title: What's the meaning of this part of code in Linux source code?
body: <pre><code>ENTRY(gdt_table)
   .quad 0x0000000000000000 /* NULL descriptor */
  
   .quad 0x0000000000000000 /* not used */
  
   .quad 0x00cf9a000000ffff /* 0x10 kernel 4GB code at 0x00000000 */
  
   .quad 0x00cf92000000ffff /* 0x18 kernel 4GB data at 0x00000000 */
  
   .quad 0x00cffa000000ffff /* 0x23 user 4GB code at 0x00000000 */
  
   .quad 0x00cff2000000ffff /* 0x2b user 4GB data at 0x00000000 */
  
   .quad 0x0000000000000000 /* not used */
  
   .quad 0x0000000000000000 /* not used */
  </code></pre>