object file
An object file is a computer file containing object code, that is, machine code output of an assembler or compiler. The object code is usually relocatable, and not usually directly executable. There are various formats for object files, and the ...
formats supported by Linux, which includes
Executable and Linkable Format
In computing, the Executable and Linkable FormatTool Interface Standard (TIS) Portable Formats SpecificationVersion 1.1'' (October 1993) (ELF, formerly named Extensible Linking Format), is a common standard file format for executable files, ...
(ELF) and
Common Object File Format
The Common Object File Format (COFF) is a format for executable, object code, and shared library computer files used on Unix systems. It was introduced in Unix System V, replaced the previously used a.out format, and formed the basis for exten ...
(COFF). The vmlinux file might be required for kernel debugging,
symbol table
In computer science, a symbol table is a data structure used by a language translator such as a compiler or interpreter, where each identifier (or symbols), constants, procedures and functions in a program's source code is associated with info ...
UNIX
Unix (; trademarked as UNIX) is a family of multitasking, multiuser computer operating systems that derive from the original AT&T Unix, whose development started in 1969 at the Bell Labs research center by Ken Thompson, Dennis Ritchie, an ...
platforms called the kernel image /unix. With the development of
virtual memory
In computing, virtual memory, or virtual storage is a memory management technique that provides an "idealized abstraction of the storage resources that are actually available on a given machine" which "creates the illusion to users of a very ...
, kernels that supported this feature were given the vm- prefix to differentiate them. The name vmlinux is a mutation of vmunix, while in vmlinuz the letter z at the end denotes that it is compressed (for example
gzip
gzip is a file format and a software application used for file compression and decompression. The program was created by Jean-loup Gailly and Mark Adler as a free software replacement for the compress program used in early Unix systems, and i ...
ped).
Location
Traditionally, the kernel was located in the root directory of the filesystem hierarchy; however, as the bootloader must use BIOS drivers to access the hard disk, limitations on some
i386
The Intel 386, originally released as 80386 and later renamed i386, is a 32-bit microprocessor introduced in 1985. The first versions had 275,000 transistorsfirst 1024 cylinders of the hard disk were addressable.
To overcome this, Linux distributors encouraged users to create a partition at the beginning of their drives specifically for storing
bootloader
A bootloader, also spelled as boot loader or called boot manager and bootstrap loader, is a computer program that is responsible for booting a computer.
When a computer is turned off, its softwareincluding operating systems, application code, an ...
bootloader
A bootloader, also spelled as boot loader or called boot manager and bootstrap loader, is a computer program that is responsible for booting a computer.
When a computer is turned off, its softwareincluding operating systems, application code, an ...
s.
By convention, this partition is mounted on the filesystem hierarchy as /boot. This was later standardised by the Filesystem Hierarchy Standard (FHS), which now requires the Linux kernel image to be located in either / or /boot, although there is no technical restriction enforcing this.
Compression
Traditionally, when creating a bootable
kernel image
The kernel is a computer program at the core of a computer's operating system and generally has complete control over everything in the system. It is the portion of the operating system code that is always resident in memory and facilitates in ...
gzip
gzip is a file format and a software application used for file compression and decompression. The program was created by Jean-loup Gailly and Mark Adler as a free software replacement for the compress program used in early Unix systems, and i ...
, or, since Linux 2.6.30,Linux 2.6.30, released the 9th of June 2009, added support to compress the kernel image with the LZMA and bzip2 algorithm /ref> using LZMA or bzip2, which requires a very small decompression stub to be included in the resulting image. The stub decompresses the kernel code, on some systems printing dots to the console to indicate progress, and then continues the boot process. Support for LZO, xz, LZ4 and zstd compression was added later.
The decompression routine is a negligible factor in boot time, and prior to the development of the ''bzImage'', the size constraints of some architectures, notably i386, were extremely limiting, making compression a necessity.
On the
SPARC
SPARC (Scalable Processor Architecture) is a reduced instruction set computer (RISC) instruction set architecture originally developed by Sun Microsystems. Its design was strongly influenced by the experimental Berkeley RISC system develope ...
architecture, the vmlinux file is compressed using simple
gzip
gzip is a file format and a software application used for file compression and decompression. The program was created by Jean-loup Gailly and Mark Adler as a free software replacement for the compress program used in early Unix systems, and i ...
, because the SILO boot loader transparently decompresses gzipped images.
The filename of the bootable image is not important, but many popular distributions use ''vmlinuz''.
bzImage
As the Linux kernel matured, the size of the kernels generated by users grew beyond the limits imposed by some architectures, where the space available to store the compressed kernel code is limited. The bzImage (''big zImage'') format was developed to overcome this limitation by splitting the kernel over non-contiguous
memory
Memory is the faculty of the mind by which data or information is encoded, stored, and retrieved when needed. It is the retention of information over time for the purpose of influencing future action. If past events could not be remembered ...
regions.
The bzImage was compressed using
gzip
gzip is a file format and a software application used for file compression and decompression. The program was created by Jean-loup Gailly and Mark Adler as a free software replacement for the compress program used in early Unix systems, and i ...
until Linux 2.6.30, which introduced more algorithms. Although the bzprefix may suggest that bzip2 compression is used, this is not the case. (The bzip2 package is often distributed with tools prefixed with bz, such as bzless, bzcat, etc.)
The bzImage file is in a specific format. It contains concatenated bootsect.o + setup.o + misc.o + piggy.o.piggy.o contains the gzipped vmlinux file in its data section. The script extract-vmlinux found under scripts/ in the kernel sources decompresses a kernel image. Some distributions (e.g. Red Hat and clones) may come with a kernel-debuginfoRPM that contains the vmlinux file for the matching kernel RPM, and it typically gets installed under /usr/lib/debug/lib/modules/`uname -r`/vmlinux or /usr/lib/debug/lib64/modules/`uname -r`/vmlinux.
Module (Linux)
In computing, a loadable kernel module (LKM) is an object file that contains code to extend the running kernel, or so-called ''base kernel'', of an operating system. LKMs are typically used to add support for new hardware (as device drivers) and/o ...
*
initrd
In Linux systems, initrd (''initial ramdisk'') is a scheme for loading a temporary root file system into memory, to be used as part of the Linux startup process. initrd and initramfs refer to two different methods of achieving this. Both are comm ...
Object file
An object file is a computer file containing object code, that is, machine code output of an assembler or compiler. The object code is usually relocatable, and not usually directly executable. There are various formats for object files, and the ...