Loadable Kernel Module

In computing, a loadable kernel module (or LKM) is an object file that contains code to extend the running kernel, or so-called base kernel, of an operating system. LKMs are typically used to add support for new hardware (as device drivers) and/or filesystems, or for adding system calls. When the functionality provided by a LKM is no longer required, it can be unloaded in order to free memory and other resources.

Most current Unix-like systems and Microsoft Windows support loadable kernel modules, although they might use a different name for them, such as kernel loadable module (kld) in FreeBSD, kernel extension (kext) in macOS,kernel extension module in AIX, kernel-mode driver in Windows NT and Downloadable kernel module (DKM) in VxWorks. They are also known as Kernel Loadable Modules (or KLM), and simply as Kernel Modules (KMOD).

Without loadable kernel modules, an operating system would have to include all possible anticipated functionality already compiled directly into the base kernel. Much of that functionality would reside in memory without being used, wasting memory, and would require that users rebuild and reboot the base kernel every time they require new functionality. Most operating systems supporting loadable kernel modules will include modules to support most desired functionality.

One minor criticism of preferring a modular kernel over a static kernel is the so-called fragmentation penalty. The base kernel is always unpacked into real contiguous memory by its setup routines; thus, the base kernel code is never fragmented. Once the system is in a state in which modules may be inserted, for example once the filesystems have been mounted that contain the modules, it is likely that any new kernel code insertion will cause the kernel to become fragmented, thereby introducing a minor performance penalty by using more TLB entries, causing more TLB misses.

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