因为内核实现了很多不同的系统调用，因此进程必须传递一个名为系统调用号（system call number）的参数来识别所需要的系统调用，eax寄存器就用作此目的。
基于52个网页-相关网页
get version number system call 取版本号系统调用
But if a call comes from a number that has not been approved, the system blocks it.
但是如果信号来自未审核的陌生号码，系统就会对其进行阻拦。
By doing this the write system call returns with the number of bytes it wrote before it got interrupted and the errno set to success.
在这样处理方式下，write系统调用返回被信号中断前已写的字节数，并将errno全局变量设置为成功。
Also note that the result of the write system call is the number of characters written — in this case, 17 plus one carriage return.
还可以注意到，write系统调用的结果是写入字符的个数数目，在这个示例中为17加上一个回车。
应用推荐