I would recommend looking at LXR. It makes it easier to follow the flow of the code (you do not have to search for each function that is called — well, you have, but the site does it for you).
Some starting points, for the current version (2.6.30):
start_kernel()— think of it as the kernel equivalent ofmain(). This function initializes almost all the kernel subsystems; follow it to see in code what you see scrolling on the screen during the boot.- entry_32.S — system calls and interrupts (x86-32 version, which should be nearer what you know; note the use of the AT&T assembly dialect instead of the Intel dialect you might be more used to).
- head_32.S — the kernel entry point. This is where the kernel starts after switching to protected mode; in the end, it will call
start_kernel(). - arch/x86/boot — the real-mode bootstrap code. It starts in assembly (boot/header.S), but quickly jumps into C code (starting at boot/main.c). Does the real-mode initialization (mostly BIOS calls which have to be done before switching to protected mode); it is compiled using a weird GCC trick (
.code16gcc), which allows the generation of 32-bit real-mode code. - arch/x86/boot/compressed — if you ever wondered where does the “Decompressing Linux…” message comes from, it is from here.