很多时候无法使用C来完成你想要做得事情，例如CPUID、处理控制寄存器等等，不过有两种方法可以使用，一是在汇编底下链接至你的c代码，或者使用InlineAssembly.
I/O access
OUTx sends a byte (or word or dword) on a I/O location. Traditionnal names are outb, outw and outl respectively. the "a" modifier enforce value to be placed in eax register before the asm command is issued and "Nd" allows for one-byte constant values to be assembled as constants and use edx register for other cases.
static __inline__ void outportb(uint16_t port,uint8_t value)
{
__asm__ volatile ("outb %0,%1"
::"a" ((char) value), "d"((uint16_t) port));
}
INx receives a byte (or word or dword) from an I/O location. Traditionnal names are inb, inw and inl respectively
static __inline__ uint8_t inportb(uint16_t port)
{
uint8_t _v;
__asm__ volatile ("inb %1,%0"
:"=a" (_v):"d"((uint16_t) port));
return _v;
}