kernel presentation at ILUG on Sat.

[Jason Clinton]

Charles Steinkuehler wrote:

> James Colannino wrote:
>
>> I live in another part of the United States and so I wouldn't have
>> been able to be there, but I have a couple questions about the kernel
>> that I have been itching to figure out and was wondering if anyone
>> could help enlighten me :)
>>
>> First, how exactly does the kernel uncompress itself?
>
>
> It's magic!
>
>> Are there a few lines of code at the beginning of the binary that
>> instruct the computer on how to uncompress the rest?
>
>
> Yes, it's very much like the way a boot-loader works.  A tiny section
> of code is responsible for extrating (or loading) the rest of the
> kernel (or boot-loader).  How many "levels" of this happen can change,
> depending on the system complexity.  For example, in a typical x86
> based PC, you have the following general boot sequence:
>
> - BIOS performs POST, loads and runs the MBR
> - MBR code (all 256 bytes!) loads the initial boot-loader (typically
> K-Bytes of code (several sectors at fixed location on the disk), but
> not the entire boot-loader)
> - The initial boot-loader loads the rest of the boot-loader
> - The boot loader determines which kernel and initial ramdisk to load,
> and which options (if any) to pass to the kernel
> - The kernel and initial ramdisk are loaded into memory
> - The boot-loader starts running the kernel
> - The kernel decompresses itself, and begins general "housekeeping"
> (initializing buses, configuring the interrupt controler(s), etc)
> - If an initial ramdisk was provided, the kernel optionally
> decompresses it (if compressed) and tries to mount it and run /linuxrc
> - The final root filesystem is mounted, and the kernel runs init.
>
Well, I stand corrected here. :)

>
> Gzip does the compression (see the gzip -f -9 line below).
>
> Binutils, including ld (the linker) and objcopy are used along with
> some custom tools (arch/i386/boot/tools/build) to turn the raw kernel
> image (vmlinux) into a bootable bzImage.
>
I'm almost completely positive that BZip2 is used if your make target is
bzImage. Doesn't make target "install" create a gzip image?

--
Jason Clinton
I don't believe in witty sigs.