The Original 64-bit x86 from Intel 

Early concept of EM64T, 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 EM64T.
The above text is copied from user 71.57.33.215, without his or her permission, but if he or she requests I would remove from my talk page.