Bootstrap loader:

The critical programs are loaded into memory by the bootstrap loader at start-up time and will remain resident as long as the computer is running. The bootstrap for most modern computers is stored in  read-only memory; on some computers, part of the resident operating system will also be contained in ROM, so that it is permanently resident in memory and always available for use. The memory resident components of an operating system are commonly known as  the kernel of the operating system. For example, the operating system program accepts user commands must always be present, as well as the programmes that handle request from the user systems and manage resources in a multitasking system. On the other hand, an operating system command that formats a new disk is only used occasionally; it can be loaded and executed only when it is required. Today, the trend is towards design of  nano kernel  or micro kernel. The essence of these kernels is to keep the basic kernel smaller.