vmlinux is a statically linked

executable In computing, executable code, an executable file, or an executable program, sometimes simply referred to as an executable or binary, causes a computer "to perform indicated tasks according to encoded instruction (computer science), instructi ...

file that contains the

Linux kernel The Linux kernel is a free and open-source, monolithic, modular, multitasking, Unix-like operating system kernel. It was originally authored in 1991 by Linus Torvalds for his i386-based PC, and it was soon adopted as the kernel for the GNU ope ...

in one of the

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 In computer programming and software development, debugging is the process of finding and resolving '' bugs'' (defects or problems that prevent correct operation) within computer programs, software, or systems. Debugging tactics can involve in ...

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 ...

generation or other operations, but must be made bootable before being used as an

operating system kernel 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 ...

by adding a

multiboot Multi-booting is the act of installing multiple operating systems on a single computer, and being able to choose which one to boot. The term dual-booting refers to the common configuration of specifically two operating systems. Multi-booting m ...

header, bootsector and setup routines.

Etymology

Traditionally,

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, and ot ...

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 l ...

, 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 In computing, BIOS (, ; Basic Input/Output System, also known as the System BIOS, ROM BIOS, BIOS ROM or PC BIOS) is firmware used to provide runtime services for operating systems and programs and to perform hardware initialization during the ...

drivers to access the

hard disk A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage device that stores and retrieves digital data using magnetic storage with one or more rigid rapidly rotating platters coated with magnet ...

, 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

were addressable. To overcome this, Linux distributors encouraged users to create a

partition Partition may refer to: Computing Hardware * Disk partitioning, the division of a hard disk drive * Memory partition, a subdivision of a computer's memory, usually for use by a single job Software * Partition (database), the division of a ...

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 ...

and kernel-related files.

GRUB Grub can refer to Grub (larva), of the beetle superfamily Scarabaeoidea, or as a slang term for food. It can also refer to: Places * Grub, Appenzell Ausserrhoden, Switzerland * Grub, St. Gallen, Switzerland * Grub (Amerang), a hamlet in Bavaria, ...

, LILO and SYSLINUX are common

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 ...

, the kernel is also compressed using

, 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 Zstandard, commonly known by the name of its reference implementation zstd, is a lossless data compression algorithm developed by Yann Collet at Facebook. ''Zstd'' is the reference implementation in C. Version 1 of this implementation was re ...

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 developed ...

architecture, the vmlinux file is compressed using simple

, because the

SILO A silo (from the Greek σιρός – ''siros'', "pit for holding grain") is a structure for storing bulk materials. Silos are used in agriculture to store fermented feed known as silage, not to be confused with a grain bin, which is used t ...

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

until Linux 2.6.30, which introduced more algorithms. Although the bz

prefix A prefix is an affix which is placed before the Word stem, stem of a word. Adding it to the beginning of one word changes it into another word. For example, when the prefix ''un-'' is added to the word ''happy'', it creates the word ''unhappy'' ...

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 Red Hat, Inc. is an American software company that provides open source software products to enterprises. Founded in 1993, Red Hat has its corporate headquarters in Raleigh, North Carolina, with other offices worldwide. Red Hat has become ass ...

and clones) may come with a kernel-debuginfo

RPM Revolutions per minute (abbreviated rpm, RPM, rev/min, r/min, or with the notation min−1) is a unit of rotational speed or rotational frequency for rotating machines. Standards ISO 80000-3:2019 defines a unit of rotation as the dimensionl ...

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.

Notes and references

External links

Boot process
{{Linux kernel Linux kernel