What are program-invisible registers?

The local and global descriptor tables are determined into the memory system. So as to access and give the address of these tables, the program invisible registers utilized. The program invisible registers are not directly addressed through software so they are specified name.

 The global descriptor table register (GDTR) and interrupt descriptor table register (IDTR) has the base addresses of the descriptor table and its boundary. The limit of all descriptor tables is 16 bits since the maximum table length is 64 Kbytes. While the protected mode operation is needed, the address of the global descriptor table and its limit are loaded in the global descriptor table register.