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BIOS
The BIOS is responsible for the initial system startup and locating the available bootable devices. When the system is powered on the processor will begin execution of the BIOS, which is split into two parts - the Power-On Self Test (POST) and runtime services. The first operation carried out by the BIOS is dependent on whether the system was powered on (cold boot) or reset (warm boot). If the system has been powered on, the first operation is the initialisation and testing of the basic system hardware components - a process known as the POST [1]. Once all hardware devices have been initialised and verified, the POST function is flushed from memory. If, however, the system has been reset a special flag is raised in memory and the BIOS will not carry out the POST in order to save time.
Once the POST has been flushed from memory, the BIOS will advance to runtime services.This function searches for bootable devices that are available to the system; the devices must also be active to be found. The order in which the devices are located is defined in the Complementary Metal Oxide Semiconductor (CMOS) settings. Typically the Linux boot device is a hard disk; it can, however, be anything from a floppy disk, network device, USB memory or CD-ROM.
When booting Linux from a hard disk, the Master Boot Record (MBR), a 512 byte sector located in the first sector of the device, contains the primary bootloader. The last task carried out by the BIOS is the loading of the MBR into memory. Once loaded, the BIOS relinquishes control [1].
First-Stage Bootloader (FSBL)
The FSBL is a section of code contained in the MBR. The remainder of the MBR is a partition table and a validation signature. The largest section of the MBR (446 bytes) is known as the primary bootloader and contains executable code and error messages [1]. The partition table is contained in the next 64 bytes, which itself contains records of four primary partitions, of 16 bytes each. The final section of the MBR is a 2 byte boot signature — 0xAA55 — for validating the MBR. The structure of the MBR is provided in Figure
.png "Structure of the Master Boot Record (MBR).png")
The primary bootloader’s job is to locate, and then load, the SSBL by searching the partition table for an active partition. Once an active partition is located, all further partitions are scanned to ensure that they are inactive. The boot record from the active partition is then loaded into memory for execution.
The SSBL will present you with a list of operating systems that are available to be booted by the system. Once the desired operating system has been selected, the kernel image is decompressed and loaded into memory before the control of the processor is passed to the OS [4].
Two main bootloaders exist: Linux Loader (LILO) and Grand Unified Bootloader (GRUB). Both LILO and GRUB are made up of the combination of both the FSBL and SSBL. LILO has been around for long time and has, in most cases, been replaced with GRUB. While LILO requires that the kernel files be stored on raw disk sectors, GRUB is able to load kernels from ext2 or ext3 file systems [1]. GRUB is able to do this by adding an extra step to the bootloader process, between the first- and second-stages, which allows it to understand a specific file system.
Other methods of booting a Linux kernel exist, such as Syslinux or Loadlin which allow you to load Linux from, and replace, the currently running Windows/DOS environment.
Kernel
With control of the CPU handed over by the second-stage bootloader, a routine will run to perform a small amount of hardware setup before decompressing the kernel image. Once decompressed, the kernel image will be transferred to high memory - the part of physical memory not mapped directly by the kernel page tables. If a RAM disk image is present this will also be moved into memory and marked for later use [1]. At this point the kernel will be called for the first time and will boot.
During the kernel boot further hardware setup is carried out, including setting up the stack, configuring the page tables, enabling memory paging and the detection of the CPU and FPU type [1].
Init
The final stage of the Linux boot process is the initialisation of init - the first user-space application. Once invoked, init looks for the file /etc/inittab and determines whether it has an entry of the type initdefault, which details the initial runlevel of the Linux system. The various Linux distributions will have different configurations for the runlevels of the system. Taking the Linux standard base specification as an example, there are 7 runlevels, as detailed in Table 24.1 [3].
Init is the first program to be invoked that has been compiled with the standard C library. No standard C applications will have been executed prior to this point [1].
![Default runlevels for Linux Standard Base [3].png](/uploads/images/editor/20200710/193155Default%20runlevels%20for%20Linux%20Standard%20Base%20[3].png "Default runlevels for Linux Standard Base [3].png")
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