System call
System call as request to hardware
"tag-Linux kernel design-monolithic kernel集成核-system call as request to hardware"
"tag-Linux kernel design-monolithic kernel集成核-system call l是process和kernel之间的interface"
System call也相当于interrupt
上面使用的是“相当于”，而不是“是”，这是因为随着技术的更新迭代，实现system call的assembly instruction也在进行更新迭代，可能原来使用的中断指令（int assembly instruction）会替换为更加高效的assembly instruction。在10.3. Entering and Exiting a System Call中对此进行了详细的说明，如下：
Applications can invoke a system call in two different ways:
1、By executing the int $0x80 assembly language instruction; in older versions of the Linux kernel, this was the only way to switch from User Mode to Kernel Mode.
2、By executing the sysenter assembly language instruction, introduced in the Intel Pentium II microprocessors; this instruction is now supported by the Linux 2.6 kernel.
使用int $0x80的方式是interrupt，使用sysenter 的方式则不是interrupt，但是它的作用其实和interrupt非常类似，我们可以将它看做是interrupt。
关于sysenter，参加：
上面描述的interrupt主要来自于hardware，其实system call的实现也是依赖于interrupt。
在chapter 4.2. Interrupts and Exceptions的“Programmed exceptions”有这样的描述：
Such exceptions have two common uses: to implement system calls and to notify a debugger of a specific event (see Chapter 10).
TODO
gitbooks Linux insides System calls in the Linux kernel. Part 1.