About Operating Mode Dear Sir or Madam I am so sorry to bother you but I ve seen your post on Processor Register the original text is copied to bellow IA32e 8 8 Early concept of EM64 T coded name Yamhill the register were extending from 32 bit to 64 bit unlike x86 64 this design do not come with additional general purpose register r8 r15 and lack of 64 bit MMU and address mode support 32 bit address mode with PAE for maximum compatibility with legacy application early Prescott Tejas and Timna were the only implement of this ISA and later is replaced by EM64 T That is so interesting and amazing Besides I wonder the information on Operating Mode Is the original IA32e mode a sub Mode of protected mode or a new mode like it in AMD64 Does the original 32 bit protected OS need to be changed from the scratch or just provides extension support for those new introduced 64 bit instructions That is most amazing thing for me I am sorry to bother you I was just a computer science student when I knew it and I am merely a computer hobbyist right now Thank you with my best wishes