Red Hat LINUX 7.2 - OFFICIAL LINUX CUSTOMIZATION GUIDE Guida di Installazione Scaricare il pdf (Pagina 78)

78 Chapter 4. GRUB

Note

Some ﬁlesystems, as well as ﬁlesystem conﬁgurations, may require a Stage 1.5 ﬁle that essentially

bridges the gap between the primary and secondary boot loaders.

For example, if the Stage 2 boot loader ﬁle is on a partition using a ﬁlesystem that the Stage 1 boot

loader cannot access, it is possible to direct the Stage 1 boot loader to load additional instructions

from the Stage 1.5 ﬁle that allows it to read the Stage 2 boot loader ﬁle. For more information, refer

to the GRUB info page.

The aforementioned boot method is called direct loading, because instructions to directly load the

operating system are used, with no intermediary code between the boot loaders and the operating sys-

tem’s main ﬁles (such as the kernel). The boot process used by different operating systems may differ

slightly from the above, however. For example, Microsoft’s DOS and Windows operating systems

tend to completely overwrite anything on the MBR when they are installed without incorporating any

of the current MBR’s conﬁguration. This destroys any other information stored in the MBR by other

operating systems, such as Red Hat Linux. The Microsoft operating systems, as well as various other

proprietary operating systems, are loaded using a chain loading boot method. With this method, the

MBR simply points to the ﬁrst sector of the partition holding the operating system, where it ﬁnds the

special ﬁles necessary to actually boot that operating system.

GRUB supports both direct and chain-loading boot methods. It is possible to use it with almost any

operating system, most popular ﬁlesystems, and almost any hard disk the system BIOS can recognize.

4.1.2. GRUB Features

GRUB contains a number of features that make it preferable to other available boot loaders. These are

some of the most important:

• GRUB provides a true command-based, pre-OS environment on x86 machines to allow maximum

ﬂexibility in loading operating systems with certain options or gathering information about the sys-

tem. Many non-x86 architectures have employed pre-OS environments for years that allows control

over how the system boots from a command line. While some command features are available with

LILO and other x86 boot loaders, GRUB contains a greater number of features.

• GRUB supports Logical Block Addressing (LBA) mode. LBA places the addressing conversion used

to ﬁnd ﬁles on the drive in the drive’s ﬁrmware, and it is used on many IDE and all SCSI hard disks.

Before LBA, hard drives could encounter a 1024-cylinder limit, where the BIOS could not ﬁnd a ﬁle

after that point, such as a boot loader or kernel ﬁles. LBA support allows GRUB to boot operating

systems from partitions beyond the 1024-cylinder limit, so long as the system BIOS supports LBA

mode (most do).

• GRUB’s conﬁguration ﬁle is read from the disk every time the system boots, preventing the user

from having to write over the MBR every time a change the boot options is made. Most boot load-

ers are not sophisticated enough to read conﬁguration ﬁles and use them to set up boot options.

For example, to change a LILO boot conﬁguration, such as changing the default operating system

to boot, users must change a LILO conﬁguration ﬁle and run a command that overwrites the sys-

tem’s MBR with the new conﬁguration data. This is more risky than GRUB’s method, because a

misconﬁgured MBR would leave the system unbootable. With GRUB, if the conﬁguration ﬁle is

erroneously conﬁgured and rebooted, it will simply default to a command line and allow the user

to manually type commands that will launch the operating system. The MBR is not touched except

to update the Stage 1, Stage 2, or menu conﬁguration ﬁle locations, and this is rarely necessary.

1 2 ... 73 74 75 76 77 78 79 80 81 82 83 ... 281 282

Commenti su questo manuale

Nessun commento

Red Hat LINUX 7.2 - OFFICIAL LINUX CUSTOMIZATION GUIDE Guida di Installazione Pagina 78

Commenti su questo manuale

Prodotti e manuali riguardandi Software di system management Red Hat LINUX 7.2 - OFFICIAL LINUX CUSTOMIZATION GUIDE