The Boot Monitor or BIOS

When you first turn on a computer system, it loads a boot monitor or Basic Input/Output system (BIOS) from storage on the motherboard. This storage is usually a programmable, read-only memory chip (PROM) or a section of flash memory that is present on the board. The BIOS or boot monitor is a very low-level, hardware-oriented application that does some basic hardware initialization, performs some amount of hardware testing and verification (often optional), waits for keyboard or console input for some predetermined period of time, and then usually begins to execute a set of predetermined instructions to load another program into memory. These instructions load another program into memory from a predetermined location such as a portion of flash memory or the Master Boot Record (MBR) of a storage device such as a hard drive or CD-ROM, and then specify the memory address at which to begin executing the program once it has been loaded successfully.

On systems with limited hardware resources, such as many embedded systems, the program loaded into memory by a boot monitor is the Linux kernel, which is described in a subsequent section. However, the boot process is slightly more involved on most desktop and server computer systems. On these types of systems, an intermediate program is loaded into memory from the MBR that provides additional configuration and customization options for the boot process. On modern x86 systems, the program that is loaded into memory at this point is known as a boot loader, which is a configurable application that provides higher-level options for loading an operating system on the computer. The boot loader that is most commonly used on modern x86 systems (including 64-bit systems) is the Grand Unified Boot Loader (GRUB) although some Linux distributions still use an older, simpler boot loader known as the Linux Loader (LiLo).

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