Bootloader

A bootloader, also spelled as boot loader or called bootstrap loader, is a computer program that is responsible for booting a computer and booting an operating system. If it also provides an interactive menu with multiple boot choices then it is often called a boot manager. When a computer is turned off, its software‍—‌including operating systems, application code, and data‍—‌remains stored on non-volatile memory. When the computer is powered on, it typically does not have an operating system or its loader in random-access memory (RAM). The computer first executes a relatively small program stored in the boot ROM, which is read-only memory (ROM, and later EEPROM, NOR flash) along with some needed data, to initialize hardware devices such as CPU, motherboard, memory, storage and other I/O devices, to access the nonvolatile device (usually a block device, e.g., NAND flash) or devices from which the operating system programs and data can be loaded into RAM. Some earlier computer systems, upon receiving a boot signal from a human operator or a peripheral device, may load a very small number of fixed instructions into memory at a specific location, initialize at least one CPU, and then point the CPU to the instructions and start their execution. These instructions typically start an input operation from some peripheral device (which may be switch-selectable by the operator). Other systems may send hardware commands directly to peripheral devices or I/O controllers that cause an extremely simple input operation (such as "read sector zero of the system device into memory starting at location 1000") to be carried out, effectively loading a small number of boot loader instructions into memory; a completion signal from the I/O device may then be used to start execution of the instructions by the CPU. Smaller computers often use less flexible but more automatic...