1.3 GRUB features
The primary requirement for GRUB is that it be compliant with the
Multiboot Specification, which is described in Multiboot Specification.
The other goals, listed in approximate order of importance, are:
- Basic functions must be straightforward for end-users.
- Rich functionality to support kernel experts and designers.
- Backward compatibility for booting FreeBSD, NetBSD, OpenBSD, and
Linux. Proprietary kernels (such as DOS, Windows NT, and OS/2) are
supported via a chain-loading function.
Except for specific compatibility modes (chain-loading and the Linux
piggyback format), all kernels will be started in much the same
state as in the Multiboot Specification. Only kernels loaded at 1 megabyte
or above are presently supported. Any attempt to load below that
boundary will simply result in immediate failure and an error message
reporting the problem.
In addition to the requirements above, GRUB has the following features
(note that the Multiboot Specification doesn't require all the features
that GRUB supports):
- Recognize multiple executable formats
- Support many of the a.out variants plus ELF. Symbol
tables are also loaded.
- Support non-Multiboot kernels
- Support many of the various free 32-bit kernels that lack Multiboot
compliance (primarily FreeBSD, NetBSD, OpenBSD, and
Linux). Chain-loading of other boot loaders is also supported.
- Load multiples modules
- Fully support the Multiboot feature of loading multiple modules.
- Load a configuration file
- Support a human-readable text configuration file with preset boot
commands. You can also load another configuration file dynamically and
embed a preset configuration file in a GRUB image file. The list of
commands (see Commands) are a superset of those supported on the
command-line. An example configuration file is provided in
- Provide a menu interface
- A menu interface listing preset boot commands, with a programmable
timeout, is available. There is no fixed limit on the number of boot
entries, and the current implementation has space for several hundred.
- Have a flexible command-line interface
- A fairly flexible command-line interface, accessible from the menu,
is available to edit any preset commands, or write a new boot command
set from scratch. If no configuration file is present, GRUB drops to
The list of commands (see Commands) are a subset of those supported
for configuration files. Editing commands closely resembles the Bash
command-line (see Bash), with <TAB>-completion of commands,
devices, partitions, and files in a directory depending on context.
- Support multiple filesystem types
- Support multiple filesystem types transparently, plus a useful explicit
blocklist notation. The currently supported filesystem types are
BSD FFS, DOS FAT16 and FAT32, Minix fs, Linux
ext2fs, ReiserFS, JFS, XFS, and VSTa
fs. See Filesystem, for more information.
- Support automatic decompression
- Can decompress files which were compressed by gzip. This
function is both automatic and transparent to the user (i.e. all
functions operate upon the uncompressed contents of the specified
files). This greatly reduces a file size and loading time, a
particularly great benefit for floppies.1
It is conceivable that some kernel modules should be loaded in a
compressed state, so a different module-loading command can be specified
to avoid uncompressing the modules.
- Access data on any installed device
- Support reading data from any or all floppies or hard disk(s) recognized
by the BIOS, independent of the setting of the root device.
- Be independent of drive geometry translations
- Unlike many other boot loaders, GRUB makes the particular drive
translation irrelevant. A drive installed and running with one
translation may be converted to another translation without any adverse
effects or changes in GRUB's configuration.
- Detect all installed ram
- GRUB can generally find all the installed ram on a PC-compatible
machine. It uses an advanced BIOS query technique for finding all
memory regions. As described on the Multiboot Specification (see Multiboot Specification), not all kernels make use of this information, but GRUB
provides it for those who do.
- Support Logical Block Address mode
- In traditional disk calls (called CHS mode), there is a geometry
translation problem, that is, the BIOS cannot access over 1024
cylinders, so the accessible space is limited to at least 508 MB and to
at most 8GB. GRUB can't universally solve this problem, as there is no
standard interface used in all machines. However, several newer machines
have the new interface, Logical Block Address (LBA) mode. GRUB
automatically detects if LBA mode is available and uses it if
available. In LBA mode, GRUB can access the entire disk.
- Support network booting
- GRUB is basically a disk-based boot loader but also has network
support. You can load OS images from a network by using the TFTP
- Support remote terminals
- To support computers with no console, GRUB provides remote terminal
support, so that you can control GRUB from a remote host. Only serial
terminal support is implemented at the moment.