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# 9.4 Kernel Drivers, Subsystems, and Modules

Before building a custom kernel, consider the reasons for doing so. If there is a need for specific hardware support, it may already exist as a module.

Kernel modules exist in the `/boot/kernel` directory and may be dynamically loaded into the running kernel using kldload(8). Most, if not all kernel drivers have a specific module and manual page. For example, the last section noted the `ath` wireless Ethernet driver. This device has the following information in its manual page:

    Alternatively, to load the driver as a module at boot time, place the
    following line in loader.conf(5):

        if_ath_load="YES"

As instructed, adding the `if_ath_load="YES"` line to the `/boot/loader.conf` file will enable loading this module dynamically at boot time.

In some cases; however, there is no associated module. This is mostly true for certain subsystems and very important drivers, for instance, the fast file system (FFS) is a required option in the kernel. As is network support (INET). Unfortunately the only way to tell if a driver is required is to check for the module itself.

> Warning: It is easy to remove support for a device or option and end up with a broken kernel. For example, if the ata(4) driver is removed from the kernel configuration file, a system using ATA disk drivers may not boot without the module added to `loader.conf`. When in doubt, check for the module and then just leave support in the kernel.

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