kernel image format
(1)vmlinux
vmlinuz is a bootable, compressible kernel image, and vm represents Virtual Memory. It is compiled from the source code of the kernel by the user. In essence, it is a file in elf format with symbol table, which can be used for debugging.
(2)Image
Image is the kernel code that contains only binary data and is processed by objcopy without compression.
(3)zImage
zImage is a kind of compressed image file commonly used in ARM linux. It is composed of vmlinux and decompressed code by gzip compression. The command format is "make zImage"
(4)bzImage
bz represents big zImage, whose format is similar to zImage, but different compression algorithms are used. gzip is used for compression. bzImage has a higher compression ratio and is suitable for large kernel.
(5)uImage
uImage is a special image file for uboot. It is a header information (tag) with a length of 0x40 added before zImage. In the header information, it describes the type, loading position, generation time, size and other information of the image file. In other words, if you start directly from the 0x40 position of uImage, there is no difference between zImage and uImage. The command format is "make" Most Linux image files in this format are stored on NAND.
There is mkimage tool in the tools directory of u-boot, which can be used to make uImage. Refer to Here.
(6)xipImage
Most of the Linux image files in this format are stored in NorFlash, and they do not need to be copied to the memory SDRAM when running. They can run directly in NorFlash
linux compression mode configuration options
In init/Kconfig, you can see the following choices about compression methods. According to the following description, you can briefly know the characteristics of various compression algorithms. For the detailed test of these algorithms, please refer to Here.
choice
prompt "Kernel compression mode"
default KERNEL_GZIP
depends on HAVE_KERNEL_GZIP || HAVE_KERNEL_BZIP2 || HAVE_KERNEL_LZMA || HAVE_KERNEL_XZ || HAVE_KERNEL_LZO || HAVE_KERNEL_LZ4
help
The linux kernel is a kind of self-extracting executable.
Several compression algorithms are available, which differ
in efficiency, compression and decompression speed.
Compression speed is only relevant when building a kernel.
Decompression speed is relevant at each boot.
If you have any problems with bzip2 or lzma compressed
kernels, mail me (Alain Knaff) <alain@knaff.lu>. (An older
version of this functionality (bzip2 only), for 2.4, was
supplied by Christian Ludwig)
High compression options are mostly useful for users, who
are low on disk space (embedded systems), but for whom ram
size matters less.
If in doubt, select 'gzip'
config KERNEL_GZIP
bool "Gzip"
depends on HAVE_KERNEL_GZIP
help
The old and tried gzip compression. It provides a good balance
between compression ratio and decompression speed.
config KERNEL_BZIP2
bool "Bzip2"
depends on HAVE_KERNEL_BZIP2
help
Its compression ratio and speed is intermediate.
Decompression speed is slowest among the choices. The kernel
size is about 10% smaller with bzip2, in comparison to gzip.
Bzip2 uses a large amount of memory. For modern kernels you
will need at least 8MB RAM or more for booting.
config KERNEL_LZMA
bool "LZMA"
depends on HAVE_KERNEL_LZMA
help
This compression algorithm's ratio is best. Decompression speed
is between gzip and bzip2. Compression is slowest.
The kernel size is about 33% smaller with LZMA in comparison to gzip.
config KERNEL_XZ
bool "XZ"
depends on HAVE_KERNEL_XZ
help
XZ uses the LZMA2 algorithm and instruction set specific
BCJ filters which can improve compression ratio of executable
code. The size of the kernel is about 30% smaller with XZ in
comparison to gzip. On architectures for which there is a BCJ
filter (i386, x86_64, ARM, IA-64, PowerPC, and SPARC), XZ
will create a few percent smaller kernel than plain LZMA.
The speed is about the same as with LZMA: The decompression
speed of XZ is better than that of bzip2 but worse than gzip
and LZO. Compression is slow.
config KERNEL_LZO
bool "LZO"
depends on HAVE_KERNEL_LZO
help
Its compression ratio is the poorest among the choices. The kernel
size is about 10% bigger than gzip; however its speed
(both compression and decompression) is the fastest.
config KERNEL_LZ4
bool "LZ4"
depends on HAVE_KERNEL_LZ4
help
LZ4 is an LZ77-type compressor with a fixed, byte-oriented encoding.
A preliminary version of LZ4 de/compression tool is available at
<https://code.google.com/p/lz4/>.
Its compression ratio is worse than LZO. The size of the kernel
is about 8% bigger than LZO. But the decompression speed is
faster than LZO.
endchoice
The kernel compression and decompression code is in the directory kernel/arch/arm/boot/compressed. It seems that there is no specific compression algorithm in this document.
More information:
linux kernel image format
