The descriptor cache is 96 bits per segment register (http://www.rcollins.org/ddj/Aug98/Aug98.html http://www.rcollins.org/ddj/Aug98/Aug98.html) so the 12 regis

The descriptor cache is 96 bits per segment register (http://www.rcollins.org/ddj/Aug98/Aug98.html http://www.rcollins.org/ddj/Aug98/Aug98.html) so the 12 registers might be the bases and limits with the extra metadata stored somewhere else. It might also be that each limit is fewer than 32 bits since the encoding for segment limits uses 21 bits in memory.