Kernel 0.01 Walkthrough.
After downloading Kernel 0.01 and unpacking the archive, you'll be facing a directory "linux" containing, in subdirectories, some 5900 lines of (more or less) ANSI C, around 2500 lines of C headers (#include files) and around 1450 lines of i386 assembler.
The linux 0.01 kernel source is at http://www.kernel.org/pub/linux/kernel/Historic/
The code directories are as follows. Click on the Details column for more information.
The header files are not the standard header files, because when you're working on a kernel you don't have the luxury of the usual C functions--you have to write your own versions.
The kernel directory contains most of the system functions, but some of them (like sys_fcntl()) are in the file system directory.