请问下面的主设备号不是应该左移20位吗怎么下面的代码是左移8位
/*
Some programs want their definitions of MAJOR and MINOR and MKDEV
from the kernel sources. These must be the externally visible ones.
*/
#define MAJOR(dev) ((dev)>>8)
#define MINOR(dev) ((dev) & 0xff)
#define MKDEV(ma,mi) ((ma)<<8 | (mi))
#endif /* _LINUX_KDEV_T_H */