E.2.1. GRUB and the Boot Process on BIOS-based x86 Systems
- The Stage 1 or primary boot loader is read into memory by the BIOS from the MBR . The primary boot loader exists on less than 512 bytes of disk space within the MBR and is capable of loading either the Stage 1.5 or Stage 2 boot loader.BIOS cannot read partition tables or file systems. It initializes the hardware, reads the MBR, then depends entirely on the stage 1 bootloader to continue the boot process.
- The Stage 1.5 boot loader is read into memory by the Stage 1 boot loader, if necessary. Some hardware requires an intermediate step to get to the Stage 2 boot loader. This is sometimes true when the
/boot/partition is above the 1024 cylinder head of the hard drive or when using LBA mode. The Stage 1.5 boot loader is found either on the
/boot/partition or on a small part of the MBR and the
- The Stage 2 or secondary boot loader is read into memory. The secondary boot loader displays the GRUB menu and command environment. This interface allows the user to select which kernel or operating system to boot, pass arguments to the kernel, or look at system parameters.
- The secondary boot loader reads the operating system or kernel as well as the contents of
/boot/sysroot/into memory. Once GRUB determines which operating system or kernel to start, it loads it into memory and transfers control of the machine to that operating system.