What's changed: Initial version (topic 2.01, subtopics 2.01.1–2.01.5)
1.3Components of the Linux Kernel
Learn the structure of the kernel image itself and its source tree: the compressed executable image bzImage, the xz data compression used inside it, the kernel source tree at /usr/src/linux/, and the accompanying documentation at /usr/src/linux/Documentation/.
Before compiling a kernel or handling modules, you need to know what the kernel artifact is actually made of. This section lays the groundwork for the next two sections (compiling and runtime management): the kernel image and the source tree.
1.3.1The bzImage kernel image
- bzImage (big zImage) is the standard format for a compressed, bootable kernel executable image on x86-family Linux. It is exactly what the boot loader loads, decompresses, and executes.
- The kernel proper inside a bzImage is compressed with one of several data-compression methods, including xz (chosen at kernel config time). A small decompression stub inside the image unpacks it at boot.
1.3.2Layout of the kernel source tree
- /usr/src/linux/ is the conventional location where kernel source is unpacked. On many distros it is a symlink to the directory named after the actual version.
- /usr/src/linux/Documentation/ holds the official documentation for each kernel subsystem's specs and configuration. It is the primary reference for what a given config option actually does.
Exam questions test the plain mapping: bootable compressed image = bzImage, conventional source tree location = /usr/src/linux/, primary reference for specs = Documentation/. This subtopic carries a lighter weight (2) but connects directly to the make bzImage target in the next section on compiling—do not skip it.
Picture the practical workflow around kernel packages. Unpacking a distro's kernel source package typically creates a real directory like /usr/src/linux-<version>/, with /usr/src/linux set up as a symlink pointing to it. This convention lets build scripts and tools like DKMS reference /usr/src/linux without caring about the exact version. To understand what a config option does (say, a filesystem driver or RAID support), the standard move is to read the relevant subsystem's writeup under /usr/src/linux/Documentation/. The final make output is bundled into a single bootable image, bzImage, whose contents are compressed with something like xz—so the on-disk image is far smaller than the uncompressed kernel.
| Component | What it is | Note |
|---|---|---|
| bzImage | Compressed bootable kernel image | Loaded, unpacked, and run by the boot loader |
| xz | One of the data-compression methods used inside bzImage | Chosen at kernel config time |
| /usr/src/linux/ | Conventional kernel source tree location | Often a symlink to the real directory |
| /usr/src/linux/Documentation/ | Official documentation | Primary reference for config options |
Trap: "bzImage is the full kernel source code" is wrong—bzImage is the compiled, compressed executable image; the source lives at /usr/src/linux/. Also wrong: "Documentation/ holds kernel module binaries"—it holds documents; modules live under /lib/modules/<kver>/.
1.3.3Section summary
- bzImage = the compressed, bootable kernel executable image (compressed internally with xz, etc.); loaded and run by the boot loader
- Source lives at /usr/src/linux/ (often a symlink) / check specs under Documentation/, the primary reference
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Quick check
(just a quick review)Q1. On x86-family Linux, what is the standard compressed, bootable kernel image format that the boot loader loads, unpacks, and executes?
Q2. You want to confirm the exact meaning of a kernel config option for a filesystem driver. Where is the most appropriate primary reference?
Q3. Which path is conventionally used across many distros as the location where kernel source code is unpacked?