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# 9.2 Why Build a Custom Kernel?

Traditionally, FreeBSD used a “monolithic” kernel. The kernel was one large program, supported a fixed list of devices, and in order to change the kernel's behavior, one had to compile a new kernel, and then reboot into the new kernel.

Today, most of the functionality in the FreeBSD kernel is contained in modules which can be dynamically loaded and unloaded from the kernel as necessary. This allows the running kernel to adapt immediately to new hardware or for new functionality to be brought into the kernel. This is known as a modular kernel.

Occasionally, it is still necessary to perform static kernel configuration. This may be because the functionality is so tied to the kernel that it can not be made dynamically loadable. Some security environments prevent the loading and unloading of kernel modules, and require that only needed functionality is statically compiled into the kernel.

Building a custom kernel is often a rite of passage for advanced BSD users. This process, while time consuming, can provide benefits to the FreeBSD system. Unlike the `GENERIC` kernel, which must support a wide range of hardware, a custom kernel can be stripped down to only provide support for that computer's hardware. This has a number of benefits, such as:

- Faster boot time. Since the kernel will only probe the hardware on the system, the time it takes the system to boot can decrease.

- Lower memory usage. A custom kernel often uses less memory than the `GENERIC` kernel by omitting unused features and device drivers. This is important because the kernel code remains resident in physical memory at all times, preventing that memory from being used by applications. For this reason, a custom kernel is useful on a system with a small amount of RAM.

- Additional hardware support. A custom kernel can add in support for devices which are not present in the `GENERIC` kernel.

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