1. Getting Started with the Kernel
國立中正大學
資訊工程研究所
羅習五　老師

2. Obtaining the Kernel Source

3. The Kernel Source Tree Directory Description
arch
Architecture-specific source
crypto
Crypto API
Documentation
Kernel source documentation
drivers
Device drivers fs
The VFS and the individual file systems
include
Kernel headers
init
Kernel boot and initialization
ipc
Interprocess communication code
kernel
Core subsystems, such as the scheduler
lib
Helper routines mm
Memory management subsystem and the VM
net
Networking subsystem
scripts
Scripts used to build the kernel
security
Linux Security Module
sound
Sound subsystem
usr
Early user-space code (called initramfs)

4. Lines of code

5. Different Natures The kernel does not have access to the C library.
The kernel is coded in GNU C. (not in ANSI C)
The kernel lacks memory protection like user-space.
The kernel cannot easily use floating point.
The kernel has a small fixed-size stack.
Because the kernel has asynchronous interrupts, is preemptive, and supports SMP, synchronization and concurrency are major concerns within the kernel.
Portability is important.

6. Different Natures NO libc support
There are multiple reasons for this, including some chicken-and-the-egg situations, but the primary reason is speed and size (upcall?).
Many of the usual libc functions have been implemented inside the kernel.
For example <linux/string.h>

7. Different Natures Inline Assembly Branch Annotation
The asm() compiler directive is used to inline assembly code.
Branch Annotation
Small, Fixed-Size Stack
Historically, the kernel stack is two pages.
Recently, the size of kernel stack is one page only!!!
if (unlikely(foo)) { /* ... */ }
if (likely(foo)) { /* ... */ }

8. Different Natures No (Easy) Use of Floating Point
Supporting FPU is very expensive for kernel developers
Using floating point inside the kernel requires manually saving and restoring the floating point registers
It is very hard to use the technique called “lazy context switch” to speedup FPU context switching

9. Note: “DO” have some floating-point computations
Recompile GNU‘s libc with option “--without-fp” to avoid using floating-point hardware while providing floating-point computations
Compile the module's .c files with gcc's
"-msoft-float" option and
"-D__NO_MATH_INLINES".

10. Different Natures Synchronization and Concurrency
“Linux” is a preemptive multi-tasking “operating system.”
The Linux kernel supports multiprocessing.
For example, Linux can use more than one core/processor to decode TCP/IP packets (named softirq)
Interrupts occur asynchronously
The “Linux kernel” is preemptive.