of having a mounted root file system, must be built

into the kernel. All other devices can be built as modules. The Debian instal 

lation kernel is built in this fashion, making it possible to fit the installation

into complex hardware configurations where blanket support causes failures.