Which device drivers are built into the kernel?

Core drivers that are considered critical to the loading of the kernel are usually built into the kernel, while other hardware drivers, etc. are built as modules or .ko files.

The .ko modules are usually stored under the /lib directory on your root partition. To use any of these, the kernel must first be able to detect and access the underlying storage device and then access its filesystem. So it's safe to assume that a kernel without SATA/SCSI and ext2/3/4 support built-into it won't really boot ;)

You can choose to switch most built-in kernel drivers into module form. The Ubuntu kernel team decides whether to modify the Linux kernel team's default configuration and include/exclude additional built-in drivers for the stock kernel images you download.

If you build your own kernel, you can do the same:

• In the above screenshot, the * indicates a built-in driver, while M indicates a module.
• Loopback device support, which is often essential to booting a system, is built-in by default.
• The low-speed USB driver (USB 1.0) is also built-in by default to allow you to boot off a USB stick, but here I have changed it into a module.

When compiling a kernel, you get to configure which components are installed. Not only that, but you get to chose whether or not they are built into the kernel or if they are a module.

For example, many people use the ext2 filesystem on their /boot partition. Because of this, the kernel must be able to read ext2 filesystems at boot time. In order to accomplish this, the ext2 module is built into the kernel itself.

Now, imagine the quantity of modules available. It wouldn't make sense to have them all built into your kernel, would it? This is why you can build these as separate .ko modules and load them at will.