3. Bootstrap

Bootstrapping(1) is the procedure by which your machine loads the microkernel and transfers control to the operating system.

3.1 Bootloader

The bootloader is the first software that runs on your machine. Many hardware architectures have a very simple startup routine which reads a very simple bootloader from the beginning of the internal hard disk, then transfers control to it. Other architectures have startup routines which are able to understand more of the contents of the hard disk, and directly start a more advanced bootloader.

Currently, GRUB(2) is the preferred GNU bootloader. GRUB provides advanced functionality, and is capable of loading several different kernels (such as Mach, Linux, DOS, and the *BSD family). See section `Introduction' in GRUB Manual.

GNU Mach conforms to the Multiboot specification which defines an interface between the bootloader and the components that run very early at startup. GNU Mach can be started by any bootloader which supports the multiboot standard. After the bootloader loaded the kernel image to a designated address in the system memory, it jumps into the startup code of the kernel. This code initializes the kernel and detects the available hardware devices. Afterwards, the first system task is started. See section `Overview' in Multiboot Specification.

3.2 Modules

Because the microkernel does not provide filesystem support and other features necessary to load the first system task from a storage medium, the first task is loaded by the bootloader as a module to a specified address. In the GNU system, this first program is the serverboot executable. GNU Mach inserts the host control port and the device master port into this task and appends the port numbers to the command line before executing it.

The serverboot program is responsible for loading and executing the rest of the Hurd servers. Rather than containing specific instructions for starting the Hurd, it follows general steps given in a user-supplied boot script.

XXX More about boot scripts.