Section 7.1. Role of a Bootloader

7.1. Role of a Bootloader

When power is first applied to a processor board, many elements of hardware must be initialized before even the simplest program can run. Each architecture and processor has a set of predefined actions and configurations, which include fetching some initialization code from an on-board storage device (usually Flash memory). This early initialization code is part of the bootloader and is responsible for breathing life into the processor and related hardware components.

Most processors have a default address from which the first bytes of code are fetched upon application of power and release of reset. Hardware designers use this information to arrange the layout of Flash memory on the board and to select which address range(s) the Flash memory responds to. This way, when power is first applied, code is fetched from a well-known and predictable address, and software control can be established.

The bootloader provides this early initialization code and is responsible for initializing the board so that other programs can run. This early initialization code is almost always written in the processor's native assembly language. This fact alone presents many challenges, some of which we examine here.

Of course, after the bootloader has performed this basic processor and platform initialization, its primary role becomes booting a full-blown operating system. It is responsible for locating, loading, and passing execution to the primary operating system. In addition, the bootloader might have advanced features, such as the capability to validate an OS image, the capability to upgrade itself or an OS image, and the capability to choose from among several OS images based on a developer-defined policy. Unlike the traditional PC-BIOS model, when the OS takes control, the bootloader is overwritten and ceases to exist.^[1]

^[1] Some embedded designs protect the bootloader and provide callbacks to bootloader routines, but this is almost never a good design approach. Linux is far more capable than bootloaders, so there is often little point in doing so.