Extreme black hole entropy obtained in an operational approach
  The entropy of anti-de Sitter Reissner-Nordstrom black hole is found to be stored in the material which gathers to form it and equals to $A/4$ regardless of material states. Extending the study to two kinds of extreme black holes, we find different entropy results for the first kind of extreme black hole due to different material states. However for the second kind of extreme black hole the results of entropy are uniform independently of the material states. Relations between these results and the stability of two kinds of extreme black holes have been addressed. 
Bin Wang, Ru-Keng Su and Elcio Abdalla