Monolithic kernel and Interfaces - The Linux Networking Architecture

Since Version 2.0, Linux has made a step towards microkernel architectures. More specifically, the possibility was created of moving certain functionalities into modules, which are loaded into the kernel at runtime, from which they can be removed again. This removed an important drawback of monolithic kernels and opened the way to loading drivers or other functionalities at runtime. In addition, modularization offers another benefit: Uniform interfaces are defined.

Table 2-1. Interfaces in the Linux kernel to embed new functionalities.

Functionality	Functions for Dynamic Registration
Character devices	(un)register_chrdev( )
Block devices	(un)register_blkdev( )
Binary formats	(un)register_binfmt( )
File systems	(un)register_filesystem( )
Serial interfaces	(un)register_serial( )
Network adapters	(un)register_netdev( )
Layer-3 protocols	dev_add_pack( ), dev_remove_pack( )
Layer-4 protocols (TCP/IP)	inet_add_protocol( ), inet_del_protocol( )
Console drivers	tty_(un)register_driver( )
Symbol tables	(un)register_symtab( )
Modules	init_module( ), cleanup_module( )

Despite its modularization, Linux has preserved a major benefit of monolithic kernels: All functions implemented in modules run in protected kernel mode, which means that they do not require any context change when called from within the kernel. This can be seen as a clever combination of the benefits from both main operating-system architectures.

Linux 2.4