为什么在这是root=hd，到第二行就成了root=sd
第一行和第二行指定root分区有什么区别

While the argument looks like a normal device node in the /dev directory, there obviously is no /dev directory before the root fs is mounted, so the kernel can not look up a dev node there. Instead, certain well known device names are hard coded into the kernel so the string can be translated to the device number. Because of this, the kernel can recognize things like /dev/sda1, but not more exotic things like /dev/mapper/vg0-root or a volume UUID.

The use of the term root file system has a different meaning in regard to GRUB. It is important to remember that GRUB's root file system has nothing to do with the Linux root file system.
1.
The GRUB root file system is the top level of the specified device. For example, the image file (hd0,0)/grub/splash.xpm.gz is located within the /grub/ directory at the top-level (or root) of the (hd0,0) partition (which is actually the /boot/ partition for the system).
2.
Next, the kernel command is executed with the location of the kernel file as an option. Once the Linux kernel boots, it sets up the root file system that Linux users are familiar with. The original GRUB root file system and its mounts are forgotten; they only existed to boot the kernel file.

Note that GRUB's root device doesn't necessarily mean your OS's root partition; if you need to specify a root partition for your OS, add the argument into the command kernel.

是不是在 set root这一行，如果设置对了，下面一行 linux /boot/vm (tab)就会自动补上核心名称，如果set root 不对，不会自动补上。
set root 设完了，在下面一行的 root=/dev/sda7中，ls ／dev 看了下，没有sda7，正常么？